Accepted to the Astrophysical Journal Preprint typeset using L A T E X style emulateapj v. 5/2/11 ALMA MULTI–LINE IMAGING OF THE NEARBY STARBURST NGC253 Meier, David S. 1,2,3 , Walter, Fabian 4,2 , Bolatto, Alberto D. 5 , Leroy, Adam K. 4 , Ott, J¨ urgen 2 , Rosolowsky, Erik 7 , Veilleux, Sylvain 8,5 , Warren, Steven R. 5 , Weiß, Axel 9 , Zwaan, Martin A. 10 , Zschaechner, Laura K. 4 Draft version January 26, 2015 ABSTRACT We present spatially resolved (⇠50 pc) imaging of molecular gas species in the central kilopar- sec of the nearby starburst galaxy NGC253, based on observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA). A total of 50 molecular lines are detected over a 13 GHz bandwidth imaged in the 3 mm band. Unambiguous identifications are assigned for 27 lines. Based on the measured high CO/C 17 O isotopic line ratio (&350), we show that 12 CO(1 - 0) has moder- ate optical depths. A comparison of the HCN and HCO + with their 13 C–substituted isotopologues shows that the HCN(1–0) and HCO + (1–0) lines have optical depths at least comparable to CO(1–0). H 13 CN/H 13 CO + (and H 13 CN/HN 13 C) line ratios provide tighter constraints on dense gas properties in this starburst. SiO has elevated abundances across the nucleus. HNCO has the most distinctive morphology of all the bright lines, with its global luminosity dominated by the outer parts of the central region. The dramatic variation seen in the HNCO/SiO line ratio suggests that some of the chemical signatures of shocked gas are being erased in the presence of dominating central radiation fields (traced by C 2 H and CN). High density molecular gas tracers (including HCN, HCO + , and CN) are detected at the base of the molecular outflow. We also detect hydrogen β recombination lines that, like their ↵ counterparts, show compact, centrally peaked morphologies, distinct from the molecular gas tracers. A number of sulfur based species are mapped (CS, SO, NS, C 2 S, H 2 CS and CH 3 SH) and have morphologies similar to SiO. Subject headings: galaxies: formation — galaxies: evolution — individual galaxy (NGC 253) — astrochemistry; ISM — radio lines 1. INTRODUCTION Characterizing the physical properties of the molecular gas phase in galaxies, the key phase for star formation (review by Kennicutt & Evans 2012), is among the main drivers in the studies of nearby galaxies. The 12 CO line is by far the brightest millimeter–wave molecular line in the interstellar medium (ISM) and has therefore been the main tracer of the molecular medium in both low– and high–redshift studies (e.g., reviews by Bolatto, Leroy & Wolfire 2013 and Carilli & Walter 2013). Even though CO emission is a reasonable tracer of the morphology and mass associated with the molecular ISM, observa- tions of other tracer molecules, that have di↵erent crit- ical densities and excitation temperatures, provide key information to constrain the physical processes in the 1 New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM, USA; E-mail: [email protected]2 National Radio Astronomy Observatory, Pete V.Domenici Array Science Center, P.O. Box O, Socorro, NM, 87801, USA 3 Adjunct Astronomer, National Radio Astronomy Observa- tory 4 Max-Planck Institut f¨ ur Astronomie, K¨ onigstuhl 17, D- 69117, Heidelberg, Germany 5 University of Maryland, College Park, Department of As- tronomy and Joint Space–Science Institute 6 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA 7 Department of Physics, University of Alberta, Edmonton AB T6G 2E1, Canada 8 Department of Astronomy, University of Maryland, College Park, MD 20742, USA 9 Max Planck Institut f¨ ur Radioastronomie, Auf dem Hgel 69, 53121 Bonn 10 European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei M¨ unchen, Germany ISM. Studying a whole suite of molecular line tracers sheds light on the chemical state of the ISM, including the gas cooling and ionization balance in the molecular ISM. Ultimately, surveying provides the means by which to characterize molecular cloud conditions that are af- fected by the galactic environment, including feedback and shocks caused by star formation, and dynamical pro- cesses within a galaxy (e.g., Meier & Turner 2005). Millimeter line surveys of nearby galaxies using single– dish telescopes have shown that their spectra are rich in molecular lines (e.g., Usero et al. 2004, Martin et al. 2006, Costagliola et al. 2011, Snell et al. 2011, Aladro et al. 2011, 2013). However, given the resolution of single–dish telescopes, most of these studies could only provide in- tegrated measurements, making it difficult to investigate spatial changes in chemical properties. Spatially resolved surveys using millimeter interferometers have been lim- ited to only a handful of transitions due to small band- widths and low sensitivity (e.g., Meier & Turner 2005, 2012, Meier et al. 2014, Martin et al. 2014). These studies demonstrate the presence of strong chemical dif- ferentiation can exist within the molecular gas in nearby galaxies and their usefulness for constraining the evolu- tionary properties of the nucleus. The newly commissioned ALMA facility is revolution- izing chemical studies of galaxies. Its sensitivity, even in ‘Early Science’ mode is unprecedented, and molecules that are typically significantly fainter than the brightest ones (i.e., CO, HCN, HCO + , CN) can be detected in rea- sonable integration times. In addition ALMA’s large in- stantaneous bandwidth of Δ⌫ =8 GHz implies that mul- tiple lines (in particular in the 3 mm band where Δ⌫ /⌫ is arXiv:1501.05694v1 [astro-ph.GA] 23 Jan 2015
Transcript
Accepted to the Astrophysical JournalPreprint typeset using LATEX style emulateapj v. 5/2/11
ALMA MULTI–LINE IMAGING OF THE NEARBY STARBURST NGC253
Meier, David S.1,2,3, Walter, Fabian4,2, Bolatto, Alberto D.5, Leroy, Adam K.4, Ott, Jurgen2, Rosolowsky,Erik7, Veilleux, Sylvain8,5, Warren, Steven R.5, Weiß, Axel9, Zwaan, Martin A.10, Zschaechner, Laura K.4
Draft version January 26, 2015
ABSTRACT
We present spatially resolved (⇠50 pc) imaging of molecular gas species in the central kilopar-sec of the nearby starburst galaxy NGC253, based on observations taken with the Atacama LargeMillimeter/submillimeter Array (ALMA). A total of 50 molecular lines are detected over a 13 GHzbandwidth imaged in the 3mm band. Unambiguous identifications are assigned for 27 lines. Basedon the measured high CO/C17O isotopic line ratio (&350), we show that 12CO(1 � 0) has moder-ate optical depths. A comparison of the HCN and HCO+ with their 13C–substituted isotopologuesshows that the HCN(1–0) and HCO+(1–0) lines have optical depths at least comparable to CO(1–0).H13CN/H13CO+ (and H13CN/HN13C) line ratios provide tighter constraints on dense gas propertiesin this starburst. SiO has elevated abundances across the nucleus. HNCO has the most distinctivemorphology of all the bright lines, with its global luminosity dominated by the outer parts of thecentral region. The dramatic variation seen in the HNCO/SiO line ratio suggests that some of thechemical signatures of shocked gas are being erased in the presence of dominating central radiationfields (traced by C2H and CN). High density molecular gas tracers (including HCN, HCO+, and CN)are detected at the base of the molecular outflow. We also detect hydrogen � recombination lines that,like their ↵ counterparts, show compact, centrally peaked morphologies, distinct from the moleculargas tracers. A number of sulfur based species are mapped (CS, SO, NS, C2S, H2CS and CH3SH) andhave morphologies similar to SiO.Subject headings: galaxies: formation — galaxies: evolution — individual galaxy (NGC 253) —
astrochemistry; ISM — radio lines
1. INTRODUCTION
Characterizing the physical properties of the moleculargas phase in galaxies, the key phase for star formation(review by Kennicutt & Evans 2012), is among the maindrivers in the studies of nearby galaxies. The 12CO lineis by far the brightest millimeter–wave molecular line inthe interstellar medium (ISM) and has therefore been themain tracer of the molecular medium in both low– andhigh–redshift studies (e.g., reviews by Bolatto, Leroy &Wolfire 2013 and Carilli & Walter 2013). Even thoughCO emission is a reasonable tracer of the morphologyand mass associated with the molecular ISM, observa-tions of other tracer molecules, that have di↵erent crit-ical densities and excitation temperatures, provide keyinformation to constrain the physical processes in the
1 New Mexico Institute of Mining and Technology, 801 LeroyPlace, Socorro, NM, USA; E-mail: [email protected]
2 National Radio Astronomy Observatory, Pete V.DomeniciArray Science Center, P.O. Box O, Socorro, NM, 87801, USA
3 Adjunct Astronomer, National Radio Astronomy Observa-tory
5 University of Maryland, College Park, Department of As-tronomy and Joint Space–Science Institute
6 National Radio Astronomy Observatory, 520 EdgemontRoad, Charlottesville, VA 22903, USA
7 Department of Physics, University of Alberta, Edmonton ABT6G 2E1, Canada
8 Department of Astronomy, University of Maryland, CollegePark, MD 20742, USA
9 Max Planck Institut fur Radioastronomie, Auf dem Hgel 69,53121 Bonn
10 European Southern Observatory, Karl-Schwarzschild-Str.2, D-85748 Garching bei Munchen, Germany
