arXiv:astro-ph/0011101v2 13 Nov 2000 Ultraviolet Signposts of Resonant Dynamics in the Starburst-Ringed Sab Galaxy, M94 (NGC 4736) William H. Waller 12 , Michael N. Fanelli 32 , William C. Keel 4 , Ralph Bohlin 5 , Nicholas R. Collins 2 , Barry F. Madore 6 , Pamela M. Marcum 7 , Susan G. Neff 8 , Robert W. O’Connell 9 , Joel D. Offenberg 2 , Morton S. Roberts 10 , Andrew M. Smith 8 , and Theodore P. Stecher 8 ABSTRACT The dynamic orchestration of starbirth activity in the starburst-ringed galaxy M94 (NGC 4736) is investigated using images from the Ultraviolet Imaging Telescope (FUV- band), Hubble Space Telescope (NUV-band), Kitt Peak 0.9-m telescope (Hα, R, and I bands), and Palomar 5-m telescope (B-band), along with spectra from the International Ultraviolet Explorer and Lick 1-m telescopes. The wide-field UIT image shows FUV emission from (a) an elongated nucleus, (b) a diffuse inner disk, where Hα is observed in absorption, (c) a bright inner ring of H II regions at the perimeter of the inner disk (R = 48 ′′ = 1.1 kpc), and (d) two 500-pc size knots of hot stars exterior to the ring on diametrically opposite sides of the nucleus (R = 130 ′′ = 2.9 kpc). The HST/FOC image resolves the NUV emission from the nuclear region into a bright core and a faint 20 ′′ -long “mini-bar” at a position angle of 30 deg. Optical and IUE spectroscopy of the nucleus and diffuse inner disk indicates a ∼10 7−8 yr-old stellar population from low-level starbirth activity blended with some LINER activity. Analysis of the Hα, FUV, NUV, B, R, and I-band emission along with other observed tracers of stars and gas in M94 indicates that most of the star formation is being orchestrated via ring-bar dynamics involving the nuclear mini-bar, inner ring, oval disk, and outer ring. The inner starburst ring and bi- symmetric knots at intermediate radius, in particular, argue for bar-mediated resonances as the primary drivers of evolution in M94 at the present epoch. Similar processes may be governing the evolution of the “core-dominated” galaxies that have been observed at high redshift. The gravitationally-lensed “Pretzel Galaxy” (0024+1654) at a redshift of ∼1.5 provides an important precedent in this regard. 1 Tufts University, Department of Physics and Astronomy, Medford, MA 02155 2 Raytheon ITSS Corporation, NASA Goddard Space Flight Center, Laboratory for Astronomy and Solar Physics, Code 681, Greenbelt, MD 20771 3 University of North Texas, Department of Physics, Denton, TX 76203 4 University of Alabama, Department of Physics and Astronomy, P. O. Box 870324, Tuscaloosa, AL 35487-0324 5 STScI, Homewood Campus, Baltimore, MD 21218 6 Infrared Processing and Analysis Center, Caltech, M/S 100-22, 770 So. Wilson Ave., Pasadena, CA 91125 7 Texas Christian University, Department of Physics, Box 298840, Fort Worth, TX 76129 8 NASA Goddard Space Flight Center, Laboratory for Astronomy and Solar Physics, Code 680, Greenbelt, MD 20771 9 University of Virginia, Department of Astronomy, P. O. Box 3818, Charlottesville, VA 22903 10 National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22903–2475 1
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Ultraviolet Signposts of Resonant Dynamicsin the Starburst-Ringed Sab Galaxy, M94 (NGC 4736)
William H. Waller12, Michael N. Fanelli32, William C. Keel4, Ralph Bohlin5, Nicholas R.Collins2, Barry F. Madore6, Pamela M. Marcum7, Susan G. Neff8, Robert W. O’Connell9, Joel
D. Offenberg2, Morton S. Roberts10, Andrew M. Smith8, and Theodore P. Stecher8
ABSTRACT
The dynamic orchestration of starbirth activity in the starburst-ringed galaxy M94(NGC 4736) is investigated using images from the Ultraviolet Imaging Telescope (FUV-band), Hubble Space Telescope (NUV-band), Kitt Peak 0.9-m telescope (Hα, R, and Ibands), and Palomar 5-m telescope (B-band), along with spectra from the InternationalUltraviolet Explorer and Lick 1-m telescopes. The wide-field UIT image shows FUVemission from (a) an elongated nucleus, (b) a diffuse inner disk, where Hα is observedin absorption, (c) a bright inner ring of H II regions at the perimeter of the inner disk(R = 48′′ = 1.1 kpc), and (d) two 500-pc size knots of hot stars exterior to the ring ondiametrically opposite sides of the nucleus (R = 130′′ = 2.9 kpc). The HST/FOC imageresolves the NUV emission from the nuclear region into a bright core and a faint 20′′-long“mini-bar” at a position angle of 30 deg. Optical and IUE spectroscopy of the nucleusand diffuse inner disk indicates a ∼107−8 yr-old stellar population from low-level starbirthactivity blended with some LINER activity. Analysis of the Hα, FUV, NUV, B, R, andI-band emission along with other observed tracers of stars and gas in M94 indicates thatmost of the star formation is being orchestrated via ring-bar dynamics involving thenuclear mini-bar, inner ring, oval disk, and outer ring. The inner starburst ring and bi-symmetric knots at intermediate radius, in particular, argue for bar-mediated resonancesas the primary drivers of evolution in M94 at the present epoch. Similar processes maybe governing the evolution of the “core-dominated” galaxies that have been observed athigh redshift. The gravitationally-lensed “Pretzel Galaxy” (0024+1654) at a redshift of∼1.5 provides an important precedent in this regard.
1Tufts University, Department of Physics and Astronomy, Medford, MA 021552Raytheon ITSS Corporation, NASA Goddard Space Flight Center, Laboratory for Astronomy and Solar Physics, Code 681, Greenbelt,MD 20771
3University of North Texas, Department of Physics, Denton, TX 762034University of Alabama, Department of Physics and Astronomy, P. O. Box 870324, Tuscaloosa, AL 35487-03245STScI, Homewood Campus, Baltimore, MD 212186Infrared Processing and Analysis Center, Caltech, M/S 100-22, 770 So. Wilson Ave., Pasadena, CA 911257Texas Christian University, Department of Physics, Box 298840, Fort Worth, TX 761298NASA Goddard Space Flight Center, Laboratory for Astronomy and Solar Physics, Code 680, Greenbelt, MD 207719University of Virginia, Department of Astronomy, P. O. Box 3818, Charlottesville, VA 22903
10National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22903–2475
Star-forming rings or “pseudorings” are commonto early and intermediate-type spiral galaxies (cf.Athanassoula & Bosma 1985; Buta, Purcell, & Crocker1995; Buta & Combes 1996), including our own MilkyWay galaxy (cf. Gusten & Mezger 1982; Clemens,Sanders, & Scoville 1988; Waller 1990a). Such ring-like accumulations of gas and associated starbirth ac-tivity may have helped to build the inner parts ofmany primeval disk galaxies (Friedli & Benz 1995),as exemplified by the recent discovery of a starburst-ringed galaxy at z ∼ 1.5 (Colley, Tyson, & Turner1996; Tyson et al. 1997). The formation and mainte-nance of these starburst rings are often attributed toorbital resonances with rotating bar or “oval” asym-metries in the stellar disks (cf. Combes 1994, Byrdet al. 1994; Combes et al. 1995; Buta & Combes1996 and references therein). However, other dynami-cal mechanisms — including gravitational instabilities(Elmegreen 1992, 1994; Kenney & Jogee 1997), out-ward propagating star formation (Walker, Lebofsky,& Rieke 1988; Waller, Gurwell, & Tamura 1992), andeven radially-driven pileups from nuclear outbursts(Waller et al. 1992; Tenorio-Tagle et al. 1997) —may play significant roles in orchestrating some of thestarburst rings that are observed. 11
As the closest early-type spiral galaxy of low incli-nation, M94 (NGC 4736) has received concentratedattention from both observers and theorists. This(R)SA(r)ab-type galaxy (de Vaucouleurs et al. 1991)is noted for its inner ring of ongoing starburst activity(R ≈ 45′′), oval stellar distribution at intermediate ra-dius (R ≈ 220′′) (cf. Mulder & van Driel 1993; Mulder1995; Mollenhoff, Matthias, & Gerhard 1995), andouter stellar ring near its de Vaucouleurs radius (R25
≈ 330′′). Figure 1 (extracted from the Digital SkySurvey using the SkyView utility [McGlynn, Scollick,& White 1996])12 shows the outermost portions ofM94, highlighting the oval disk and outer pseudoring.
The inner starburst ring is a prominent source ofHα, H I, and CO emission (Smith et al. 1991; Mulder& van Driel 1993; Gerin, Casoli, & Combes 1991).
11Collisionally-induced “ring galaxies” such as the CartwheelGalaxy are thought to be morphologically and dynamicallydistinct from the more common “ringed galaxies” consideredherein (cf. Athanassoula & Bosma 1985; Marcum et al. 1992).
12NASA’s SkyView facility (http://skyview.gsfc.nasa.gov) wasdeveloped and is maintained under NASA ADP Grant NAS5-32068 at NASA’s Goddard Space Flight Center.
The discovery of compact thermal & nonthermal ra-dio sources in the ring (Duric & Dittmar 1988) indi-cates the presence of dense H II regions and youngSNRs. The ring’s velocity field can be described bya combination of circular rotation with velocities oforder 200 km/s and residual non-circular motions oforder 15 km/s (Mulder 1995) to 25 km/s (Buta 1988),depending on the adopted inclination and major axisposition angle.
Interior to the ring, the bright bulge and innerdisk show twisted isophotes at red and near-IR wave-lengths, indicative of a weak bar-like distortion (Beck-man et al. 1991; Shaw et al. 1993; Mollenhoff et al.1995). FIR and CO observations interior to the ringreveal a rich ISM with gas surface densities exceedingthat of the ring (Smith & Harvey 1994; Garman &Young 1986; Gerin et al. 1991; Wong & Blitz 2000).
Optical spectroscopy of the nuclear region yieldsLINER-type emission lines along with absorption linesfrom the circumnuclear stellar population, consistentwith an early main-sequence stellar turnoff (A4–A7)and corresponding age of ∼500 Myr ( Pritchett 1977;Keel 1983; Taniguchi et al. 1996). Further supportfor a young central population comes from NIR spec-troscopy which shows deep CO absorption bands fromred giant and asymptotic giant branch stars of sim-ilar age (Walker et al. 1988). These authors haveproposed an outward propagating mode of star for-mation, whereby NGC 253, M82, M94, and M31 rep-resent increasingly evolved versions of the same star-bursting sequence. Although the kinematics of thering show very little evidence for outward expand-ing motions (contrary to prior claims of bulk expan-sion [van der Kruit 1974; 1976]), they also do notpreclude a scenario for radially propagating star for-mation. Other investigators have modeled the innerand outer rings in terms of resonant dynamics medi-ated by bar or “oval” potentials interior to the rings(Gerin et al. 1991; Shaw et al. 1993; Mollenhoff etal. 1995; Mulder & Combes 1996), with the observednon-circular motions resulting from dispersion orbitsnear the Lindblad resonances (Buta 1988).
In this paper, we present and discuss new obser-vational clues to the dynamical mechanisms govern-ing the star formation in M94. The ultraviolet im-ages obtained by the Ultraviolet Imaging Telescope,in particular, reveal hitherto unrecognized patternsof recent star formation whose presence lends fur-ther support to the hypothesis of galaxy evolutionvia bar-mediated resonances. The various imaging
and spectroscopic observations and reductions are de-scribed in Section 2. The resulting FUV, NUV, Hα,R, and I-band emission morphologies are presentedand compared in Section 3. Radial intensity profilesand other photometric comparisons are discussed inSection 4. UV and optical spectroscopy of the innerdisk and nucleus is presented in Section 5. Kinematicproperties and inferred dynamical scenarios are con-sidered in Section 6. Our summary of the wavelength-dependent morphological, spectro-photometric, anddynamical properties of M94 appears in Section 7,wherein evolutionary implications are discussed.
In the following Sections, we assume a distance toM94 of 4.6 (75/H) Mpc, based on the galaxy’s re-cession velocity of 345 km/s with respect to the Lo-cal Group ( Sandage & Tammann 1981). The corre-sponding spatial scale is 22.3 pc/arcsec. Unresolvedsources imaged by the HST/FOC provide additionalconstraints on the distance, as discussed in Section 4.We adopt a nominal inclination of 40 and major-axisposition angle of 120, while recognizing that bothof these quantities may vary significantly with radiusand with measuring technique (e.g. morphological vs.kinematic determinations) (Bosma, van der Hulst, &Sullivan 1977; Buta 1988; Mulder & van Driel 1993;Mulder 1995; Mollenhoff et al. 1995; Wong & Blitz2000).
2. Observations and Reductions
A log of the ultraviolet and visible imaging is pre-sented in Table 1. A listing of complementary UVand visible spectra is shown in Table 2.
2.1. Ultraviolet Imaging
The Ultraviolet Imaging Telescope (UIT) imagedM94 in the far-ultraviolet (λ = 1521 A, ∆λ =354 A) on 1995 March 12 as part of the 16-daySpacelab/Astro-2 mission aboard the Space Shut-tle Endeavour. This wide-field telescope images 40-arcmin fields of view at ∼3 arcsec resolution. In thecase of M94, the UIT image represents the only ex-tant UV image of the entire galaxy (see Figure 2a(Plate xxx)). The 1040-sec exposure was obtainedwith a dual-stage image intensifier with CsI photo-cathodes and was recorded on carbon-backed IIaOKodak film. After processing of the film and scan-ning of the emulsion, the resulting digitized “densityimage” was fog-subtracted, flat-fielded, linearized to“exposure units,” and calibrated to flux units using
IUE observations of standard stars (cf. Stecher et al.1992, 1997; Waller et al. 1995 and references therein).Correction for image distortion produced by the mag-netically focused image intensifiers was carried out ac-cording to the procedures described by Greason et al.(1994). The resulting corrections amounted to a fewarcsec in the field center (which includes all of M94’sFUV emission) increasing to 10–20 arcsec near theedge of the 40 arcmin field of view. Astrometry wastied to 10 compact knots evident in both the FUVand B-band images (see next subsection). Positionsin the resulting distortion-corrected image are goodto ∼3 arcsec, and the spatial resolution is of similarmagnitude.
The Hubble Space Telescope’s Faint Object Cam-era (FOC)13 imaged the center of M94 in the near-ultraviolet (λ = 2300 A, ∆λ = 500 A) on 18, July1993 — before the optical repair mission — as partof a UV imaging survey of 110 large nearby galax-ies (Maoz et al. 1995, 1996). After standard STScIpipeline processing, the 596 sec NUV exposure has a22′′×22′′ field of view at 0.022′′/pixel (see Figure 2a(Plate xxx)). The spherical aberration of HST’s pri-mary mirror resulted in a point-spread function (PSF)featuring a sharp core with FWHM ≈ 0.05′′ and about15% of the total light surrounded by an extensive haloof several arcsec radius containing most of the energy.Following Maoz et al. (1996), our calibration of thedetected FOC counts into flux densities assume a con-version of 1.66× 10−17 erg s−1 cm−2 A−1/count s−1,while noting that the PHOTFLAM conversion in theimage header is 2.017×10−17 erg s−1 cm−2 A−1/counts−1. Flux uncertainties are estimated at ∼5% overlarge areas increasing to ∼20% for compact sources(Maoz et al. 1995, 1996, and references therein).
2.2. Visible Imaging
A wide-field (9.66′ × 9.66′) B-band image was ob-tained with the Palomar 5-m telescope and Tek3 CCDcamera (1024 × 1024 pix) on 1994 February 11 underhazy skies. This 600-sec exposure is saturated in thecentral 2 arcmin, but contains high S/N detections ofthe oval disk and parts of the outer ring. Astrometryof this image is tied to the positions of several fore-ground stars, as measured on a corresponding image
13Data from the NASA/ESA Hubble Space Telescope were ob-tained at the Space Telescope Science Institute, which is oper-ated by the Association of Universities for Research in Astron-omy, Inc., under NASA contract No. NAS5-26555.
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in The Digitized Sky Survey (STScI 1994)14.