ISM. Studying a whole suite of molecular line tracerssheds light on the chemical state of the ISM, includingthe gas cooling and ionization balance in the molecularISM. Ultimately, surveying provides the means by whichto characterize molecular cloud conditions that are af-fected by the galactic environment, including feedbackand shocks caused by star formation, and dynamical pro-cesses within a galaxy (e.g., Meier & Turner 2005).Millimeter line surveys of nearby galaxies using single–
dish telescopes have shown that their spectra are rich inmolecular lines (e.g., Usero et al. 2004, Martin et al. 2006,Costagliola et al. 2011, Snell et al. 2011, Aladro et al.2011, 2013). However, given the resolution of single–dishtelescopes, most of these studies could only provide in-tegrated measurements, making it di�cult to investigatespatial changes in chemical properties. Spatially resolvedsurveys using millimeter interferometers have been lim-ited to only a handful of transitions due to small band-widths and low sensitivity (e.g., Meier & Turner 2005,2012, Meier et al. 2014, Martin et al. 2014). Thesestudies demonstrate the presence of strong chemical dif-ferentiation can exist within the molecular gas in nearbygalaxies and their usefulness for constraining the evolu-tionary properties of the nucleus.The newly commissioned ALMA facility is revolution-
izing chemical studies of galaxies. Its sensitivity, evenin ‘Early Science’ mode is unprecedented, and moleculesthat are typically significantly fainter than the brightestones (i.e., CO, HCN, HCO+, CN) can be detected in rea-sonable integration times. In addition ALMA’s large in-stantaneous bandwidth of �⌫=8GHz implies that mul-tiple lines (in particular in the 3mm band where �⌫/⌫ is
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highest) are covered in each observation. The study pre-sented here does not intend to discuss the details of everydetected line but instead present the data and highlight therich chemistry provided by ALMA’s new capabilities, inthe nearby starburst galaxy NGC253. This paper com-plements two other studies of NGC253 using the samedataset, one describing the molecular outflow revealed in12CO emission (Bolatto et al. 2013, hereafter B13) andthe other constraining the molecular clump properties inthe central starburst region using the brightest dense gastracers available (Leroy et al. 2014, hereafter L14).The paper is structured as follows: Sec. 2 briefly sum-
marizes the observations, data reduction and data prod-ucts. In Sec. 3 we present our identification of the richsuite of molecular line emission in the galaxy. We thenproceed to sketch a simple picture of the conditions in thecentral region of NGC253, by focusing on several groupsof key molecular gas tracers (Sec. 4). We conclude byproviding a short summary in Sec. 5. Throughout the pa-per we adopt a distance to NGC253 of 3.5Mpc (Rekolaet al. 2005), i.e., 1” corresponds to 17 pc at that distance.The systemic velocity of NGC253 is ⇠250 km s�1 (e.g.,Houghton et al. 1997).
2. OBSERVATIONS AND DATA REDUCTION
Observations: We have mapped NGC253 with ALMAin cycle 0 (16 antenna array) in two di↵erent frequencysetups and configurations. Details regarding the obser-vations and data reduction can be found in B13 andL14 and we here summarize the observational parametersthat are most relevant for the current study. Both se-tups covered an instantaneous bandwidth of 8GHz each:The first frequency setup, covering 85.6–89.6GHz (lowerside–band, LSB) and 97.4–101.4GHz (upper side–band,USB), is a central 3–point mosaic along the major axisin the extended configuration (average beam size: ⇠2”,⇠35 pc). This mosaic covers the central 10 (1 kpc) ofNGC253’s starburst. The second setup covers 99.8–103.7GHz (LSB) and 111.8–115.7GHz (USB) and is a 7–point mosaic of NGC253’s center in the compact configu-ration. This yields an average beam size of ⇠4” (⇠70 pc)and a field of view of roughly 1.50 (⇠1.5 kpc).Data reduction: All of the calibration and imaging of
the data cubes was completed within CASA, includingself–calibration to remove residual phase and flux calibra-tion errors. After imaging, much of the remaining analy-sis was performed with IDL scripts with some use of theMiriad and Matlab software packages. Most of the datapresented here is based on the continuum–subtractedcube, where the continuum was defined in those spec-tral regions where no line emission was evident. Ourfinal cube has 20 km s�1 velocity resolution and a typicalRMS of ⇠2.0mJy beam�1 (extended, LSB: 1.9mJy, ex-tended, USB: 2.1mJy, compact, LSB: 2.0mJy, compact,USB: 3.3mJy).Final data cubes: We display the information in the
data cubes in two complementary fashions: through aposition–velocity (pV) diagram along the major axis ofNGC253 in Fig. 1; a representation similar to long–slitspectroscopy in optical/NIR astronomy (Fig. 1). Thesecond (Fig. 2) is a single spectrum taken toward a cen-tral molecular peak (position 5). As outlined above, thedata consist of four final data cubes, the LSB and USBcubes for the extended configuration, and those for the
compact configuration. Figs. 1 and 2 also show our lineidentification (see also Tab. 1), as further discussed inSec. 3.Integrated line maps: We created line cubes for each
line, and then blanked those with a mask derived fromthe CO(1-0) line (by far the brightest line in the band-pass). The integrated intensity map for a given line wasthen derived by simply adding all data in the respectivedata cube (without any further flux cuto↵). Integratedline maps are corrected for primary beam attenuationand are shown in Fig. 3.Uncertainties: We calculated error maps by taking the
number of channels per pixel into account. An inspec-tion of the data cubes revealed that these error mapsprovide too optimistic uncertainties, as they do not ac-count for artifacts (in particular the negative ‘bowl’ dueto the missing short spacings) in the current data. Wethus adopt a conservative 10% error for pixels that havebeen detected at high S/N (> 10), where the noise Nis from taken from our error maps. For pixels that aredetected at lower S/N (5<S/N<10) we adopt an evenmore conservative 30% uncertainty, to also account forpossible issues in baseline determination. We ignore allpixels that have a S/N <5 in the analysis that follows.We present our line intensity measurements towards 10positions in NGC253 (Fig. 4, coordinates in Tab. 1) inTab. 1. Toward the center of the galaxy (in particu-lar region 6, Fig. 4) line emission is observed againststrong continuum (L14). Some resulting absorption willdecrease the integrated line signal at these locations. Be-cause of this the fluxes toward this location should beconsidered highly uncertain.Spatial filtering: As ALMA is an interferometer it acts
as a spatial filter, sampling only a range of spatial scales.So the observations will potentially resolve out some flux.In the compact (extended) configuration spatial scales of12 – 47 k� (23 – 100 k�) [90th percentile] were sampled,corresponding to 4.400 – 1800 (200 – 900). Hence the observa-tions should adequately sample fluxes uniform over ⇠1000
(in one channel). The one exception to this is 12CO(1–0),which has been zero-spacing corrected and therefore de-tects all flux (see discussion in B13). It is not possible toestimate the percentage of detected flux for every line inthe survey because suitable single-dish observations areoften not available. We do, however, determine this frac-tion for a number of transitions where possible. For thefollowing transitions we calculate detected flux percent-ages over single-dish beam of ⇠22 00 – ⇠2800 of: HCN(1–0) — 100 % (Paglione et al 1995), HCO+(1–0) — 55 %(Martin et al. 2009), C17O(1–0) — 60 % (Henkel et al.2014), SiO(2–1) — 110 % (Martin et al. 2009), CN(1–0; 32– 1
% (Nakajima et al. 2011) and HNCO(404–303) — 50 %(Nguyen-Q-Rieu et al. 1991) [though this value is highlyuncertain because HNCO does not peak where Nguyen-Q-Rieu et al. pointed]. Therefore, it appears that thedata consistently detect at least 50 % of their respectivesingle dish fluxes. Furthermore our discussion focuses onthe compact clumps of emission, where much higher frac-tions of the flux are detected (&90 %). (In fact, spatialfiltering of the interferometer actually a↵ords advantagesbecause it allows the separation of these compact struc-tures from any extended di↵use medium that can domi-
ALMA Multi–line Imaging of the Nearby Starburst Galaxy NGC253 3
112.247 comp, USB tent. IDC17O(1–0) 112.35928 comp, USBHCOOH 112.459 comp, USB tent. ID, blend: (53,3 � 43,2) & (53,2 � 43,1)CN(1-0;1/2-1/2) 113.19128 comp, USB secondary CN FS groupCN(1-0;3/2-1/2) 113.49097 comp, USB main CN FS groupU 114.218 comp, USB poss. IDs: CO(1-0; v=1);
C4H(12-11; 25/2-23/2& 23/2-21/2)U 114.605 comp, USB no obvious IDCH3CHO(60,6 � 50,5)A++E 114.952 comp, USBNS(5/2-3/2;7/2-5/2) 115.15394 comp, USB blended with COCO(1–0) 115.27120 comp, USB Bolatto et al. 2013NS(5/2-3/2;7/2-5/2) 115.55625 comp, USB
Note. — Note: species in italics represent tentative identifications. In these cases, the frequency listed is the observed one corrected to restfrequency based on the known velocity field and does not reflect the rest frequency of the tentatively identified line. Lines labeled U are the onesthat we could not securely identify, and in some cases we give possible ID’s in the last column.
nate single-dish observations.) Finally, we avoid compar-ing line intensities derived from di↵erent array configura-tions wherever possible in this study, to further mitigateagainst di↵erences in resolved flux. Hence we concludethat uncertainties in line ratios due to di↵erent degreesof missing flux are .10 %.
3. RESULTS
3.1. The Nucleus of NGC253
The nucleus of NGC253 is characterized by the innerportion of NGC253’s large–scale bar: The highly inclined(i ' 78 deg) nuclear disk extends from the very center outto a radius, r & 370 pc. In some transitions emission isseen out to the edge of the mosaic. The outer part ofthe nuclear disk (the ‘outer nuclear disk’) is suggested torepresent the location where gas flowing radially inwardalong the large-scale bar collects between the outer andinner Lindblad resonance (Garcia-Burillo et al. 2000).Embedded within the outer disk is a compact (r ' 170
4 Meier, Walter, Bolatto et al.
Fig. 1.— Position–velocity cut along the major axis of the central bar of NGC253 (equivalent to an optical/near–infrared long–slitspectrum). Angular o↵sets are given with respect to the galaxy center. The left two panels show the lower and upper sidebands of theextended ALMA configuration, the right two panels the corresponding sidebands of the compact configuration. Identified molecular spectrallines are labeled (secure: bold, tentative: italics, see Tab. 1 for a full description of transitions). The CO line is completely saturated in thispresentation and the HCN, HCO+ and CN lines are partially saturated. Table 1 lists the coordinate about which the o↵sets are referenced.
Note. — Coordinates of the 10 regions in NGC253 (indicated inFig. 4) for which intensities have been measured (see Tab. 1). Thecoordinates of (0, 0) reference position of the maps is also given.
pc) region exhibiting a large quantity of high density gasand intense star formation, which is discussed in detail
in Section 4.2. A molecular gas outflow/wind is beingdriven from this inner nuclear disk, as discussed in B13.We have selected 10 individual locations that span the
nuclear disk and the base of the molecular outflow inNGC253 (Tab. 1). Regions 3–9 trace the inner starburstdisk from west to east and regions 5 and 6 are coincidentwith the detected millimeter continuum emission in thegalaxy (L14). Regions 1 and 10 are located at the west-ern and eastern base of the southern molecular outflow(B13). Region 2 indicates the shocked region towards thenorth. The GMC physical properties in these regions arediscussed in detail in L14. We further caution that someof our line measurements towards region 6 (the center)may be a↵ected by absorption and continuum subtrac-tion uncertainties (as discussed in Sec. 2).In Tab. 1 we provide a summary of all detected lines
(column 1), their rest frequencies (column 2) and obser-vational setup (column 3). For each of our 10 regions,we measure the peak integrated intensity (in Kkm s�1)at that position and report the values in Tab. 1. We
ALMA Multi–line Imaging of the Nearby Starburst Galaxy NGC253 5
Fig. 2.— Extracted spectrum near the center of NGC253 (position 5 in Fig. 4), on a logarithmic scale. To deal with subtle issuesregarding the bandpass calibration, the data have been independently continuum subtracted for each spectral window. After that, theaverage continuum flux density in a given spectral window was added to the spectrum. The flux density o↵set between the observationsobtained in the compact and extended configurations is real; the observations in the compact configuration recover more of the extendedflux. As in Fig. 1, identified molecular spectral lines are labeled (secure: bold, tentative: italics, see Tab. 1 for full description of transitions).
measure all values from beam–matched maps, i.e., all thecompact and extended configuration data have been con-volved to a common resolution, respectively (this com-mon resolution is given in column 4 of Tab. 1, whereascolumn 3 gives the original resolution). We have dis-cussed our uncertainty estimates in Sec. 2. Tab. 1 alsoincludes the intensity measurement in each transition forthe entire galaxy (columns 5 and 6).
3.2. Molecular Emission Across NGC 253’s Nucleus
Fig. 1 shows the position–velocity (pV) diagram ori-ented along the molecular bar (i.e., along the major axisof the galaxy) of all four data cubes. This figure showsthe richness of the ALMA data cubes, and the challengesin finding the correct line identification. Already fromthis presentation it is clear that the emission of the dif-ferent line species emerge from di↵erent regions in thegalaxy.We detect at least 50 emission lines in total of which
27 have unambiguous identifications. This correspondsto a detection ratio of one line every ⇠0.25 GHz. Speciesprinted in italics are tentative IDs (13 lines). These lineshave clear candidate transitions from a species not yetwell established in extragalactic systems (see Sec. 3.3).The candidate IDs are given in column 4 of Tab. 1. Insome cases we were not able to find a plausible line identi-fication or the broadness of the spectral line prohibited aunique identification. Those lines are labeled U (uniden-tified; 10 cases). In those cases we give our best estimatefor each feature’s rest frequency in column 2.We show line maps for 27 of the bright, unblended lines
in Fig. 3. In Fig. 4 we select out six species that summa-rize the basic morphological patterns seen in the sample.Each represent key tracers of the di↵erent phases of themolecular gas, as discussed below. These are: 12CO, atracer for the overall molecular gas distribution, 12C17O,
an optically thin tracer of molecular gas emission, C2H, aphoton-dominated region (PDR) tracer, H40↵, a hydro-gen recombination line and H II region tracer, HCN, ahigh–density gas tracer and HNCO, a weak shock tracer.From Figs. 3 and 4 it is apparent that the morpholo-
gies change significantly between species (as is alreadyevident from Fig. 1). Molecular gas column densities re-main quite large across the entire nuclear disk, maintain-ing N(H2) & 1023 cm�2 over much of the disk (Sec. 3.4).The wide range of dense gas tracers observed demon-strates that dense gas is present across much of this re-gion, but the dense gas fraction increases toward the in-ner disk (L14). The millimeter hydrogen recombinationlines, including for the first time H� lines, show thatdense H II regions associated with the young starburst(or possibly AGN; Mohan et al. 2002) are confined tothe innermost part of this inner disk. Species like C2H,CN and CH3C2H also dominate from this inner nucleardisk. HNCO, on the other hand, is dominated by theouter nuclear disk.The brightest and most widespread transitions (HCN,
HCO+ and CN) show weak emission extended verticallybetween 1 and 2, following the base of the molecularoutflow, suggesting there is dense molecular gas here.However, we do not discuss the dense gas tracers in theoutflow in detail here due to the di�culties dealing withthe morphology of the maps above and below the disk inthe presence of the negative ’bowl’.
3.3. Tentative and Unidentified Lines
There are a number of detected lines for which no clearidentification was possible. We have listed tentative iden-tifications in Tab. 1 – in some cases no plausible specieswere found, and these lines constitute unidentified (U)lines. The majority of these tentative identifications (butnot tentative detections) match prominent transitions
6 Meier, Walter, Bolatto et al.
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88.63 GHz
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10
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20
Fig. 3.— Integrated intensity maps of selected molecules in our observations (Tab. 1). The respective intensity scale (in units of K km s�1)is shown in each panel. The name of the line and its rest frequency are given in the top right corner of each panel. The beam size is shownin the bottom right corner. Tentatively identified lines are labeled in italics.
from larger molecules, including the aldehydes CH3CHOand NH2CHO, cyanides CH2CN and CH3CH2CN anda number of organics of similar complexity, CH2CO,HCOOH and CH3CH2OH. These molecules are abun-dant in the Galactic center (e.g., Cummins et al. 1986)and are expected to be detectable at our sensitivity inNGC253. If follow–up, multi-line studies confirm theseIDs then they would represent their first extragalacticdetections.
3.4. Excitation and Abundances
When appropriate, an estimate of abundances is madefor the detected species. Molecular column densitiesNmol are determined assuming optically thin LTE emis-sion:
Nmol =
✓3kQeEu/kTex
8⇡3⌫Sulµ20gu
◆Imol, (1)
where Sul the line strength, and gu’s and Eu are theupper state degeneracy and energy, respectively, Q isthe partition function, µo is the dipole moment in debyeand Tex is the excitation temperature associated withthe transition (e.g., Turner 1991). For the symmetric(CH3SH) and asymmetric (HNCO, H2CS) tops Q is pro-
portional to T3/2ex , whereas Q for the linear rotors (the
rest) is proportional to Tex.To calculate abundances it is necessary to adopt an
Tex. Since this survey covers only one band, gas exci-tation is not the focus of this project. The only lines
ALMA Multi–line Imaging of the Nearby Starburst Galaxy NGC253 7
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H50"
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101.48 GHz
−40−30−20−10010203040
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0.8
R.A. Offset (")
NH2CHO?