Groundbased Hα, R, and I-band images of M94were obtained with the now-retired KPNO 0.9-m tele-scope and RCA-1 CCD camera (508 × 316 pix) on17, February 1986.15 These images have 7.28′ ×4.53′ fields of view at 0.86′′/pix. Sky conditionsvaried from photometric to hazy, yielding PSFs ofabout 2′′ (FWHM). Astrometry is tied to 5 fore-ground stars that are common to the KPNO (Hα,R, I-band) and Palomar (B-band) images. For cali-bration purposes, spectrophotometric standard stars(BD262606 & HD84937) were imaged before and af-ter the target imaging. Subtraction of the red contin-uum from the Hα-band image was carried out withthe R-band image according to the formulations inWaller (1990b), whereby corrections were made forthe ∼38% [N II]λλ6548, 6584 contribution to the to-tal Hα + [N II] line emission and resulting ∼15%contamination of the Hα image after transmission bythe 36 A bandwidth Hα filter. A pure red-continuumimage was also produced, by removing the contam-inating Hα + [N II] line emission from the R-bandimage (Waller 1990b) (see Figure 2a (Plate xxx)).
Photometry of the Hα emission from the 7.3′×4.5′
field yields a total flux of 9.9 × 10−12 erg cm−2 s−1,14% higher than that determined by Kennicutt &Kent (1983) within a 7′-diameter aperture (after cor-recting for a 38% [N II] contamination within their100 A bandwidth). The starburst ring (R = 30′′
→ 60′′) is measured to have f(Hα) = 9.6 × 10−12
erg cm−2 s−1, which is nearly 1.8 times higher thanthat obtained by Smith et al. (1991) from the 75A bandwidth image of Pogge (1989). Some of thesediscrepancies can be attributed to the varying band-passes and corresponding uncertainties in the [N II]emission being transmitted, vagaries in the continuumsubtraction, problematic Hα absorption produced byA-type stars in the inner disk (see Section 4.2), andfinal calibration (Smith et al. 1991). Because of hazyconditions during the I-band exposure, calibration ofthe I-band image was done by bootstrapping to theI-band radial intensity profiles resulting from the pho-
14The Digitized Sky Survey was produced at the Space TelescopeScience Institute (STScI) under U. S. Government grant NAGW-2166. The digitized images are based on photographic datafrom the Palomar Observatory Sky Survey (produced by theCalifornia Institute of Technology & Palomar Observatory, andfunded by the National Geographic Society)
15Kitt Peak National Observatory is operated by the Associationof Universities for Research in Astronomy, Inc., under contractwith the National Science Foundation.
toelectric photometry of Munoz-Tunon et al. (1989)and the (R−I) color profiles of Beckman et al. (1991).
2.3. UV Spectroscopy
FUV (1200 A to 2000 A) and NUV (2000 A to 3200A) spectra of M94’s inner disk were obtained fromthe International Ultraviolet Explorer (IUE) archive.The IUE data were accessed via the IUE Data Anal-ysis Center at NASA/GSFC16 and are representa-tive of the NEW Spectral Image Processing System(NEWSIPS).
Table 2 lists the image numbers, dates, exposuretimes, nominal positions and roll angles of the low-resolution FUV (SWP) and NUV (LWP & LWR) IUEspectra. Here, SWP, LWP, and LWR respectively re-fer to IUE’s short-wavelength prime, long-wavelengthprime, and long-wavelength redundant cameras. Fig-ure 3 shows the IUE apertures (20′′ × 10′′) on grey-scale images of the FUV emission, where the mappingis based on the nominal positions and roll angles of theFUV (SWP) and NUV (LWP & LWR) observations.Although the accuracy of the nominal positions is oforder ±10′′, Figure 3 indicates that the apertureswere most likely sampling the inner disk rather thanthe starburst ring. Table 2 notes which aperturesare filled with disk (d) and/or nuclear (n) emission,based on visual inspection of the overlays. The limit-ing spectral resolution of these data is about 6 A.
2.4. Visible Spectroscopy
Groundbased visible spectra (3800 A to 7500 A)were obtained from the Lick 1-m Nickel telescope andImage Dissecting Spectrograph (IDS). The summedspectrum of the central region (8.1 arcsec circularaperture) is a mean-flux average of 64 min in blueand red grating settings, with 16 min in an inter-mediate setting to ensure the overlap area was wellcalibrated. The spectrum has been rebinned to 2.5-Apixels from the original, which still oversamples theresolution of ≤10 A FWHM. We note that adjacentpixels in the IDS spectrum are not statistically in-dependent, yielding detections similar to those of anon-centroiding photon counter which spreads singlephotons across several output pixels.
We complement this composite spectrum with anal-ysis of the image-dissector scanner data presented by
16The IUE database is currently available athttp://archive.stsci.edu/iue/
Keel (1983) and discuss a new high-resolution spec-trum, obtained with the 2.5-m Isaac Newton Tele-scope (INT) on La Palma, using the Intermediate-Dispersion Spectrograph with image photon-countingsystem (IPCS) detector.17 The 1” slit was orientedapproximately along the major axis (PA 135) for this1000-second exposure. There is useful signal over 45spatial increments of 0.6′′ each. We consider equiva-lent widths of the lines along with spectral slopes anddiscontinuities of the continuum emission.
3. Ultraviolet and Visible Morphologies
Figure 2a (Plate xxxx) shows the dramaticallydifferent morphologies that are detected at ultravio-let and visible wavelengths. The inner disk and bulgecomponent that is so prominent at R-band completelydisappears in the FUV image. The oval disk at inter-mediate radius also has no FUV counterpart. Instead,the FUV image is characterized by (a) an extendedand elongated nucleus, (b) a diffuse inner disk, (c) abright inner ring at the perimeter of the inner disk(R = 48′′ = 1.1 kpc), and (d) two 500-pc size knotsexterior to the ring (R = 130′′ = 2.9 kpc).
3.1. Nuclear Region
The insert in Figure 2a (Plate xxxx) contains theHST/FOC pre-COSTAR image of the nuclear region(Maoz et al. 1995, 1996). This near-UV image showsa marginally-resolved nucleus (FWHM ≈ 0.1′′) em-bedded in a bright core of ∼1′′ (22 pc) diameter alongwith a faint “mini-bar” that has a total length of 20′′
(450 pc) at a position angle of approximately 30.Low-level ripples in the emission (which are includedin the estimate of the bar’s total length) are evident2′′, 5′′ and 9′′ to the SW of the nucleus. Maoz et al.(1995) attribute these features to bow shocks or tidalarms resulting from a recent merger event. However,we see that the ripples overlap with the larger con-centric arcs that are evident in the UIT/FUV imageat projected radii of ∼9′′ and 15′′ (see next subsec-tion). Similar ripples are also present in an archivalHST/WFPC2 V-band image of this region – with con-nections to larger spiral arcs. To the north of thenucleus are two point sources whose flux densities areconsistent with those of single B-type supergiant stars(see Section 4).
17The Isaac Newton Telescope is operated by the Royal Green-wich Observatory on behalf of the SERC at the Spanish Ob-servatoriao del Roque de los Muchachos.
3.2. Inner Disk
The UIT/FUV emission from the inner disk in-cludes concentric FUV arcs to the SW of the nucleuswhich can be traced for ∼π radians (∼700 pc). Thesearcs are of low contrast and, because of the noise char-acteristics of the UIT imaging, are not amenable totypical contrast enhancement techniques (e.g. medianfiltering). However, they do show up as enhancementsin the radial distribution of intensities (see Figure2b). Typical spacing between the arcs is 9′′ (200 pc),with additional features at 15′′ and 34′′. The arcsthemselves appear to show some substructure at thelimits of resolution.
Contrary to a merging scenario (Maoz et al. 1995),which would seem to be precluded by the lack of sig-nificant tidal effects beyond the inner few hundredparsecs, these features more likely indicate an orbitaldynamic at work in the inner disk. At an interpolatedorbital velocity of 140 km/s and corresponding shearrate of -1360 km−1 kpc−2 (see Section 6), such fea-tures could have been differentially swept out over atimescale of ∼20–100 Myr. This estimated timespanis consistent with a population of late B-type starswhose main-sequence lifetimes are of similar duration.
The UIT/FUV image of the inner disk also showsa brighter arc to the north, just inside the starburstring at a radius of 40′′ (see Figure 2b), along withwidespread diffuse emission at a level amounting to∼15% the mean surface brightness of the ring. Thenature of these resolved and unresolved FUV compo-nents is uncertain, although some clues can be ob-tained from the longer wavelength imagery.