102.06 GHz
−40−30−20−10010203040
−0.1
0
0.1
0.2
0.3
0.4
0.5
Fig. 3.— Cont.
that are detected and directly constrain gas excitation isa pair of SO transitions, SO(32 � 21) and SO(22 � 11).From this pair, an excitation temperature is estimated,via:
Tex =10.09 K
ln( R2.18 )
, (2)
where R is the intensity ratio, SO(32 � 21)/SO(22 � 11).Towards the inner nuclear disk this ratio has a value ofR'2.5. There is some evidence for the ratio to increasetowards the outer nuclear disk (R.4), but the faintnessof SO(22 � 11) limits what can be said. For these ratios,we estimate the inner disk to have an excitation temper-ature of 74 K in SO, dropping to Tex .17 K. Indirectmethods of estimating Tex can also be achieved by ra-diative transfer modeling (see section 4.2). There Tex isfound to range between ⇠6 – 45 K. Therefore, Tex likely
falls between at least 10 – 75 K, across much of the nu-clear region. Ott et al. (2005) use NH3 to estimate gaskinetic temperatures, Tkin, across the inner nuclear diskof approximately twice the 74 K measured here, howeverKnudsen et al. (2007) (and this paper) favor Tkin nearerthis value for HCN and HCO+ (see section 4.2). Thedrop in Tex toward the outer nuclear disk region reflectsSO becoming more strongly sub-thermal in lower densitygas.The relevant excitation temperature for a given species
/ transition depends on a number of factors, includinggas density, opacity, molecular structure and locationwithin the nucleus. For transitions with e↵ective criticaldensities significantly above the gas density, Tex will belower than the Tkin (subthermal excitation). Changes innH2 and Tkin with position in the nucleus imply changesin excitation. The e↵ective critical density of a species
8 Meier, Walter, Bolatto et al.
De
c.
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se
t ("
)
CH3C2H
102.55 GHz
−40
−30
−20
−10
0
10
20
30
40
0
1
2
3
4
H2CS
103.04 GHz
−0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
C17O
112.36 GHz
0
0.5
1
1.5
2
De
c.
Off
se
t ("
)
CN
113.19 GHz
−40
−30
−20
−10
0
10
20
30
40
0
10
20
30
40
CN
113.49 GHz
0
20
40
60
80
CO(v=1)/CH4?
114.22 GHz
−0.2
0
0.2
0.4
0.6
0.8
1
1.2
R.A. Offset (")
De
c.
Off
se
t ("
)
CH3CHO?
114.94 GHz
−40−30−20−10010203040−40
−30
−20
−10
0
10
20
30
40
−0.2
0
0.2
0.4
0.6
R.A. Offset (")
CO
115.27 GHz
−40−30−20−10010203040
100
200
300
400
500
600
700
800
R.A. Offset (")
NS
115.56 GHz
−40−30−20−10010203040
0
0.5
1
1.5
2
Fig. 3.— Cont.
depends on line opacity, ⌧ , (photons become an e↵ec-tive ’collision’ partner) and hence abundances, with highopacity lowering critical densities, ncr, roughly in pro-portion to ⌧ . Finally, even in the case that a transitionis thermalized, the LTE calculated columns are strict up-per limits because it assumes that all transitions of themolecule are thermalized up to arbitrarily high energies.With these caveats stated, abundances are the impor-
tant astrochemical variable and so we report ’reference’ranges. In Table 1, abundances are reported for Texranging from 10 K (first number in each entry) to 75 K(second number). Also included in the the table are ncr(not including opacity e↵ects) for the transitions. It isexpected that for the inner nuclear disk positions tran-sitions with moderate ncr (e.g., C17O, HNCO, H2CS,C2H, HCO+, and SO) will have Tex at the high end ofthe range. High ncr transitions and outer nuclear disk
locations likely will have Tex (and hence abundances) atthe low end of the range. Future, follow up, multi-linestudies of di↵erent species are necessary to narrow thesequoted ’reference’ value ranges.To convert Nmol to fractional abundances we also re-
quire H2 column density, N(H2). N(H2) is most easilyobtained from the CO(1-0) brightness and an empiricalconversion factor, XCO. We adopt a CO-to-H2 conver-sion factor of XCO = 0.5 ⇥ 1020 cm�2 (K km s�1)�1.This is a factor of four lower than typical for the Galac-tic disk (Strong et al. 1988; Hunter et al. 1997; Bolattoet al. 2013a), but consistent with what has been previ-ously estimated for NGC 253 (e.g. Paglione et al. 2001,B13, L14). Uncertainties in XCO are significant, likelyat the ±2⇥ level, and in section 4.1 some discussion ofits validity in the context of optical thin C17O(1–0) isdiscussed.
ALMA Multi–line Imaging of the Nearby Starburst Galaxy NGC253 9
De
c.
Off
se
t ("
)
CO
115.27 GHz
total molecular gas
−30
−20
−10
0
10
20
30
100
200
300
400
500
C17O
112.36 GHz
!<1 molecular gas
0
0.5
1
1.5
C2H
87.32 GHz
PDR
0
1
2
3
4
5
6
7
1
2
3
45
67
8910
R.A. Offset (")
De
c.
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se
t ("
)
H40"99.02 GHz
RRL
−30−20−100102030−30
−20
−10
0
10
20
30
0
0.2
0.4
0.6
0.8
1
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R.A. Offset (")
HCN
88.63 GHz
dense gas
−30−20−100102030
0
2
4
6
8
10
12
14
R.A. Offset (")
HNCO
87.93 GHz
weak shocks
−30−20−100102030
0
0.5
1
1.5
2
Fig. 4.— Collection of key molecular gas tracers in NGC253. From top left to bottom right: CO(1–0) (main molecular gas tracer),C17O(1–0) (optically thin tracer of the molecular gas), C2H (PDR tracer), H40↵ (radio recombination line), HCN (high density gas tracer),HNCO (shock tracer). The crosses and labels (bottom left panel) mark the di↵erent regions defined in the galaxy. Logarithmic contoursstart at 1Kkm s�1 and increase/decrease (white/grey) by a factor of 3 from one contour to the next. The beam size is indicated in thebottom left corner of each panel.
4. DISCUSSION
In the following we present a broad discussion of thechemistry in the nuclear region of NGC253. The primarygoal is to obtain spatially resolved views of the di↵erentmolecular environments in and around the starburst cen-ter.We begin our discussion by studying the main tracer of
the molecular gas, 12CO, its isotopologues, and impliedmolecular gas opacity in Sec. 4.1. In Sec. 4.2 we con-tinue with a similar discussion of the isotopologues of themain tracers of the dense molecular gas phase (H13CN,H13CO+, HN13C). This is followed by a discussion oftracers of PDRs, (Sec. 4.3), shock tracers (HNCO andSiO; Sec. 4.4), Sulfur species (Sec. 4.5), hydrogen recom-bination lines (Sec. 4.6), and other tentatively identifiedspecies (Sec. 4.7) detected in our study. A schematicpicture summarizes the overall chemistry of the centralregion of NGC 253 in the conclusions/summary section(Sec. 5).
4.1. CO Isotopologues and Gas Opacity
Our data cover a number of isotopologues (i.e.molecules where one atom is replaced with an isotope).Given the fact that atomic isotopes are much less abun-dant than the main atom, many of these isotopologuelines have low optical depth compared to the main
species. Isotopologue ratios thus provide insights into theoptical depth of the main lines and can also be used toconstrain isotopic abundance ratios. The isotopic abun-dance ratio ultimately can provide insights into stellarnucleosynthesis and possible variations of the stellar ini-tial mass function (IMF). In the discussion that followswe do not interpret the sometimes anomalous line ratiosfound for region 6 (i.e. the region that is coincident withthe central continuum emission) as it is likely that insome cases our measurements are a↵ected by absorption(Sec. 2).A comparison of the brightness of the C17O(1–0) tran-
sition to the 12CO(1–0) transition [hereafter CO(1–0)]can provide important constraints on the total H2 col-umn. Being an optically thin version of CO, even inthis extreme star formation environment, it permits COabundances to be determined by ‘counting molecules’ ifthe [CO/C17O] abundance ratio is known.The [CO/C17O] abundance ratio is constrained based
on existing measurements and nucleosynthetic expecta-tions. In the Galaxy, [CO/C17O] is .1900 at the solarradius and drops to ⇠900 in the Galactic center (e.g.,Wilson & Rood 1994, Ladd 2004, Wouterloot et al. 2008).This decrease in the [CO/C17O] abundance is consistentwith 17O being a secondary chemical evolution productformed from reactions between primary 16O and a pro-