At Hα the inner disk appears in absorption withrespect to the continuum emission from the underly-ing population of stars. The strongest absorbers atHα are the photospheric atmospheres of B and A-type stars, whose temperatures are sufficiently coolfor Hydrogen to remain neutral and sufficiently warmfor significant population of the H Balmer (n=2)electronic energy level (cf. Mihalas 1978). In theUIT/FUV image, the diffuse light from the innerdisk probably arises from these same stars. Indeed,the UIT/FUV image represents the first view of thisyoung stellar population, unconfused by the longer-wavelength emission from the older inner disk andbulge components.
Figure 4 compares a spatially-filtered R-band im-age with an (R−I) color image of the inner disk. Thespatially-filtered R-band image (Fig. 4a) was cre-
6
ated by median smoothing the R-band image over a13′′×13′′ window and then subtracting the smoothedimage from the original. The resulting fine-scalestructure includes the nuclear “mini-bar” previouslynoted by Mollenhoff et al. (1995) and Mulder (1995),along with a “dark” spiral arc of diminished emis-sivity to the west and other flocculent spiral struc-tures associated with the starburst ring. The archivalHST/WFPC2 V-band image of this region resolvesthe flocculent structure into spiral dust lanes of highpitch angle which cross through the nearly circularring.
The (R − I) color image (Fig. 4b) shows redden-ing along the western spiral arc seen in (Fig. 4a) andin another arc to the NE, along with blue knots in thestarburst ring. None of the fine-scale structures andreddened features in the optical inner disk have coun-terparts in the UIT/FUV image. Therefore, neitheremitting nor absorbing structures, nor reddening dustlanes in the old disk can be defining the observed con-centric FUV arcs to the SW and NE. We infer that theFUV arcs are recently-generated structures of emit-ting B and A-type stars and/or scattering clouds oflow dust optical depth. We further speculate that theemitting and/or scattering FUV sources, being rela-tively young, are distributed in a much thinner diskthan that associated with the optical fine-scale struc-tures and reddened features, thus explaining the mor-phological differences between these two wavelengthregimes. Further insights on the stellar content of theinner disk can be gained from the IUE spectroscopypresented in Section 6.
3.3. Starburst Ring
The inner starburst ring in M94 is the single dom-inant emission feature in the FUV, Hα, and radiocontinuum bands. Rectification of the galaxy to itsnominal face-on orientation (PA = 120 i = 40 [Mul-der & van Driel 1993]) shows that the ring is almostperfectly circular, with a mean radius of 47′′ (1.1kpc) and FWHM of 21′′ (0.49 kpc). The latter mea-surement is based on doubling the measured HWHMof the emission beyond the mean radius, thereby ex-cluding the diffuse contribution interior to the ring.Adopting an inclination of 35, as in the dispersionorbit model of Buta (1988), would yield a finite butsmall ellipticity of 0.063, with a 16 offset betweenthe kinematic line of nodes and projected major axis.We are not able to further constrain the ring’s orien-tation, and so shall continue to assume an inclination
of 40, based on the H I study of Mulder & van Driel(1993).
A comparison of the FUV and Hα morphologiesreveals strong similarities. Figure 5a shows the Hαemission, and Figure 5b shows the ratio of Hα andFUV intensities in the inner disk and ring. Exceptfor the spiral-like ridges of enhanced Hα emission tothe WNW and ESE, very little coherent variation inthis ratio is evident in the ring. In particular, theintensity ratios do not show any radial displacementin the FUV emission relative to that at Hα. From thislack of radial structuring in the Hα/FUV ratio, wecan conclude that little evidence is found for outwardor inward propagating starburst activity, as explainedbelow.
A morphological comparison of Hα and FUV emit-ting regions provides a useful means of tracking se-quential patterns of star formation in disk galaxies.The Hα emission is dominated by the most massiveand short-lived stars (M ≥ 20 M⊙), while the FUVemission arises mostly from less massive, longer- livedstars (20 ≤ M/M⊙ ≤ 2.5). For an evolving star clusterwith an IMF typical of local star-forming regions, theHα emission typically reaches a maximum within amillion years of the star-forming episode, and quicklydecays with an e-folding timescale of about 3 Myr.By contrast, the FUV emission reaches a maximumat about 3 Myr (due to the onset of B-type super-giants), decaying to 1/e in yet another 3 Myr. TheHα/FUV ratio is seen to decrease by 2 orders of mag-nitude after 10 Myr (e.g. Hill et al. 1995). Subse-quently, the Hα emission will vanish, while the FUVdeclines slowly for the next few 100 Myrs.
In the outward propagating starburst scenario, theHα emission would concentrate where the propagat-ing wavefront is located — on the outer perimeterof the FUV-emitting ring. Inward propagating sce-narios would have the Hα emission interior to theFUV ring. Spatial displacements of Hα and FUVemission have been observed across the spiral armsof M51 (O’Connell 1997; Petit et al. 1996), M74(Marcum et al. 1997), and NGC 4258 (Courtes etal. 1993). These displacements have been interpretedas the result of spiral density waves concentrating gasalong the insides of the spiral arms and subsequentmigration of the evolving clusters past the spiral wavefronts. Given the resolution of our images (∼100 pc)and characteristic timescale between Hα and FUVmaxima (3 Myr), any residual propagation of star-burst activity would have to proceed at a speed less
7
than 35 km/s to avoid detection. Implications of thispropagation speed limit are discussed in Section 7.
The arm-like enhancements in the Hα/FUV ratiocorrespond to similar features in a recent CO mappingwith the BIMA interferometer (Wong et al. 2000).These arm-like extensions away from the ring havefurther analogues at B-band, where a complex spiralpattern is evident (see next subsection).
3.4. Bi-Symmetric Knots and Associated Spi-ral Structure
One of the most remarkable aspects of M94’s FUVemission morphology is the pair of knots on diamet-rically opposite sides of the nucleus. Both knotshave projected radii of approximately 130′′ (2.9 kpc).A line drawn through the knots intersects the nu-cleus at a position angle of 105, intermediate be-tween the PAs of the inner starburst ring (127) andthe oval disk (95) (Mollenhoff et al. 1995). Thetwo knots are roughly 20′′ (500 pc) in size, withthe eastern knot showing the more complex struc-ture. Only the eastern knot shows significantly in ourcontinuum-subtracted Hα image (Figure 5). Othermuch smaller knots of FUV emission appear at lowS/N to the SW, SSW, ENE, and NNE. The SW, SSW,and ENE knots have faint counterparts in the light ofHα, while all of these features have counterparts atB-band (see below).
Figure 6a and Figure 6b respectively show theB-band image before and after spatial filtering. Thefiltering in this case involves median smoothing overa 30′′ × 30′′ window and then dividing the originalimage by this smoothed image. Such median normal-ized spatial filtering reveals fine-scale structure overa wide range of surface brightnesses (e.g. Waller etal. 1998). Here, it highlights the starburst ring andbisymmetric knots as significant enhancements abovethe disk at B-band. The filtered image also revealsmany flocculent spiral arms outside of the ring, whoserelation with the knots is somewhat ambiguous. Thewestern knot appears part of a dominant arm with a“shingled” morphology connecting to the ring in thenortheast. The eastern knot appears to be associatedwith several arms, including one which would be thesymmetric counterpart to the arm that links the west-ern knot to the ring. The marginally detected FUVknots to the SW, SSW, ENE, and NNE all show en-hanced B-band emission from associations of massivestars among the myriad spiral arms.
In summary, the bi-symmetric FUV knots have B-band counterparts which appear to be part of thecomplex spiral arm structure at these radii. Theirsymmetric prominence on opposite sides of the nu-cleus, however, requires a dynamical explanation thatis spatially more specific than that of the arms. Sim-ilarly, the starburst ring can be regarded as an espe-cially bright, tightly wound component of the overallspiral pattern, whose prominence also indicates spe-cial dynamical circumstances.
Bi-symmetric knots or plumes have been notedin other ringed-barred galaxies, including NGC 1433(Buta 1986), NGC 7020 (Buta 1990), NGC 7098(Buta 1995), and IC 4214 (Buta & Combes 1996).As discussed in these papers, the symmetric featuresare likely tracing dynamical resonances that are con-nected with the general ring-bar phenomenon.
4. Photometric Results
4.1. Nuclear Photometry
From the UIT and HST/FOC images, circularaperture photometry of the nucleus out to a radius of5′′ yields m(FUV ) = 14.7 mag and m(NUV ) = 13.5mag, respectively. The resulting (FUV −NUV ) colorof 1.2 mag is significantly redder than those derivedfrom IUE spectra of the inner disk (see next Section).Between 5′′ – 10′′ radii, the non-nuclear emission issignificantly bluer than those derived using IUE, with(FUV −NUV ) ≈ −0.3. The latter colors indicate thepresence of late B and hotter stars, depending on theamount of reddening (Fanelli et al. 1992). Estimatesof the reddening have been derived from the visiblecolors (Smith et al. 1991; see also next subsection),visible spectra and subsequent modelling of the stellarpopulations ( Taniguchi et al. 1996; Pritchett 1977).The resulting estimates of visual extinction are 0.5–1.0 mag in the nuclear region, corresponding to UVcolor excesses of E(FUV − NUV ) = 0.1–0.2 mag.Therefore, the corrected (FUV −NUV ) color of thenucleus could be close to 1.0 mag, or the equivalent ofa stellar population with a late A-type stellar cutoff(Fanelli et al. 1992).
The non-nuclear point sources in the HST/FOCnear-UV image of the nuclear region provide help-ful checks on the distance to M94. For the brightestpoint source, Maoz et al. (1996) lists a monochro-matic flux of fλ(2300 A) = 2.6 × 10−16 erg s−1 cm−2
A−1 (mλ(2300A) = 17.9 mag). This source is unre-solved at a resolution of 0.1′′ and corresponding linear
8
scale of <2.2 pc at the assumed distance. Although anextremely compact star cluster cannot be ruled out,this exceptional UV source is most likely dominatedby a single hot supergiant star. In the absence of ex-tinction, a B0-2 Ia-O supergiant star has M(NUV ) ≈−10.7 mag (Fanelli et al. 1992), which would imply adistance modulus of (m − M) = 28.6 mag and corre-sponding distance of 5.2 Mpc. Assuming nuclear colorexcesses of E(B−V ) = 0.15–0.27 mag ( Taniguchi etal. 1996) and Galactic-type extinction law, the corre-sponding near-UV extinctions would be A(NUV ) ≈
1–2 mag, and the revised distances would be 3.3–2.1Mpc, respectively. The adopted distance of 4.6 Mpccan thus be regarded as a reasonable estimate, giventhe uncertainties in stellar spectral type and extinc-tion. The second source, 1-arcsec to the E, is 6 times(1.9 mags) fainter – consistent with it being an A0-2 Ia-O supergiant at this distance, if subject to thesame amount of extinction. Were the galaxy signif-icantly closer, more abundant OBA-type giant andmain-sequence stars would be resolved – which is notthe case.
4.2. Surface Photometry of the Disk
Figure 7 shows radial profiles of the FUV surfacebrightness and cumulative flux. These profiles werederived from annular-averaged photometry using el-liptical annuli consistent with the adopted positionangle (120) and inclination (40). Since Galactic ex-tinction is negligible towards M94 (de Vaucouleurset al. 1991), no correction was made. The result-ing surface brightness profile (Fig. 7a) is dominatedby light from the nucleus, inner disk, starburst ring,and an exponentially-declining disk that is punctu-ated by an enhancement from the bi-symmetric knotsat R = 130′′. The exponentially declining componentbetween 50 and 80-arcsec radii has an e-folding scale-length of only 10′′ (223 pc), similar to that found atR and I-bands (cf. Munoz-Tunon et al. 1989). TheFUV emission beyond the bi-symmetric knots alsoshows a steep decline with an estimated scalelength of20′′ (446 pc), significantly shorter than the 70′′ scale-length measured at R and I-bands in the outer disk(Munoz-Tunon et al. 1989).
The cumulative flux profile (Fig. 7b) indicatesthat the half-light radius matches that of the star-burst ring, and that more than 80% of the total FUVemission is contained within a radius of 60′′ (1.3 kpc).The total FUV flux from M94 is fλ(1520 A) = 6.02 ×
10−13 erg cm−2 s−1 A−1, corresponding to m(FUV) =
9.45 mag, or M(FUV) = −18.91 mag at the adopteddistance of 4.6 Mpc.
The corresponding FUV luminosity of 1.5 × 1039
erg s−1 A−1 is the photometric equivalent of 2 × 104
Orion nebulae (Bohlin et al. 1982) or about 90 30-Doradus regions (as measured on an UIT/FUV im-age of 30 Dor out to a radius of 5 arcmin [67 pc]),before correcting for the extinction in these sources.Adopting a Salpeter IMF with lower and upper masslimits of 0.1 and 100 M⊙, respectively, yields an un-corrected star formation rate of 0.15 M⊙/yr. ThisSFR estimate assumes continuous star formation andincludes the strong contribution of B supergiant starsto the overall luminosity. An overall visual extinctionof 1 mag would increase the global luminosity and in-ferred SFR by about a factor of 11 (Hill et al. 1997).At this rate, it would have taken ∼10 Gyrs to trans-form the dynamical mass of 1.6 × 1010 M⊙ that ispresent within 60′′ of the nucleus(Garman & Young1986) into the dominant stellar component that wesee today.
As a check on these starbirth estimates, we notethat the measured Hα flux of 9.6 × 10−12 erg cm−2
s−1 converts to a luminosity of L(Hα) = 2.45 × 1040
erg/s, or only 5600 equivalent Orion nebulae. Theorigin of this discrepancy is deferred to the next sub-section. Assuming Case B recombination and mul-tiplying the Hα luminosity by 7.4 × 1011 yields aphotoionization rate of Ni = 1.8 × 1052 photons/sec,and corresponding star formation rate of 0.22 M⊙/yr.The similarity of the FUV and Hα-based SFRs, be-fore correcting for extinction, suggests either that in-significant obscuration is present in the photometri-cally dominant starburst ring, or that significant ob-scuration exists with other mechanisms making upfor the greater attenuation of the FUV emission rel-ative to the longer-wavelength Hα emission. Suchmechanisms include (1) an excess contribution of non-ionizing B-type stars to the total FUV emission, (2)a reduction of the total Hα emission due to Hα ab-sorption by the atmospheres of B and A-type stars,and (3) the absorption of EUV photons before theyionize the gas and induce Hα emission. Likely EUVabsorbers include the nebular dust associated withthe H II regions as well as the various metal speciesin the O-type stellar atmospheres themselves (cf. Hillet al. 1997; Waller, Parker, & Malumuth 1996). Weconclude in the next subsection that modest obscura-tion plus non-ionizing contributions to the total FUVemission best explain the observed levels of Hα and
9
FUV emission. Stellar absorption at Hα is proba-bly less than a 10% effect overall, given Hα emissionequivalent widths of order 100 A in the ring, and stel-lar absorption equivalent widths peaking at less than10 A.
The global (total) (FUV −V ) color of M94 is 1.21mag, which is characteristic of early-type disk galax-ies with circumnuclear starburst activity (e.g. NGC1068, NGC 3351 [Waller et al. 1997]). The (R−I) col-ors shown in Figure 4b do not vary as much as thosefound by Beckman et al. (1991) in their photometricstudy. Bootstrapping our measured intensity ratio inthe nuclear region to a color of (R− I) = 0.45 mag asreported by Beckman et al. (1991), we obtain colorsthat range from (R− I) = 0.3 mag in the rings’ star-burst knots to (R − I) = 0.5 mag in the arcs interiorto the ring. Beckman et al. (1991) obtain much bluercolors of order 0.1 mag near the ring. We attributethis discrepancy to our having used the Hα-band im-age to remove contaminating line emission from theR-band image ( Waller 1990b), thereby reducing thered flux from the knots in the ring by 20%. Compari-son of the contaminated and uncontaminated R-bandimages confirm our attribution.
If the reddened arcs are caused by dust, the corre-sponding color excess relative to that of the starburstknots would amount to E(B − V ) ≈ E(R − I) = 0.2mag, or the equivalent visual extinction of roughly 0.6mag in excess of the knots. Allowing for bluer stellarpopulations in the knots could reduce the estimateof excess extinction in the arcs to negligible levels,while inclusion of some internal reddening in the knotswould increase the total estimated extinction by ≤1mag. Such low estimates for the extinction are simi-lar to the spectroscopic results obtained by Pritchett(1977) and by Taniguchi et al. (1996) in their stud-ies of the nuclear region, where color excesses of 0.27mag and 0.15 mag, corresponding to AV = 0.86 magand 0.48 mag, were respectively obtained.