10 Meier, Walter, Bolatto et al.
ton in intermediate mass stars. Therefore, in a stronglyprocessed location like the center of a starburst, a lowvalue of the CO/C17O abundance ratio is expected, sowe adopt [CO/C17O] = 1000, which is approximately theGalactic center value referenced above, as our nominalvalue.As shown in Fig. 5, this isotopologue line ratio is fairly
constant with position in NGC253 and large (&350). Fora ratio 16O/17O of 1000 in NGC253, the measured rationominally implies a 12CO opacity of ⇠2.5, a moderatevalue for the main CO line. Raising the intrinsic 16O/17Oabundance to 2000 gives a 12CO opacity of order ⇠5.5.Various non-LTE e↵ects can occur in a starburst en-
vironment that could alter the observed CO/C17O lineratio. Examples that artificially inflate the CO/C17Oline ratio include: 1) CO has a higher excitation tem-perature than C17O because CO has high opacity andits emission is dominated by warmer, externally heatededges of the GMCs while C17O is dominated by thecooler interiors. 2) Isotope–selective photo-dissociationmakes the cloud sizes smaller in C17O than CO. Or,3) due to the absence of radiative trapping in C17O,C17O is subthermal relative to CO and therefore fainter(e.g., Meier & Turner 2001). The first two would beexpected to be pronounced close to the nuclear star for-mation. However, interestingly, no evidence is seen foran increase in the CO/C17O ratio towards the inner disk,where PDRs are enhanced (Sec. 4.3) and warm gas dom-inates. But if there is a decrease in density away fromthe inner disk then the third option can operate in theouter disk, partially compensating for the first two. ForCO/13CO abundances [CO/13CO] & 60 (Martin et al.2010), CO opacities estimated from the CO/C13O lineratios are ⇠5 (e.g., Paglione et al. 2004, Sakamoto et al.2011). This scenario is consistent with CO/C17O data(and CO/C18O) if non-LTE e↵ects are at most a factor of⇠2. We conclude that, despite the large column densitytowards NGC 253’s nucleus, CO(1–0) is only modestlyopaque. It is likely that this low optical depth is due toincreased line widths due to turbulence, the molecularoutflow, non-circular motions and/or elevated gas tem-peratures that reduce the CO opacity per unit velocity.The moderate CO optical depth is also reflected in the
implied sub-Galactic CO-to-H2 conversion factor in star-bursts like NGC 253 (Harrison et al. 1999, Downes &Solomon 1998, Meier & Turner 2001; Bolatto, Leroy &Wolfire 2013). A comparison of the LTE C17O column tothe H2 column estimated by an XCO of 0.5⇥ 1020 cm�2
(K km s�1)�1, gives a value for [C17O/H2] constant (towithin 20%) of 1.9 ⇥ 10�7 across positions 3-10 (usingEq. 1 and Tex=75K, Sec. 3.4). If [CO/C17O] = 1000and [CO/H2] = 10�4, then we would expect a value of[C17O/H2] ' 10�7. The level of agreement between thetwo methods of constraining N(H2) is reasonable, beingless than the uncertainties in each method. The smalldi↵erences are an indicator of the uncertainty in N(H2).Agreement could be made exact by raising [C17O/H2]and/or by lowering the adopted Tex.
4.2. Dense Gas Tracers and their Isotopologues
The commonly observed dense gas line ratios,HCO+/HCN, HNC/HCN and HCO+/HNC ratios are
predicted to be sensitive to gas density, cosmic ray ion-ization and possibly the X-ray versus UV ionization rateor the present of mechanical heating (e.g., Kohno etal. 2001, Meijerink & Spaans 2005, Gracia-Carpio et al.2006, Knudsen et al. 2007, Papadopolous 2007, Krips etal. 2008, Baan et al. 2008, Loenen et al. 2008, Meijerinket al. 2011, Kazandjian et al. 2012). HCO+(1–0) ef-fective critical densities are nearly a factor of five lowerthan HCN, so to first order, elevated HCO+/HCN andHCO+/HNC ratios are expected in moderate density gas(nH2 ⇠ 104.5 cm�3). As densities increase, both transi-tions thermalize and the ratio tends to unity. Moreover,HNC/HCN ratios are expected to drop in energetic en-vironments because HNC is preferential destroyed in hotgas (e.g., Schilke et al. 1992, Meier & Turner 2005). Ifthese transitions have high optical depths, which is prob-able given the large columns of dense gas, all ratios aredriven toward unity. As a result of the large inferredoptical depths, changes in these main isotopic line ra-tios due to interesting physical/chemical changes will bepartially hidden.Here we focus on the dense gas properties in the cen-
tral starburst region of NGC253. The HCO+/HCNintensity ratio (Fig. 5) is observed to be slightly lessthan unity and exhibits very little change over the in-ner disk (except for position 2 and 3). This sug-gests that both HCN(1–0) and HCO+(1–0) are opticallythick and slightly subthermal across much of the nucleus(Knudsen et al. 2007). Here we add new HCN/H13CN,HCO+/H13CO+, H13CO+/H13CN, and HN13C/H13CNintensity ratios to further constrain dense gas properties.The isotopologues of HCN, HCO+, and HNC have loweroptical depths and allow estimates of the line opacity andchanges in the abundance ratio of these species.The observed HCN/H13CN and HCO+/H13CO+ both
have values of 10 - 15 over much of the inner nuclear disk(Fig. 5). The fact that these ratios are much lower thanthe expected [12C/13C] ratio of &60 (Martın et al. 2010)clearly demonstrates that both HCN(1–0) and HCO+(1–0) have opacities greater than one. This confirms theconclusions of earlier work (Knudsen et al. 2007). ForA =12C/13C abundance ratios of 80 above and LTE as-sumptions, both HCN and HCO+ have ⌧ ' 5� 8. Inter-estingly this is an optical depth similar or slightly largerthan inferred for CO(1–0) from C17O. This is expectedbecause the HCN/H13CN and HCO+/H13CO+ ratios arequite similar to the observed CO/13CO line ratios (e.g.,Paglione et al. 2004; Sakamoto et al. 2011). Such opticaldepths for HCN and HCO+ are large. For a Tex=75 K,inferred optically thin HCN abundances are ⇠ 2.5⇥10�8
(Table 4). However, if the isotopologues of HCN, HCO+
and HNC have a lower excitation temperature than themain species (likely true, see below), then LTE inferredopacities are lower limits.The H13CO+/H13CN ratio in particular shows larger
variation (Fig. 5) than seen in the main isotopic ratios.Moreover the average values of the ratios are di↵erent.Toward the two main inner disk GMCs (positions 5 and7) HCO+/HCN and H13CO+/H13CN have very similarratios, but toward the outer nuclear disk the isotopicdense gas ratios decrease relative to their main versions.Likewise the average HN13C/H13CN ratio of ⇠0.35 issignificantly lower than the single-dish value for the main
ALMA Multi–line Imaging of the Nearby Starburst Galaxy NGC253 11
Fig. 5.— CO and dense gas line ratios (Secs. 4.1 and 4.2). The respective line ratio is given in each panel. The x–axis number correspondsto the region number as defined in Sec. 3.2 and Fig. 4. The horizontal line in each panel shows the value for the entire galaxy (i.e., theglobal ratio). Uncertainties are determined from Table 1.
species ratio (⇠0.74; Baan et al. 2008). The fact that theisotopologue substituted species have ratios farther fromunity is consistent with the expectation that high opticaldepth in the main lines artificially drives the ratio closerto unity. These isotopologue ratios tend to move NGC253 further into the PDR-like ratio parameter space ofBaan et al. (2008), consistent with dense gas abundanceratios influenced by warm, PDR across the inner nucleardisk.Given the lower opacities of the isotopologues of HCN,
HCO+, and HNC, it is expected that subthermal exci-tation will be the most relevant non-LTE consideration,therefore it is worth modeling the physical conditionsimplied by these lines. Knudsen et al. (2007) have mod-eled the main isotopologues, including selected �J tran-sitions. Here we carry out an independent large velocitygradient (LVG) radiative transfer modeling focusing onthe isotopic line ratios. Line ratios are modeled as theyare to first order independent of the unknown areal fill-ing factor, and therefore constrain the parameter spacemore precisely. The model used is patterned after themodels of Meier et al. (2008), with Jmax extended to20 and collisional coe�cients adapted from the LeidenLAMDA database (van der Tak et al. 2007; see Table4 for references to the individual molecular rate coe�-
cients). To model the radiative transfer, assumptionsmust be made about the abundance per velocity gradi-ent. Models with a wide range of abundance per velocitygradient (10�11.33 � 10�7.67 km�1 s pc) were calculated.In general it is not easy to obtain agreement for the lineratios of HCN/HCO+, HCN/H13CN, HCO+/H13CO+,H13CO+/H13CN, HN13C/H13CO+ and HN13C/H13CN.HCO+’s e↵ective critical density is significantly lowerthan HCN and so HCO+ abundances have to be low com-pared to HCN to obtain ratios of HCO+/HCN lower thanunity. But pushing HCO+ abundances too low causesdisagreement with the isotopic ratios. If we focus onlyon the five isotopic line ratios, then reasonable solutionsthat match all are found over a fairly narrow range of pa-rameter space. We adopt XHCN/dv/dr = 10�8.1 km�1 spc and XHCO+/dv/dr = XHNC/dv/dr = 10�8.85 km�1
s pc, consistent with XHCN ' 2.3 ⇥ 10�8, XHCO+ 'XHNC ' 4.2 ⇥ 10�9 and a velocity gradient of ⇠3 kms�1 pc�1. For the isotopologues, a 12C/13C ratio of 80 isadopted. These values are in reasonable agreement withthe values listed in Table 4 and the observed gas kinemat-ics. There is room to adjust the abundances per veloc-ity gradient of the five species individually, but keepingHCO+ and HNC abundances equal does an acceptablejob. The low abundance of HCO+ demanded relative to
12 Meier, Walter, Bolatto et al.
HCO /HCN+
n(H
_2)
T_KIN
20 40 60 80 100 120 140 160 180 200
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6.0
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_2)
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_2)
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_2)
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n(H
_2)
T_KIN
20 40 60 80 100 120 140 160 180 200
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Fig. 6.— LVG modeling of the inner nuclear disk. The acceptable model predicted parameter space for (clockwise) HCN/HCO+
HCN/H13CN, HCO+/H13CO+, H13CO+/H13CN, HN13C/H13CO+, and HN13C/H13CN1–0 transitions. Contours are the ±1� observedline ratio for the average of position 5 and 7. Tick marks indicate the direction of the favored parameter space if it is not obvious. Adoptedabundance per velocity gradient values from the six di↵erent species are discussed in section 4.2. The cross marks an example set ofparameters (Tkin = 80 K, nH2 = 104.75 cm�3) that is within 1� of with all five isotopic ratios. The HCN/HCO+ line ratio is not matched,but is within 30 % of the predicted ratio.
HCN is a direct consequence of its lower e↵ective criticaldensity.Figure 6 displays the acceptable parameter space for
the ratios observed toward the inner nuclear disk (av-erage of the ratios toward positions 5 and 7) displayedin Figure 5. When admitting uncertainties, good agree-ment to all five ratios is obtained for nH2 = 104.5�5.0
cm�3 and Tkin = 60 � 120 K. The cross in Figure 6marks the location of a representative good fit to alllines. Its values are nH2 = 104.75 cm�3 and Tkin = 80K. Tkin below 60 K challenges the observed ratios forH13CO+/H13CN and HN13C/H13CN , and Tkin > 120 Kbegins to disagree with HCO+/H13CO+. Strictly speak-ing the HCO+/HCN ratio is not matched, however thedisagreement is small (⇠30 %), and at the level that weexpect other complications such as line width variationand transition depend filling factors to become impor-tant. The degree of internal consistency obtained in thesesimple uniform physical condition models is encouraging.The favored densities and kinetic temperatures of the
dense gas isotopologues match well those previouslyfound in Knudsen et al. (2007), with a range overlap-
ping but favoring about 0.25 dex lower densities. Theseresults confirm Knudsen et al. (2007)’s conclusions thatHCN, HNC and (to a somewhat lesser degree) HCO+
are moderately subthermally excited. The dense gas iso-topologues are even more strongly subthermal. For theabove favored model, we obtain Tex = 34 K and 44 Kfor HCN and HCO+, respectively. Corresponding opac-ities ⌧ are 13 and 2.4 for HCN and HCO+. For theisotopologues, Tex [⌧ ] are 7.2 K [2.4], 16 K [0.27] and 5.3K [0.71] for H13CN, H13CO+ and HN13C, respectively.LVG modeling demonstrates that while the main tran-sition line opacities crudely match those predicted fromLTE, the isotopologue opacities are significantly largerthan 12⌧/A. This stems from the fact that the e↵ectivecritical density of a transition is dependent on its opac-ity (radiative trapping), so the isotopologues are morestrongly subthermal. As a result, the level populationsof the isotopologues settle into the lowest J transitionsmore e�ciently, raising their opacity. This explains thewell known e↵ect that isotopic line ratios are gas den-sity probes (e.g., Meier et al. 2001). Furthermore thisis strong evidence that the main species, particularly of
ALMA Multi–line Imaging of the Nearby Starburst Galaxy NGC253 13
Fig. 7.— Same as Fig. 5, but for PDR tracer line ratios (Sec. 4.3).
HCN, must have opacities well in excess of unity.The slightly larger HCN/H13CN and HCO+/H13CO+
ratios together with somewhat lower HN13C/H13CN ra-tios toward the outer nuclear disk are consistent withwarm gas that is ⇠0.5 dex lower in density. However,the decrease in the H13CO+/H13CN ratio toward theouter disk is somewhat unexpected as gas density is de-creasing in this region. Raising Tkin significantly willlower the H13CO+/H13CN ratio slightly even at lowerdensity but only by ⇠10 % for Tkin up 300 K. Thise↵ect does not appear to be strong enough to explainthe observed ratio, and so this ratio is evidence for asmall, but genuine abundance enhancement of HCN andits isotopologue relative to HCO+ and HNC across thenuclear disk — or conversely a decrease of both the iso-topologues of HCO+ and HNC relative to H13CN. In-voking mechanical heating may be a viable candidate forelevating HCN at the expense of HCO+ and HNC (Loe-nen et al. 2008, Mejierink et al. 2011, Rosenberg et al.2014), because hotter gas converts HNC to HCN (e.g.,Schilke et al 1992). If mechanical heating is more pro-nounced at the base of the outflow in the outer disk thenthis may explain the further drop in H13CO+/H13CNhere. However this possibility is speculative so we con-sider the explanation for the lower H13CO+/H13CN ratiounsettled.
4.3. PDR Tracers
Given the strong radiation field in NGC253’s center,the presence of PDR tracers are expected and observed.PDRs warm gas and maintain significant amounts of car-bon in ionized form (e.g., Tielens & Hollenbach 1985), sotracers include species that form rapidly from C+ andH or H2. These include simple hydrocarbons, such asCH, C2H, c-C3H2 and molecules that form directly fromthese species, like CN (e.g. Fuente et al. 1993, Sternberg
& Dalgarno 1995, Rollig et al. 2007). In molecular gasstrongly irradiated by UV photons we expect such speciesto exhibit elevated abundances. The two main PDR trac-ers in our dataset are CN and C2H. C2H has a criticaldensity that is significantly lower than CN (see Table 4),and therefore traces somewhat more di↵use PDRs.Both CN and C2H exhibit fine/hyperfine structure
splitting and so their opacity can be determined directly.For C2H the theoretical LTE C2H(3/2-1/2)/C2H(1/2-1/2) ratio should be 2.3 in the optically thin limit (forthe subset of blended lines included), dropping to 1when optically thick. We observe a global ratio of ⇠2.4(Fig. 7). Likewise for CN, the CN(3/2-1/2)/CN(1/2-1/2)fine structure ratio is 2 (Fig. 7), which is what we observefor the inner nuclear disk of NGC253. Both CN and C2Hare likely optically thin and therefore their intensities areproportional to their column densities.The maps of these two species are strongly dominated
by the innermost disk locations, 4–7, where star forma-tion is most intense and where the ionized/molecularoutflow originates. Fig. 7 displays both the CN(1–0;3/2–1/2)/CO(1–0) (hereafter CN/CO) and the C2H(1–0;3/2–1/2)/HCN(1–0) (hereafter C2H/HCN) line ra-tios11. Both ratios decrease significantly from the in-ner (CN/CO ' 0.11; C2H/HCN ' 0.45) to the outer(CN/CO ' 0.035; C2H/HCN ' 0.18) nuclear disk. Thisbehavior is explained by a combination of an increasedPDR fraction and di↵erential optical depth. To sep-arate these e↵ects, ratios were calculated between op-tically thin proxies for CO and HCN, CN/C17O andC2H/H13CN (Fig. 7). In both cases the trend for en-hanced PDR tracers in the inner disk is seen, though
11 These two ratios are chosen instead of the more common ratio,CN/HCN (e.g., Boger & Sternberg 2005), because only they canbe obtained from matched array configurations in our dataset.
14 Meier, Walter, Bolatto et al.
less pronounced. The average CN/C17O ratio is .20 inthe outer disk and a factor of ⇠2 higher towards the in-ner disk region). Likewise, for C2H/H13CN, the outerdisk average is ⇠2 and the inner disk average is 3.5-4.5. The fact that C2H/H13CN and CN/C17O both showthe same magnitude of an e↵ect suggests that densitychanges across the nucleus are not controlling the lineratio. Hence it appears that PDRs constitute a largerfraction of the dense gas towards the inner nuclear disk.Simple hydrocarbons exhibit elevated abundances in thisenvironment. We note that the more complex hydrocar-bon symmetric top, CH3C2H, has a rather similar mor-phology to CN and C2H, suggesting that its abundancecan also be elevated in PDRs.We also note that the PDR tracers are observed in the
base of the molecular outflow. This is consistent withthe recent detection of C2H in the molecular outflow inthe nucleus of Ma↵ei 2 (Meier & Turner 2012).