4.3. Photometry of the Starburst Ring andKnots
Photometry of the FUV and Hα emission from thestarburst ring (between projected radii of 30′′ and60′′) yields a mean Hα/FUV flux ratio (in equivalentwidth units) of 22 A. By way of comparison, we notethat this flux ratio is somewhat lower than that foundin the Orion nebula (38 A[Bohlin et al. 1982]), withinthe range of flux ratios evident in M33’s giant H IIregions (19 A to 69 A [Parker, Waller, & Malumuth
1996]), and close to the mean of M51’s wide-rangingGHRs (29 ± 23 A [Hill et al. 1997]).
A model of steady-state star formation with aSalpeter-type IMF yields a somewhat lower ratio of 16A, while a modeled 2 Myr-old starburst would matchour result (Hill et al. 1995 [see their Fig. 10]). Sucha short burst timescale is probably untenable, thusindicating some need to correct our FUV flux for ex-tinction. For 1 mag of visual extinction, the resulting“corrected” Hα/FUV flux ratio would be lower bya factor of ∼5, leading to a modeled burst age of 6Myr — close to the age limit of the ionizing O stars.Based on these considerations alone, we surmise thatthe FUV and Hα emission from the starburst ring aresubject to no more than 1 mag of visual extinction(on average). We also can conclude that the simi-larity in uncorrected SFRs based on the global FUVand Hα fluxes is probably coincidental. After correc-tion for 0.5–1.0 mag of visual extinction, the Hα fluxwould be increased by a factor of 1.4–2.1, while theFUV emission would increase by a factor of 3.3–11.In other words, the FUV emission is tracing an ad-ditional non-ionizing Population I component in theinner disk that is missed at Hα and, after correctionfor extinction, is contributing to the overall calculatedrate of star formation.
The bi-symmetric FUV knots to the west and eastof the ring have fluxes of 3.0× 10−15 and 3.8× 10−15
erg cm−2 s−1 A−1, respectively. This translates to theequivalent of 100 and 127 Orion nebulae, respectively.Only the eastern knot shows significant Hα emission,with a flux of 2.57 × 10−13 erg cm−2 s−1 and hencean Hα/FUV flux ratio of 67 A— about twice that ofthe Orion nebula — thus indicating greater FUV ob-scuration than is present in Orion or a hotter ionizingcluster. Examination of the spatially-filtered B-bandimage (Figure 6b) reveals dark spiral-arm featuresat the position of the eastern knot which may cor-respond to obscuring dust clouds. The western knotshows similar structures to one side. The lack of sig-nificant Hα emission from this knot is difficult to ex-plain with dust, and is most likely due to the presenceof an older (τ > 10 Myr), non-ionizing stellar popu-lation.
5. Spectroscopic Results
5.1. FUV Spectroscopy
Figure 8a shows an average of 5 IUE/SWP spec-tra of the extranuclear emission from the inner disk.
10
Based on the aperture mapping shown in Figure 3asome emission from the nucleus may be present, butat low levels. The wavelengths of commonly observedFUV absorption and emission lines are indicated onthe spectrum for comparison. Although the aver-aged spectrum is of generally low S/N, several fea-tures can be identified. Of these, the most prominentare the absorption blends of S II (1250-1259) and Si II(1260,1265), the P-Cygni profile of S IV (1394,1443),the absorption blends of Fe III (1601-1611), Al III(1600-1612), C II (1720-1722) and Al II (1719-1725),and part of the broad absorption complex of Fe III(1891-1988). The low-ionization lines, in particular,are most characteristic of B and A-type stars. Thestrength of the blueshifted S IV absorption feature rel-ative to that of C IV (1550) along with the strong Si IIabsorptions at 1260,1265 A and 1527,1533 A indicatea composite spectral type later than B3 but earlierthan B8 (Fanelli et al. 1992; Kinney et al. 1993;Walborn, Parker, & Nichols 1995). The absorptionsat 1470 A, 1780 A, and 1790 A remain unidentified.
In emission, there is some evidence for C IV (1548-1551), He II (1640), N III] (1730,1750), and C III](1909) — much of which may be the result of LINERactivity being scattered by the ISM in the inner disk.The C III] (1909) line emission may be responsible forfilling in part of the broad stellar absorption complexof Fe III (1891-1988).
5.2. NUV Spectroscopy
Figure 8b shows an average of 2 IUE/LWP spec-tra of the disk. The most prominent spectral fea-tures are the absorption lines of Fe II (2609, 2750),Mg II (2800), and Mg I (2852). Comparison withstellar spectra from the IUE Spectral Atlas (Wu etal. 1991) shows that Mg II (2800) is unusually weakrelative to Mg I (2852); the line ratios in this spec-tral range being consistent with light dominated bylate G-type stars, based on this cursory comparison.However, the spectrum is too blue and the ampli-tude of the 2800 A break too small for this to be thewhole story. Comparison with the nuclear spectra(IUE LWR 12221,12238 – as listed in Table 2) showsevidence for a spectrally smooth blue component offthe nucleus proper. Also, the Mg II 2800 A break isroughly 50% smaller than that evident in the nuclearspectrum (see below).
All the absorption features are broader in the off-nuclear spectrum, because the light there almost uni-formly fills the aperture (so centering shifts are not
an issue). The off-nuclear spectrum shown here isnot only bluer but shows a diminished 2800 A breakamplitude. Measured as a ratio of flux Fλ above andbelow 2800 A, the ratio is 2.5 at nucleus and 1.8 awayfrom it. Using the spectral-break indices developedby Fanelli et al. (1992), we obtain for the off-nucleusaveraged spectrum (Mg II 2800 – Mg I 2852) = 0.31mag, (2609/2660) = 0.45 mag, and (2828/2921) =0.31 mag — all of which are consistent with a (B−V )color of 0.2–0.3 mag, or the equivalent of a late A-type main sequence star or late A to early F-typegiant/supergiant star.
The off-nuclear continuum is likewise flatter in theSWP range, with little trace of the emission-like fea-tures around 1900 A seen in the nuclear spectrum(see Kinney et al. 1993). These properties can beaccounted for, if recent star formation (few times 108
years ago) contributes relatively more light off thenucleus than on it, consistent with an aging burstthat is spatially more extended than the centrally-concentrated stars of the older disk and bulge.
5.3. Optical Spectroscopy
Figure 8c shows the spectrum of optical emissionfrom the central D = 8.1 arcsec, as obtained with theLick 1-m telescope and IDS. Absorption lines includethe Ca II K&H lines at 3934 A and 3968 A, Hδ (4101),CH G-band (4300), Hγ (4340), He II (4686), Hβ(4860), Mg I (5170), Fe I (5270), Na D (5890,5896),Hα (6563), and atmospheric absorption bands of O2
(6867) and H2O (7186). Emission lines are restrictedto [N II] (6584) and [S II] (6731).
The Hβ equivalent width of about 5.4 A is huge foran old population typical of spiral bulges, even beforeany correction for line emission. There has been starformation in this region not long ago. The 4000-Abreak is also suppressed, indicating the effective ageis much younger than for typical early-spiral bulges.Following the Dressler and Shectman (1987) defini-tion (ratio of Fν between 3950-4050 and 3750-3850A), the observed break has Fν = 1.6, whereas valuesof 1.9–2.0 are more usual.
From the IDS/IPCS on the 2.5-m Isaac NewtonTelescope, we see that the nucleus (inner 1” or so) isbluer at optical wavelengths than its immediate sur-roundings. Misalignment in the IPCS detector is lessthan 0.1 pixel, so this is not an obvious instrumen-tal effect. This nuclear blueing effect is substantial,amounting to about 30% over the 4400–5200 A range
11
(more or less equivalent to E(B − V ) = −0.3 mags,where the mean (B − V ) color of the bulge is 0.91mags, which is already bluer than ordinary Sa/Sbbulges [Keel & Weedman 1978]). Outside this area,the color along the slit is quite constant out to 15′′
from the nucleus, where the signal begins to die out.This effect has no observed counterpart in the IUEspectra, because of their courser spatial resolution.
The prominent Hβ absorption line in the nuclearINT/IPCS spectrum has FWHM 18–20 A, consistentwith values seen in mid-to-late A stars. Thus, thedata are consistent with seeing the main-sequenceturnoff near this spectral type, rather than the super-giant dominance of a much younger population givingsuperficially similar spectral features (as seen in thestarbursting nucleus of NGC 4569 [Keel 1996]).
The optical spectra confirm our imaging result thatHα is in absorption with respect to the continuumfrom the underlying population of A and B-type stars.By contrast, the neighboring line of [N II] (6584) isseen in emission in the Lick/IDS spectrum. As pre-viously noted, Hβ is also dominated by photosphericabsorption. The IDS/IPCS spectrum shows some Hγemission at least at the nucleus, that is not swallowedby the stellar absorption line.