4.4. Shock Tracers
Given the intense star forming environment inNGC253’s center and its associated outflow of molec-ular gas (B13), we expect the presence of shocked gastracers. Our dataset includes two common extragalac-tic shock tracers, SiO (e.g., Garcia-Burillo et al. 2000)and HNCO (Meier & Turner 2005). HNCO is thoughtto come from dust ice mantle sublimation (where dueto filling factor reasons ice mantle evaporation in hotcores is invisible on larger scales), while SiO is enhancedby ejection of significant Si from sputtered silicate graincores in high velocity shocks (e.g., Meier & Turner 2005,Rodriguez-Ferndandez et al. 2010, Tideswell et al. 2010,Martin-Pintado et al. 1992).SiO emission is distributed across the nucleus in a se-
ries of compact sources with some relative enhancementseen toward the eastern outer nuclear disk. CalculatedSiO abundances (X(SiO) ⇠ 10�9) confirm that SiO iselevated across the entire nuclear disk. This SiO value iscomparable to that seen locally on GMC scales in ‘shockregions’ in other nearby spiral galaxies (e.g., Usero etal. 2006, Meier & Turner 2012) and slightly larger thanfound in previous NGC 253 SiO measurements (di↵er-ences are due to di↵erent adopted Tex and N(H2) values;Garcia-Burillo et al. 2000). These abundances are muchlarger than observed toward quiescent Galactic molecularclouds (. 10�11; e.g., Zuirys et al. 1989, Martin-Pintadoet al. 1992).Interestingly, HNCO has the most distinctive morphol-
ogy of all the bright lines, being completely dominatedby the outer locations of the disk. Such an enhance-ment at the outer edges of the nuclear disk is also seenin several other nearby barred nuclei, IC 342, Ma↵ei 2and NGC6946 (Meier & Turner 2005; 2012). EstimatedHNCO abundances reach⇠ 10�8 across the outer nucleardisk and decrease to ⇠ 10�9 towards the inner nucleardisk. HNCO abundance in quiescent Galactic clouds ap-proach this inner disk value (e.g. Marcelino et al. 2009,Tideswell et al. 2010).Assuming SiO traces shocks, then the SiO/HCN line
ratio can be considered to first order an indicator ofthe fraction of the dense gas that is experiencing strongshocks. HNCO/SiO ratio is often considered a tracer ofshock strength, because the shocks need to be strongerto elevate the SiO abundance (e.g., Usero et al. 2006).
In Fig. 8 we present these two ratios across the nucleardisk of NGC 253. The SiO/HCN intensity ratio is fairlyflat across the disk with slight elevations seen towards re-gions 2 and 3. This indicates that shocks are uniformlypresent over much of the nuclear disk in NGC 253. TheHNCO/SiO ratio in the outer nuclear disk matches theratio observed in bar shock regions of nearby spirals likeIC 342 (Meier & Turner 2005, Usero et al. 2006) and Maf-fei 2 (Meier & Turner 2012) well. Towards the inner diskthe ratio drops dramatically to a value of ⇠0.17. OnlyM 82 has a ratio anywhere near this low (Garcia-Burilloet al. 2000, Martin et al. 2009).At face value this extremely low HNCO/SiO ratio in
the inner disk could be taken to indicate that shockstrengths decrease away from the core. However, the SiOmorphology and abundance suggests that, even in theouter disk, shock strengths are strong enough to keepSiO high relative to HCN. Therefore other e↵ects thatfurther alter the HNCO morphology besides simple shockstrength must be considered. Two important second or-der e↵ects that can alter the HNCO/SiO ratio (see Meier& Turner 2012 for a thorough discussion): 1) HNCO hasa significantly higher photodissociation rate than SiO, soits shock signatures are erased more easily in the pres-ence of PDRs/UV radiation fields. 2) SiO is a linear topmolecule so its partition function is / Tex, while HNCOis an asymmetric top molecule, with a partition func-tion / T1.5
ex . So in hot gas, HNCO’s lower energy statesdepopulate more rapidly than SiO. For HNCO this ef-fect is mitigated at Tex ⇠ 5 � 20 K by the fact thatEu(HNCO) is ⇠ 11K compared to SiO’s value of 6.3K,but if Tex ⇠ 75 K in the inner disk then this could be apronounced e↵ect.In the inner nuclear disk, the gas is hot and PDR dom-
inated (see Sec. 4.3), so we expect that both act to drivedown the HNCO/SiO ratio in the center. Therefore itappears we are observing the starburst (and outflow) ac-tively in the process of erasing the signatures grain man-tles species in the inner disk (Martın et al. 2009a,b). Inthis context it is not surprising that its HNCO/SiO ratiois similar to M 82’s, a nucleus where PDRs have com-pleted the task of erasing the weak shock tracers (Takanoet al. 1995, Garcia-Burillo et al. 2001)
4.5. Sulfur Species
We detect a significant number of sulfureted species,including CS, SO, NS, H2CS, CH3SH and C2S. Whenaccounting for the di↵erent array configurations used toobserve them, CS, SO, H2CS and NS all have quite sim-ilar morphologies. They tend to be dominated by thedense GMCs, with a preference toward the clouds on thenortheast side of the continuum peak (7 and 8), whereasthe PDR/hydrocarbons tend to favor the southwesternside of the inner disk (position 5). Interestingly, the mor-phology of the sulfuretted species, particularly CS andSO are very similar to SiO. Assuming SiO traces shockedgas (Sec. 4.4), this hints at a possible connection betweenthe S species and shocked gas. This is somewhat unex-pected, at least for CS, given that in other nearby nucleiit has been argued that CS is primarily a PDR tracer(e.g., Meier & Turner 2005; Martın et al. 2009). Ev-idently a mix of shocks and strong radiation fields arecapable of maintaining high abundances of SiO and CS,though not HNCO.
ALMA Multi–line Imaging of the Nearby Starburst Galaxy NGC253 15
Fig. 8.— Same as Fig. 5, but for shock tracer ratios (Sec. 4.4)
Fig. 9.— Same as Fig. 5, but for sulfur species line ratios (Sec. 4.5).
The CS/SO abundance ratio is sensitive to the C+/Oabundance ratio and it has been argued to be either atracer of the C/O abundance ratio (⇠ 0.4 in the Galaxy),or early vs. late time chemistry (e.g., Heithausen et al.1998, Nilsson et al. 2000). So when C or C+ is nearlyas abundant as atomic O, CS is strongly favored overSO. Our observed SO/CS intensity ratio is ⇠0.1–0.18over much of the nuclear disk (Fig. 9). This correspondsto a CS/SO abundance ratio of >1.5–3 (higher if CS isoptically thick), in good agreement with Galactic starforming regions. Nilsson et al. (2000) find values rangingbetween 0.4–5, with most at ⇠2.Likewise, the NS/SO (and NS/CS) abundance ratios
are particularly sensitive to atomic C and O abundances.NS is formed from the neutral-neutral reactions, S + NH! NS + H and N + SH ! NS + H. Atomic O destroysboth NS and its parent SH, converting them to NO andSO, respectively. Likewise, reactions with C/C+ driveNS into CS and CN. In star forming regions the radia-tion field can keep a significant amount of C and O innon-molecular form. As such, standard gas-phase modelspredict NS cannot not attain abundances above X(NS)⇠ 10�11 (e.g., McGonagle & Irvine 1997, McElroy et al.2013). In the inner nuclear disk we observe NS abun-dances in the range of 1 – 6⇥10�9, at least two orders ofmagnitude larger than the maximum gas-phase modelscan accommodate. NS/SO abundances ratios are alsohigh, reaching 0.2 - 0.5 (the exact values are uncertain
due to non-matched array configurations). These ele-vated abundances are consistent with that determinedfrom single-dish measurements toward NGC 253 (Martınet al. 2003, 2005), except the present observations furtherdemonstrate that the high abundances are attained to-ward the same inner disk GMCs where PDR conditionsare pronounced.To explain the similar (high) abundances of CS, SO
and NS toward the same region requires non-standardchemistry. The atomic abundances of species like O mustbe kept very low, despite the PDR conditions. Two pos-sible scenarios appear feasible. The first, introduced byViti et al. (2001) (see also Charnley 1997, Harada et al.2010 and Izumi et al. 2013), invokes a high tempera-ture gas phase chemistry due to the passage of a shock.This high temperature gas drives O into the more inertH2O form so it does not destroy NS as rapidly. A sec-ond possibility is that in the pre-starburst phase of NGC253’s nucleus, much of the atomic species condensed ontothe dust grains where they are converted into their sat-urated form (e.g., Brown et al. 1988). At a later timeshocks liberated the saturated species (e.g., CH4, NH3,H2O and possibly H2S). The gas-phase chemistry thenevolved from molecular initial conditions, keeping atomicC and O low. Both of the scenarios invoke the presenceof shocks, and so are consistent with the tight morpho-logical connection between the sulfur species and SiO.We observe both ortho-H2CS at 101GHz and a mix
16 Meier, Walter, Bolatto et al.
of para transitions at 103 GHz. The ortho-to-para ra-tio of H2CS, the sulfur analog to formaldehyde H2CO, isuseful for constraining the formation conditions of H2CS(Minh et al. 1991). Under optically thin, high-T (>15 Kin the case of H2CS), LTE case, the ratio of the orthospecies of H2CS (those with Ka [K�1] even, see Tab. 1)to the para (those with Ka odd) should be 3.0. If thetemperature at the time of formation is 15 K then thetwo forms of H2CS with not be thermalized to their sta-tistical weights and hence have an abundance ratio lessthan 3 — approaching unity at T=0K. The intensityratio of the two types of transitions (Fig. 9) is approxi-mately constant across the disc at a value of ⇠1.4. ForTex = 75 K the measured ratio implies an ortho-to-pararatio of 2.9. Therefore every position is consistent withstandard LTE, high temperature limit. Thus, at leastin the case of H2CS, this suggests that H2CS formed inthe high temperature gas phase after evaporation, or ifit was formed on the grain surfaces in an earlier epoch,those surfaces were still significantly warmer than 15 K.
4.6. Millimeter Recombination Lines
We have detected three hydrogen recombination linesin our observations (H40↵, H50� and H52�). The mor-phology of these recombination lines is very di↵erentfrom all the molecular gas tracers (e.g., Figs. 1 and 3):The emission is centrally concentrated, coincident withthe location of the underlying continuum emission (L14).H� radio recombination lines are reported outside theLocal Group for the first time here. The error bars aresignificant for the � lines, but the global H40↵/H50� ra-tio is around 3.5, in good agreement with the LTE valueof 3.5 (=1/0.285) for ne = 104 cm�3 and Te = 104 K.The � lines would be fainter than the � line by a factor of2.3. Indeed, we do not clearly detect any � lines. We alsodo not detect any He↵ lines. Other radio/mm recombi-nation lines mapped in NGC253 have been discussed ex-tensively elsewhere (e.g., Anantharamaiah & Goss 1996,Mohan et al. 2002, 2005, Rodriguez-Rico et al. 2006,Kepley et al. 2011). The morphology of both the ↵ and� lines mapped here are in excellent agreement with theones reported previously.
4.7. Tentatively Identified Species
We here briefly comment on other selected species de-tected in our observations.A large number of tentatively identified lines can
be attributed to carbon-rich complex species (e.g.,CH3CH2CN, CH3CH2OH, CH2CN, CH2CO, CH3CHOand NH2CHO (Tab. 1). These species represent thenext step in complexity beyond the molecules detectedin external galaxies up until now, and suggest that thesemolecules may be fairly extended in NGC 253.One unidentified feature detected is of particular inter-
est. This is the feature U-114.22 at ⇠114.218 GHz. Thisspectral feature is within <4 MHz (<11 km s�1) of bothC4H(N=12–11; J=23/2–21/2; F=12–11) and the v = 1transition of CO(1–0). The upper energy state of theCO(1–0; v = 1) line is 3089K above ground, so the exci-tation temperature of the gas must be >450 K in order toexplain the observed line ratio. The morphology of thistransition is very similar to the RRLs (accounting forresolution di↵erences), as would be expected if it is trac-ing molecular gas not associated with the cold molecular
phase. The C4H transition has a much lower upper en-ergy level of 35.6 K, typical of the other transitions seenhere. Moreover, it is plausible to expect that the C4Hmight trace PDR emission (like C2H), which are also con-centrated towards the inner disk, though why it would besignificantly more compact than C2H is unexplained. Weconsider C4H as the most likely identification, howeverthe observed morphology indicates that the possibilitythis feature could be vibrationally excited CO deservesfurther investigation.
5. SUMMARY AND CONCLUSIONS
We present the detection of a total of 50 molecular linesin the nearby starburst galaxy NGC253, based on earlyscience (cycle 0) observations taken with the AtacamaLarge (Sub–)Millimeter Array (ALMA). For 27 lines wehave an unambiguous identification, and 13 lines havetentative IDs (no plausible line identification was possiblein 10 cases).We here describe a schematic picture of the central
starburst region of NGC253 mapped by ALMA (Fig. 10):As described in Sec. 3.1, the nucleus of NGC 253 is char-acterized by the inner portion of NGC253’s large–scalebar. The outer part of the nuclear disk (the ‘outer nu-clear disk’), likely represents the location where the gasis flowing radially inward along the large-scale bar. In-side this is a compact region exhibiting a large quantity ofhigh density gas and intense star formation, as evidencedby the presence of dense gas (Sec. 4.2), PDR (Sec. 4.3)and shock tracers (Sec. 4.4). This component (the ‘in-ner nuclear disk’), dominates the morphology of most ofthe spectral lines detected by the observations. We de-tect high density molecular gas tracers (HCN, HCO+ andCN) at the base of the molecular outflow first detectedin the 12CO emission (B13).In detail, we find moderate 12CO opacities of ⇠2–5,
despite the large column density towards NGC253’s nu-cleus. This may be due to increased line–widths dueto turbulence, non–circular motions and/or elevated gastemperatures that reduce the CO opacity per unit veloc-ity. Comparing HCN(1–0) and HCO+(1–0) with their13C–substituted isotopologues yields that the HCN(1–0) and HCO+(1–0) are also optically thick, with similar(moderate) opacities to CO. These high HCN and HCO+
opacities imply that the main isotopologue HCN/HCO+
(and HCN/HNC) line ratios have less diagnostic power inthis starburst environment. Using the isotopically sub-stituted versions of these high density tracers we measuremuch more robust values for the popular ”HCO+/HCN”,”HNC/HCN” and ”HNC/HCO+” ratios. All five of theisotopic dense gas tracer ratios can be well fit by LVGmodels with nH2 = 104.75 cm�3, Tkin = 80 K andX(HCN) ⇠ 5X(HCO+) ⇠ 5X(HNC).The weak shock tracer/ice mantle species HNCO has
the most distinctive morphology of all the bright lines be-ing completely dominated by the outer locations of thedisk. This is at odds with what is seen in the strongshock tracer, SiO. Attributing the dramatic variation inthe HNCO and SiO maps across the nucleus to chang-ing shock strength is unsatisfactory because SiO is en-hanced across the nucleus at a level that should implyshocks are strong throughout the inner disk. A possibleexplanation for the faintness of HNCO is that becauseof its higher photodissociation rate relative to SiO, the
ALMA Multi–line Imaging of the Nearby Starburst Galaxy NGC253 17
Fig. 10.— A schematic picture of the central starburst region of NGC253 (see discussion in Sec. 5). The morphology is based on Figure1 of (B13) with the chemistry added.
ice mantle evaporation shock signatures are being pref-erentially erased in the presence of the intense centralradiation fields. This is consistent with the presence ofan enhanced PDR fraction towards the region of HNCO’sdramatic decrease as determined form the CN/C17O andC2H/H13CN ratios. The sulfureted species appear alsoto be (indirectly) connected to the presence of shocksacross the inner disk.We have detected three hydrogen recombination lines
(H40↵, H50� and H52�) that show a centrally concen-trated morphology (similar to the underlying continuumemission) that is distinctly di↵erent from all moleculargas tracers.Finally, we report some tentative identifications, in-
cluding the aldehydes CH3CHO and NH2CHO, cyanidesCH2CN and CH3CH2CN and a number of organics ofsimilar complexity, CH2CO, HCOOH and CH3CH2OH.The first vibrational state of the 12CO(1–0) line is alsopossibly detected. If follow–up, multi-line studies con-firm these IDs then they would represent the first extra-
galactic detections of the above species.
DSM acknowledges partial support by the NationalScience Foundation through grant AST-1009620. ADBacknowledges support by the Alexander von HumboldtFoundation and the Federal Ministry for Education andResearch through a Humboldt Fellowship, support by theNational Science Foundation through CAREER grantAST-0955836 and AST-1412419, as well as a CottrellScholar award from the Research Corporation for Sci-ence Advancement. We thank the anonymous refereefor a thorough and helpful review. This paper makesuse of the following ALMA data: ADS/JAO.ALMA#2011.0.00172.S. ALMA is a partnership of ESO (rep-resenting its member states), NSF (USA) and NINS(Japan), together with NRC (Canada) and NSC andASIAA (Taiwan), in cooperation with the Republic ofChile. The Joint ALMA Observatory is operated byESO, AUI/NRAO and NAOJ. The National Radio As-tronomy Observatory is a facility of the National Sci-ence Foundation operated under cooperative agreementby Associated Universities , Inc.Facilities: ALMA
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Note. — Integrated intensity measurements at positions 1–10 and intensity of the entire galaxy. Column 1: The transition. column 2: The original resolution of the ALMA data/commonresolution used in this study. column 3-12: The integrated intensity for regions #1–#10 (see Fig. 4 for location of regions) with uncertainty. Uncertainties are set to 10%, representing absoluteflux calibration uncertainties only, unless S/N is low then they are conservatively set to 30 % (see section 2). Small errors associated with missing flux are not included. We caution that valuesfor region 6 may be a↵ected by absorption towards the central continuum source. All upper limits are 5�.
20Meier,
Walter,
Bolatto
etal.
TABLE 4LTE ’Reference’ Molecular Abundances for NGC 253a
(Bolatto et al. 2013a). Calculation of the column densities of individual species are calculated assuming LTE excitation. The range quoted for each entry corresponds to 10 - 75 K. Overallsystematic uncertainties, including the adopted excitation temperature (range shown), galactic position (positions 1-3 and 7-10 likely have excitation temperatures at the low end of the range),and the CO conversion factor (uncertain to a factor of three; Bolatto et al. 2013a), dominate the error budget, so separate error bars are not presented for each position. Upper limits are 5�.b The critical density (not including radiative trapping e↵ects), log(ncr = Aij/Cij [100K]). Rates are adapted from the Leiden LAMDA database, van der Tak et al. 2007, with Cij ’s:HCN/H13N/HN13C: Dumouchel et al. (2010); HCO+/H13CO+: Flower (1999); SiO: Dayou & Balanca (2006); C2H: Spielfiedel et al. (2012); HNCO: Green (1986); CS: Lique et al. (2006a); SO:Lique et al. (2006b); HC3N Green & Chapman (1978); H2CS: Wiesenfeld & Faure (2013); CN: Lique et al. (2010); C17O: Yang et al. (2010).