6. Kinematics and Dynamics
Figure 9a shows the rotation curve based on the HI observations of Mulder and van Driel (1993), wherekey morphological features are noted. This curve isqualitatively similar to that obtained from a recentinterferometric mapping of CO (Wong & Blitz 2000)but is on-average 12% lower, due to the adoption ofa 40 inclination compared to their 35. Figure 9bshows the corresponding radial profiles of H I and H2
gas, the latter being derived from the CO observationsof Gerin et al. (1991).
The molecular gas component clearly dominatesthe inner disk’s ISM and may continue to prevailat higher radii, where the CO emission has yet tobe measured. The plotted extrapolation beyond thelast reliable measurement of CO emission is intendedto provide an upper limit on the total gas and thusa lower limit on the Q stability index (see below).The interferometric measurements by Wong & Blitz(2000) yield even lower extrapolated surface densi-ties of gas – consistent with our extrapolated upperlimit. Moreover, FIR measurements with IRAS andthe KAO (Smith et al. 1991) reveal negligible dust
emission beyond 60 arcsec radius – further corrobo-rating the gas upper limit used here. The H I profile,though of lower amplitude, shows enhancements atthe radii of the starburst ring and the bi-symmetricknots.
Figure 9c shows the corresponding radial profileof the gravitational stability index (Q). Here, we haveconsidered the simplest case of the gaseous stabil-ity, without any coupling with the stellar component,such that
Q = Σcrit/Σgas = κσ/πGΣgas,
where the epicyclic frequency κ is closely linked to therotation curve v(R) through
κ2 =2v
R(v
R+
dv
dR),
and where σ is the gas velocity dispersion — a quan-tity which is not well constrained but is probably oforder 5–10 km/s. Here, we leave σ as an unknown.The resulting radial profile of Q/σ out to the limits ofthe CO observations shows a shallow minimum at theradius of the starburst ring. Although this minimumis too broad and shallow to explain the more discreteand prominent starburst ring, it may help to explainthe relatively young (107 − 108 yr) stellar populationpervading the inner disk.
Figure 10 shows the orbital resonance diagramthat results from the rotation curve along with theradii of key morphological features. A single wavepattern speed of 35 km s−1 kpc−1 (where d = 4.6Mpc) would place the nuclear bar inside the Inner-Inner Lindblad Resonance (IILR), the starburst ringbetween the IILR & Outer Inner Lindblad Resonance(OILR), the bisymmetric knots at the 4:1 “ultra-harmonic” resonance, the oval disk terminating at co-rotation (CR), and the outer pseudo-ring at the OuterLindblad Resonance (OLR). Alternatively, a higherpattern speed of 56 km s−1 kpc−1 (as modeled byMulder & Combes (1996)) would move the starburstring just outside the OILR, the bisymmetric knots tothe CR radius, and the edge of the oval disk to theOLR. This latter model, however, fails to account forthe outer ring without invoking additional patterns.
Further support for the proposed sequence of res-onances comes from specific ratios of the correspond-ing radii (cf. Buta 1986). Given a flat rotationcurve, the modeled ratio of OLR and UHR radii isr(OLR)/r(UHR) = 2.6. If these OLR and UHR reso-
12
nances are respectively traced by the outer pseudor-ing and bi-symmetric knots, then we obtain a ratioof radii equaling 2.54 – closely matching the modeledratio. The theoretical ratio of OLR and CR radii,being r(OLR)/r(CR) = 1.7, is also well matched bythe observed relative dimensions of the outer ring andoval disk, where a ratio of 1.5 is obtained.
Admittedly, morphological tracers and rotationcurves are insufficient to discriminate between theseand other possible kinematic patterns (cf. Buta &Combes 1996). Further progress on constraining theresonant dynamics in M94 will require analysis of thecomplete velocity field in the disk (cf. Westpfahl &Adler 1996; Canzian & Allen 1997). A complete H Imapping with the VLA has been made recently, whosespectral and spatial resolution would be sufficient toderive a complete velocity field and its associated res-onant states (D. Westpfahl, private communication).Until such an analysis is carried out, we think thatthe resonant state diagrammed in Figure 10 bestexplains the most features with the fewest conditions.
7. Conclusions and Implications
Through UV-Optical imaging and spectroscopy, wehave found new evidence for bar-mediated resonancesas the primary drivers of evolution in M94 at thepresent epoch. Our observational results include evi-dence for
(1.) A 450-pc long nuclear “mini-bar” at both opticaland near-UV wavelengths.
(2.) An inner disk with diffuse FUV emission inconcentric arcs that do not match the fine-scalestructures or reddened structures at visible wave-lengths. Since Hα is observed in absorption here,the UIT/FUV image represents the first view ofthis non-ionizing but relatively young disk compo-nent.
(3.) UV-Optical colors and spectral indices in thenucleus and inner disk that indicate B and A-typestars in the presence of modest extinction (AV ≤1mag) along with some LINER activity from thenucleus itself.
(4.) A 2.2 kpc diameter starbursting ring at theperimeter of the inner disk that is bright at FUV,Hα, and radio-continuum wavelengths. The levelof starbirth activity in this inner ring rivals the lev-els observed in starbursting irregular galaxies suchas NGC 1569 and NGC 4449. The inferred star for-mation rate within the ring and inner disk amounts
to 1.5 M⊙ yr−1 — sufficient to build up the stellarmass of the inner disk and bulge in ∼1010 yr.
(5.) No detectable radial offsets between the Hα andFUV rings, thus indicating a 35 km/s speed limit toany outward or inward propagating star formationin the ring, if such a mode is present.
(6.) Two 500-pc size FUV-emitting knots exterior tothe ring on diametrically opposite sides of the nu-cleus. The bisymmetric knots and starburst ringappear to be especially prominent parts of a com-plex spiral arm structure, as revealed in a spatially-filtered B-band image.
(7.) The starburst ring, bi-symmetric knots, ovaldisk, and outer pseudo-ring as signposts of reso-nant dynamics in the disk of M94. More specifi-cally, the radii of these features match those of var-ious orbital resonances, given a pattern speed of 35km s−1 kpc−1 at our adopted distance and incli-nation. These orbital resonances are most likelydriven by some combination of the nuclear mini-bar and oval distortion in the disk.
(8.) A shallow minimum of gravitational stability atthe radius of the starburst ring that extends inwardinto the inner disk. Although too broad to explainthe discrete starburst ring, the shallow minimummay help to explain the 107 − 108-yr old stellarpopulation interior to the ring.
Although we can set a limit on the speed of out-ward or inward propagating star formation in thering, we cannot preclude the existence of such a mode.At a propagation speed of 35 km/s, a wave initiated inthe nucleus could traverse the inner disk to the radiusof the current starburst ring in only 31 Myr. There-fore, it is possible that the 107−108-yr old stellar pop-ulation detected in the inner disk is the result of suchan outward propagating wave. The striking differ-ence in emission morphologies at FUV and red wave-lengths provides further support for the starburst ringbeing a transient phenomenon which does not persistat any one radius for very long. Either these reso-nant phenomena come and go, as the oval distortionsundergo secular evolution, or their operating radii mi-grate in response to other dynamical influences on thestars and gas (cf. Combes 1994; Combes et al. 1995;Friedli & Benz 1995). Otherwise, one must invokestrong radial inflows of stars from the starburst ringto populate the inner disk and bulge, a feat requiringunusual circumstances – e.g. mergers.
13
The results reported herein may have importantimplications with regard to observations of the mostdistant observable galaxies. At redshifts of 1–5, the2-kpc diameter starburst ring in M94 would subtendangles of only (0.7′′ – 1.0′′)H/75 in an Einstein-deSitter Universe (q = 1/2) and (0.3′′ – 0.2′′)H/75in an open (Milne) universe (q = 0) (cf. Narlikar1983). The UV-bright nuclear rings evident in NGC1097, NGC 1317, NGC 1433, NGC 1512, NGC 2997,NGC 4321, and NGC 5248 (Maoz et al. 1995; Maoz etal. 1996; Kuchinsky et al. 2000; Marcum et al. 2000)would subtend even smaller angles at the same red-shifts. Moreover, nuclear rings tend to have higherFUV surface brightnesses than their larger counter-parts – the inner ring in M94 being a remarkableexception. Therefore, some of the “core-halo” mor-phologies that are evident at high-redshift in the rest-frame FUV (cf. Giavalisco et al. 1997) may, in fact bemarginally-resolved representations of galaxies withstarburst rings in their centers.
Gravitationally-lensed galaxies are fortuitously mag-nified, enabling resolutions of their structure at highS/N. An important precedent in this regard is thegravitationally-lensed “Pretzel Galaxy” which lies be-hind the galaxy cluster 0024+1654 at an estimatedredshift of 1.2 – 1.8 (Colley et al. 1996; Tyson et al.1997). Detailed reconstructions of the multiply-lensedgalaxy show a clear annular morphology on a scale ofseveral kpc. If M94 and other nearby ringed galaxiescan be used as current-epoch analogues, the “Pret-zel Galaxy” and perhaps other marginally-resolved“core-halo” galaxies at high redshift may representyouthful inner disks and bulges growing under the or-ganizing influence of oval or bar asymmetries(Friedli& Benz 1995; Waller et al. 1997). Conversely, if ev-idence for starburst rings at high redshift proves tobe sparse, then massive inner disks featuring ring-bardynamics have yet to form in most systems, or star-bursting bulges are masking their presence.
We thank David Adler, Gene Byrd, FrancoiseCombes, Daniel Friedli, and David Westpfahl for gen-erously providing consultation on the dynamics ofringed-barred spiral galaxies. WHW is grateful toJohn Huchra and the OIR division of the Harvard-Smithsonian Center for Astrophysics for their kindhospitality during his visiting appointment at theCfA. WHW also thanks Eric Murphy and Chris-tine Winslow, Tufts undergraduates who helped craftsome of the graphics. UIT research is funded through
the Spacelab Office at NASA Headquarters underProject number 440-51. We are deeply grateful to thecrew of STS-67 and the many people who helped makethe Astro-2 mission a success. WHW acknowledgespartial support from NASA’s Astrophysics Data Pro-gram (071-96adp).
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Fig. 1.— A wide-field J-band image (extracted fromthe Digital Sky Survey) shows the outermost por-tions of the (R)SA(r)ab galaxy M94, highlighting theovoid disk and outer stellar ring. The field of viewis 22.67′ × 22.67′. North is up and East to the left.Corresponding fields of the 9.66′×9.66′ B-band imageand the 7.28′×7.28′ R-band, Hα and FUV images areindicated.
Fig. 2.— (a.) — UV and R-band imaging of M94.The field of view is 7.28′ × 4.53′. North is up andEast to the left. The UIT’s FUV image shows thestarburst ring in high contrast against a mostly darkdisk. Exterior to the ring are two hitherto unrecog-nized 500-pc size bi-symmetric knots on diametricallyopposite sides of the nucleus. The HST’s NUV image(see inset) shows a 450-pc long nuclear mini-bar thathad been previously inferred from photometric anal-yses of optical-band images. By comparison, the R-band image shows the underlying bulge and oval diskmade up of cooler and typically older stars. (b.) —Radial distribution of FUV intensities (surface bright-nesses) reveals strong enhancements at the nucleusand starburst ring, along with regularly-spaced low-level enhancements that are associated with arcs ofFUV emission interior to the ring.
Fig. 3.— Location of IUE apertures with respect tothe inner disk and nucleus of M94. Numbers refer tothe entries in Table 2. Positional accuracy is esti-mated at ±10′′. (a.) — Overlay of IUE’s far-UV(SWP) apertures on the UIT/FUV image of M94,showing both nuclear and disk-dominated observa-tions. (b.) — Overlay of the IUE’s near-UV (LWPand LWR) apertures on the UIT/FUV image of M94.
Fig. 4.— (a.) — The nuclear mini-bar and innerdisk: Spatial filtering of the R-band image (shownin Fig. 2) shows a mini-bar with the same approx-imate extent and position angle as that seen in theHST’s NUV image. Optical and UV spectroscopyof this region indicates a stellar population with anearly main-sequence turnoff (A4-A7) superposed onan older G-type population belonging to the centraldisk and bulge. The inner disk shows dark spiralarcs of relatively lower surface brightness that connectwith the starburst ring. (b). — Color morphology:The (R − I) color image is coded so that dark fea-tures denote relatively red colors and bright featuresdenote bluer colors. The image shows reddened arcsto the west and northeast along with especially blue
knots in the starburst ring.
Fig. 5.— (a.) — Inner disk in the light of Hα. Afterscaling and subtracting the underlying red-continuumemission, the residual Hα line emission shows concen-trations in the starburst ring and in (only) one of thebi-symmetric FUV knots. The nucleus and innermostdisk show a net deficit, due to Hα absorption by theatmospheres of the B and A-type stars that domi-nate the light in these regions. (b.) — Ratio of Hαand FUV emission in the inner disk: No radial dis-placement in the Hα/FUV intensity ratio is evidentacross the starburst ring, contrary to outward or in-ward propagating starbirth scenarios.
Fig. 6.— (a.) — Deep B-band image, taken withthe Palomar/Hale 5-m telescope. The field of viewis 9.66′ × 9.66′ with the central 2 arcmin saturated.(b.) — Complex of spiral structure. The same B-band image has been spatially filtered to highlightthe intermediate and small-scale structure. The nu-clear region was saturated and hence shows no struc-ture. In the disk, however, a complex spiral pattern isevident. The starburst ring and bi-symmetric knotsappear to be especially prominent parts of this over-all spiral structure. The knots are resolved into as-sociations of hot stars, thus confirming their relativeyouth.
Fig. 7.— (a.) — Radial distribution of annular-averaged FUV surface brightness in M94, expressedin magnitudes. (b.) — Radial distribution of thecumulative FUV flux in M94, showing most of theFUV light arising from the central R = 1.3 kpc.
Fig. 8.— (a.) — FUV spectrum of the inner disk,based on the signal-weighted average of 5 IUE/SWPdisk-dominated spectra. (b.) — NUV spectrum ofthe inner disk, based on the signal-weighted averageof 2 IUE/LWP disk-dominated spectra. (c.) — Op-tical spectrum of the central D = 8.1 arcsec, as ob-tained with the Lick/Shane 1-m telescope and ImageDissecting Spectrometer (IDS).
Fig. 9.— (a.) — Rotation curve (radial distribu-tion of orbital velocities), based on the H I obser-vations of Mulder and van Driel (1993), where keymorphological features are noted. (b.) — Radialprofile of gas surface densities. The H I gas densitiescome from Mulder and van Driel (1993), and the H2
are derived from the CO observations of Gerin et al.(1991). Beyond a radius of 75′′, the H2 surface densi-
17
ties have been extrapolated and most likely representupper limits (see text). This was done to provide a(probable) lower limit on the gravitational stabilityindex (Q). (c.) — Gravitational stability index (Q),as normalized by the gas velocity dispersion V(disp)(= σ in text). Beyond a radius of 75′′, the extrapo-lated value of Q represents a (probable) lower limit.As shown, the starburst ring and inner disk appearto be situated in a shallow minimum of Q.
Fig. 10.— Radial profile of orbital frequencies andcorresponding resonances. From the H I rotationcurve shown in Fig. 9a., the resulting orbital res-onance diagram shows key morphological features co-incident with important resonances. Here, the solid,short-dashed, and long-dashed lines respectively tracethe angular frequencies Ω, Ω±κ/4, and Ω±κ/2. A sin-gle pattern speed of 35 km s−1 kpc−1 (where d = 4.6Mpc and i = 40) would place the nuclear bar insidethe Inner Inner Lindblad Resonance (IILR) [whereΩp = Ω − κ/2], the starburst ring between the IILR& Outer Inner Lindblad Resonance (OILR), the bi-symmetric knots at the “ultra-harmonic” resonance(UHR) [where Ωp = Ω − κ/4], the oval disk termi-nating at the “co-rotation” resonance (CR) [whereΩp = Ω], and the outer pseudo-ring at the OuterLindblad Resonance (OLR) [where Ωp = Ω + κ/2].
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Table 1
Log of Images (As Presented)
Telescope Camera/Filter Image # RA(2000) Dec(2000) t(exp) Date
Note.—(1) — The numbers refer to the apertures shown in Figure 3a (FUV) and Figure 3b (NUV). (5) — Position angle of20′′ × 10′′ aperture (in degrees). (8) — Notes refer to inclusion of disk (d) and/or nucleus (n) within the spectroscopic aperture– with the dominant component appearing first – as ascertained from visual inspection of the overlays in Figure 3.
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This figure "wwaller.fig1.jpg" is available in "jpg" format from:
