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9Astrophysical Journal, acceptedPreprint typeset using LATEX style emulateapj v. 08/22/09

GAS SLOSHING AND BUBBLES IN THE GALAXY GROUP NGC 5098

S. W. Randall1, C. Jones1, M. Markevitch1, E. L. Blanton2, P. E. J. Nulsen1, W. R. Forman1

Astrophysical Journal, accepted

ABSTRACT

We present results from Chandra observations of the galaxy pair and associated galaxy groupNGC 5098, and find evidence for both gas sloshing and AGN heating. The X-ray brightness im-ages show diffuse emission with a spiral structure, centered on NGC 5098a, and a sharp edge in thediffuse emission surrounding much of the galaxy at about 30 kpc. The spiral structure in the X-raysurface brightness and temperature maps, the offset between the peak of the cool gas and the centralAGN, and the structure of the cold front edges all suggest gas sloshing in the core. The most likely per-turber is the nearby galaxy NGC 5098b, which has been stripped of its gaseous atmosphere. Detailedimages of the core reveal several X-ray cavities, two of which, just north and southeast of the centralAGN, correlate with radio emission and have bright X-ray rims, similar to buoyant bubbles seen inthe ICM of other systems. We estimate the pressures in the bubbles and rims and show that theyare roughly equal, consistent with these being young features, as suggested by their close proximityto the central AGN. We assume that the other X-ray cavities in the core, which show no correlationwith existing radio observations, are ghost cavities from previous AGN outbursts. An estimate ofthe mechanical energy required to inflate the cavities indicates that it is sufficient to offset radiativecooling of the gas for 15 Myr. Therefore, for a typical cycle time of 107 yrs, the central AGN energyoutput is enough to balance cooling over long timescales.Subject headings: galaxies: clusters: general — galaxies: clusters: individual (RGH80, NSCS

J132014+330824) — X-rays: galaxies — galaxies: individual (NGC5098)

1. INTRODUCTION

A major surprise from early Chandra and XMM-Newton observations was that gas in cool core clustersdoes not reach the low central temperatures predicted byradiative cooling models, in disagreement with the pre-viously accepted cooling flow model (Peterson & Fabian2006). The implication is that the central gas must expe-rience some kind of heating. The source of this heating,and understanding when and how it takes place, has re-cently been a major topic of study in extragalactic astro-physics. A promising candidate is feedback from energyinjection by the central AGN of the cD galaxy (McNa-mara & Nulsen 2007). However, the details of this inter-action, and how the energy is transferred from the jets tothe ambient ICM, are poorly understood. Galaxy groupsprovide an excellent opportunity to study heating andother non-gravitational processes in the ICM. Althoughnot as X-ray luminous as clusters, the effects of heat-ing are more readily seen in groups, due to their lowermass and central density. For example, the gas fractionin groups shows a relatively large scatter (∼ 2 at anyfixed temperature) within r2500, with the scatter beingtightly correlated with the central entropy (Gastaldelloet al. 2007; Sun et al. 2009), reflecting the greater roleof non-gravitational processes in the centers of groups ascompared to clusters.Another discovery from Chandra was the existence of

contact discontinuities, or cold fronts, where a cool, dense

1 Harvard-Smithsonian Center for Astrophysics, 60 GardenSt., Cambridge, MA 02138, USA; srandall@cfa.harvard.edu,cjf@cfa.harvard.edu, mmarkevitch@cfa.harvard.edu,wrf@cfa.harvard.edu

2 Institute for Astrophysical Research, Boston University, 725Commonwealth Ave., Boston, MA 02215, USA; eblanton@bu.edu

subclump of gas exhibits a temperature and density jumpat the interface with warmer gas, such that the pressureprofile across the interface is continuous (Markevitch &Vikhlinin 2007). More recently, it has been shown thatsuch cold fronts can be generated not only from sub-group cores in a merger, but also by gas sloshing arounda potential minimum, caused by an off-axis interactionwith a perturber (Ascasibar & Markevitch 2006). Coldfronts are found in clusters and groups with relativelyhigh frequency, and sloshing cold fronts have been iden-tified in a handful of systems (e.g., Mazzotta et al. 2001;Dupke et al. 2007; Gastaldello et al. 2009). Understand-ing cold fronts and sloshing is of interest as they can havea significant impact on cluster cores through gas heating,ICM mixing and enrichment, turbulence, constraints onconduction and magnetic fields, etc.In this paper we report on Chandra observations of

the NGC 5098 galaxy group (RGH 80), originally iden-tified by Ramella et al. (1989), which shows evidence forboth AGN heating and gas sloshing. Studies of ROSATand ASCA observations found average temperatures andmetallicities for this system of ∼ 1 keV and ∼ 30% so-lar (Davis et al. 1999; Hwang et al. 1999; Buote 2000;Mahdavi et al. 2000). More recently, XMM-Newton ob-servations were used to derive radial profiles for variousproperties of the X-ray gas, including temperature, pres-sure, entropy, total mass, gas mass, and cooling time(Xue et al. 2004; Mahdavi et al. 2005). A joint analy-sis of the XMM-Newton and Chandra data we considerhere was performed by Gastaldello et al. (2007) as partof a sample of relaxed galaxy groups. The Chandra datawere used to study the global properties of the gas in thissystem as part of the group sample studied by Sun et al.(2009). X-ray observations imply a total group mass of

2 RANDALL ET AL.

4− 6× 1013M⊙, and optical studies find a group line-of-sight velocity dispersion of σlos = 602 km s−1 (Xue et al.2004; Mahdavi et al. 2005).The NGC 5098 galaxy group is dominated by the

central galaxy pair NGC 5098a and NGC 5098b (firstidentified by Ramella et al. 1995, their “group 80”).NGC 5098a is the brighter (and presumably larger) of thetwo, with absolute optical magnitudes of MB = −21.131and MV = −22.097, as compared to MB = −20.845 andMV = −21.770 for NGC 5098b. Absolute magnitudeswere calculated from magnitudes given in the Sloan Dig-ital Sky Survey (SDSS; Adelman-McCarthy et al. 2008)catalog and transformed to the Johnson filter system us-ing the relations provided by Smith et al. (2002). Therelative line-of-sight velocity of the pair is 360 km s−1,slightly less than the group velocity dispersion. Thewestern galaxy, NGC 5098a, hosts the extended radiosource B2 1317+33, which has been detected at severalfrequencies (Colla et al. 1970; Parma et al. 1986; Mor-ganti et al. 1997; Condon et al. 1998).We report here on Chandra observations of NGC 5098.

As the global structure of the gas has already been stud-ied using XMM-Newton (Xue et al. 2004) and Chandra(Sun et al. 2009), we focus on detailed structure in thecentral regions, a task for which Chandra is well-suited.The observations and data reduction techniques are de-scribed in § 2. The X-ray image is presented in § 3, andresults on temperature and abundance structure fromspectral analysis are given in § 4. In § 5, we argue that thecentral gas is currently sloshing due to a recent interac-tion with a nearby galaxy, most likely with NGC 5098b.We also report on the detection of several X-ray cavitiesin the central region and use these to show that the en-ergy output by the central AGN is sufficient to balanceradiative cooling of the gas. Our results are summarizedin § 6.We assume an angular diameter distance to NGC 5098

of 153.1 Mpc, which gives a scale of 0.74 kpc/′′ forΩ0 = 0.3, ΩΛ = 0.7, and H0 = 70 km s−1 Mpc−1. All er-ror ranges are 68% confidence intervals (i.e., 1-σ), unlessotherwise stated.

2. OBSERVATIONS AND DATA REDUCTION

NGC 5098 was originally observed with Chandra onAugust 4, 2001, for 11 ksec with the Chandra CCD Imag-ing Spectrometer (ACIS) in Very Faint mode, pointedsuch that the galaxy was visible on the front-side illu-minated ACIS-I CCD array. It was again observed onNovember 5, 2005, for 39 ksec with ACIS in Very Faintmode, pointed such that the galaxy was visible on theback-side illuminated ACIS-S3 CCD. Due to the longerexposure time and better sensitivity at soft energies, weconsidered only data from the more recent ACIS-S3 ob-servation. These data were reduced using the methoddescribed in Randall et al. (2008). All data were repro-cessed from the level 1 event files using the latest calibra-tion files (as of CIAO4.0). CTI and time-dependent gaincorrections were applied where applicable. LC CLEANwas used to remove background flares3. The mean eventrate was calculated using time bins within 3σ of the over-all mean, and bins outside a factor of 1.2 of this mean

3 http://asc.harvard.edu/contrib/maxim/acisbg/

were discarded. The resulting cleaned exposure time was38.4 ksec.The emission from NGC5098 and the surrounding

group fills the ACIS-S3 image field of view. We there-fore used the standard CALDB4 blank sky backgroundfiles appropriate for each observation, normalized to ourobservations from the ACIS-S1 chip in the 10-12 keV en-ergy band. To generate exposure maps, we assumed aMEKAL model with kT = 1 keV, Galactic absorption,and abundance of 30% solar at a redshift z = 0.0379,which is consistent with typical results from detailedspectral fits (see § 4).

3. THE X-RAY IMAGE

The exposure corrected, background subtracted,smoothed image is shown in Figure 1 (the optical DSSimage of the same field is shown for comparison). Theimage shows several interesting features associated withNGC 5098a:

• A bright point source, coincident with the centralAGN, near the center of the diffuse emission.

• A plume of emission extending to the northeast(the “tail” noted previously by Gastaldello et al.2007). The plume exhibits a spiral arm morphol-ogy, originating east of NGC 5098a and wrappingaround to the north. The presence of this featureindicates that the system is not dynamically re-laxed. NGC 5098a is most likely currently inter-acting with NGC 5098b.

• A sharp surface brightness edge to the west, south-west, and south, roughly 30′′ (28 kpc) from the cen-tral AGN. The edge appears to continue to the eastand define the outer boundary of the arm, addingto the overall impression of a spiral pattern in thediffuse emission. The edges are similar to featuresseen from cold fronts generated by gas sloshingin observations (Dupke et al. 2007; Gastaldello etal. 2009) and simulations (Ascasibar & Markevitch2006) of galaxy clusters and groups.

• An asymmetry in the brightest (central) diffuseemission, which extends farther west of the AGNthan to the east, also suggesting that this systemis disturbed.

• Two small cavities, surrounded by bright rims ofemission, roughly 4′′ (3 kpc) to the north andsouthwest of the AGN. They are morphologicallysimilar to “bubbles” seen in X-ray observations ofother galaxies and clusters, some of which are asso-ciated with radio emission (e.g., Abell 2052, Blan-ton et al. 2003; Perseus cluster, Fabian et al. 2006;NGC 4552, Machacek et al. 2006; MS0735.6+7421,McNamara et al. 2007; M84, Finoguenov et al.2008; NGC 5044, Gastaldello et al. 2009), thoughthey are relatively small in size. The proximityof the bubbles to the AGN suggests that they areyoung features, possibly currently being inflated byjets from the AGN.

4 http://cxc.harvard.edu/caldb/

N5098 FROM CHANDRA 3

Also of note is the complete lack of X-ray emissionassociated with NGC 5098b. Although the galaxy is wellwithin the northeastern arm of NGC 5098a, there is noindication of enhanced surface brightness centered on thegalaxy. This suggests that NGC 5098b may have beenstripped of its gas during an interaction with NGC 5098aor the ambient group ICM.In order to examine the faint, diffuse, group emission

at larger radii, we made a more heavily smoothed image,shown in Figure 2. Point sources have been removedby filling in source regions using a Poisson distributionwhose mean was equal to that of a local annular back-ground region. Diffuse emission is seen beyond the coreand the edges noted in Figure 1, in all directions. Theextended halo also shows hints of a spiral structure, con-tinuing from the inner arm noted in Figure 1 and wrap-ping around from the north out to a radius of ∼ 120′′

(89 kpc), through the west, and back to the south outto ∼ 132′′(98 kpc), though the reality of this structure isunclear, and may just be an artifact of the brighter, in-ner arm reaching into the extended emission in the north.There appears to be a sharp, linear edge in the extendedemission to the south. The orientation of the edge issuch that it cannot be due to a chip node boundary.We searched the NASA/IPAC Extragalactic Databasefor a foreground absorber that could be responsible forthis feature, but found none. To the southwest, thereis an extended source associated with the galaxy tripletNGC 5096, which has been included as a member of theNGC 5098 group (Ramella et al. 1995). Using opticaland XMM-Newton X-ray observations, Mahdavi et al.(2005) identify this source as an independent subgroupthat has not yet interacted with, nor is bound to, themain group.

3.1. Unsharp-Masked Image

To better visualize faint surface brightness fluctua-tions, particularly near the core, we made a (0.3–5.0 keV)unsharp-masked image. It was created by dividing theimage smoothed with a 0.98′′ radius Gaussian by onesmoothed with a 9.8′′ Gaussian. The resulting image isshown in Figure 3. Two bubbles are clearly seen to thenorth and southeast of the central AGN, which are alsodetected in the radio (see Figure 4). The surface bright-ness profiles in four sectors, two of which contain thebubbles, are compared to the average profile in Figure 5.This figure shows that the northern and southern bubblescorrespond to ∼ 60% and ∼ 40% deficits respectively, ascompared to both the average profile and the peaks atlarger radii which correspond to the bright X-ray rims.In the radio, 6 cm observations clearly show a centralcore with two radio lobes corresponding to the bubblesseen in the Chandra images, with comparable radio fluxfrom each lobe and the core (see B2 1317+33 in Morgantiet al. 1997). In Figure 4 we overlay radio contours fromVLA L-band 1.45 GHz data taken from the VLA imagearchive over a close-up of the core in Figure 3. Radioemission fills the X-ray bubbles.The unsharp-masked image also reveals complex struc-

ture in the diffuse emission, most notably southwest ofthe AGN, with several surface brightness depressionssimilar to those seen in the bubbles (but without the sur-rounding bright rims), all within the outer edge noted inFigure 1. There is no obvious correlation between the ra-

dio observations and these other X-ray cavities. A com-parison of the net counts in these depressions to thosefrom adjacent regions shows that most are statisticallysignificant at the 2-3σ level. In the bubbles, the countrates in the rims are higher than those in the central de-pressions by 4.6σ and 2.3σ, for the northern and southernbubbles respectively. For one of the more significant cav-ities to the southwest, the deficit is significant at 3.8σ.Therefore, the statistical significance of the cavities is onthe order of that for the bubbles, which are seen in theradio and clearly real features. The large, dark regionto the east, just outside the bubbles, is an artifact ofthe relatively sharp drop-off in surface brightness in thisregion, possibly indicating an edge in the central brightdiffuse emission (although the bright rims of the bubblesalso contribute to this deficit).

4. SPECTRAL ANALYSIS

The X-ray image (Figure 1) shows diffuse emission as-sociated with NGC 5098a, as well as fainter group emis-sion filling the field of view. We generated a tempera-ture map as a guide for detailed spectral fitting to disen-tangle the various components and study the structureseen in the ICM. We assume a galactic absorption ofNH = 1.31× 1020 cm−2 throughout.

4.1. Temperature Map

The temperature map was derived using the samemethod as developed in Randall et al. (2008; 2009). Foreach temperature map pixel, we extracted a spectrumfrom a circular region containing 1000 net counts (aftersubtracting the blank sky background). The resultingspectrum was fit in the 0.6 – 5.0 keV range with anabsorbed APEC model using XSPEC, with the abun-dance allowed to vary. The resulting temperature map,with X-ray surface brightness contours overlaid, is shownin Figure 6. Unfortunately, due to the small number ofnet counts, the extraction regions for the temperaturemap pixels were relatively large. Faint regions had ex-traction radii on the order of 1.6′ (71 kpc), while thebrightest regions, near the core of NGC 5098a, had radiiof 7.9′′ (5.8 kpc). As a result, each pixel in the tempera-ture map is highly correlated with nearby pixels, and thetemperature map is effectively smoothed on large scales,particularly in regions far from the core.Nevertheless, there are several interesting features in

the temperature map. There is an elliptical clump of coolgas west of the AGN, which appears to be doubly peakedto the north and south. The cool gas wraps around theAGN, but does not overlap it. The outer edge of thiscool region roughly corresponds to the surface bright-ness edge noted in Figure 1, suggesting that this featureis a cold front. A long arm of cool gas extends east ofthe AGN and wraps around to the north, with the outeredge connecting to the edge of the cool elliptical regionin the south. The orientation and morphology of the coolarm is very similar to the arm seen in the surface bright-ness map (Figure 1), though it extends well beyond theapparent outer boundary of the surface brightness arm.We investigate this feature further in § 4.2. As in the sur-face brightness map, there is no structure correlated withNGC 5098b. As a test of the robustness of the tempera-ture measurements in faint regions, we re-fit the spectrafor a few temperature map pixels near the tip of the cool

4 RANDALL ET AL.

arm in the temperature map and varied the backgroundnormalization by ±10%. We found no significant effecton the resulting temperature. The average radial temper-ature structure is consistent with that found previouslyfrom analyses of Chandra and XMM-Newton data (Xueet al. 2004; Gastaldello et al. 2007), with kT ≈ 0.8 keVnear the core of NGC 5098a, rising to ∼ 1.3 keV at 1.5-2′

(67-89 kpc), and dropping off at larger radii.To search for correlation with the detailed structure

shown in Figure 3, we made a higher-resolution temper-ature map of the central region of NGC 5098a, shownin Figure 7, with the same X-ray surface brightness con-tours as in Figure 6. The smoothing scales are too largeto show any structure at the level of the X-ray cavitiesshown in Figure 3, although the double-peaked natureof the elliptical region, and its anti-correlation with theAGN, can be sen more clearly. It is also clear that thetemperature in the inter-arm region, between the arm’sinner edge and the cooler central gas, is consistent withthe ambient temperature of ∼ 1.2 keV. This is also con-sistent with what we find from detailed spectral fits (see§ 4.2).The structure of the temperature map, in particular

the cool spiral arm morphology and the offset of the coolgas from the central AGN, supports our conclusion from§ 3 that this is a disturbed system, possibly due to aninteraction with the nearby galaxy NGC 5098b. In par-ticular, these features are very similar to those seen fromsloshing of the central gas about the potential minimum(see Figure 7 in Ascasibar & Markevitch 2006, especiallyat 1.7 - 1.9 Gyr). We discuss this possibility further in§ 5.

4.2. Detailed Spectra

4.2.1. Diffuse Emission

Based on the derived temperature map and the X-rayimage, we defined 7 regions for detailed spectral analy-sis. A summary of the spectral model for each region isgiven in Table 1. R1 is centered on the AGN and coversthe central projected 26′′ (19.2 kpc) of NGC 5098a. R2and R3 correspond to the southern and northern coolspots in the larger elliptical region of cool gas seen inthe temperature map, respectively. R4 covers the innerpart of the cool arm seen in the temperature map, whilethe adjacent region R5 is the hotter area just outside ofthe temperature map arm (though R5 overlaps with thearm seen in the surface brightness map). The regionsare shown overlaid on the X-ray image and temperaturemap in Figure 8, where in the left panel point sourceshave been removed as in Figure 2.A single-temperature thermal model was an adequate

fit to each region considered in Table 1. The best-fittingtemperature for the “inter-arm” region seen in the tem-perature map (R5) was greater than that in the adjacentregion R4, with a significance of 3.7σ. The best-fittingabundance was also higher in R5, by more than a factorof two. As a test, we re-fit the spectrum from R5 withthe abundance fixed to the best-fitting value from regionR4 (row 6 in Table 1). The chi-squared per degree-of-freedom was significantly worsened by fixing the abun-dance, and the best-fitting temperature was still foundto be higher, with a significance of 2.3σ. This suggeststhat the arm structure seen in the temperature map in

the vicinity of R4 and R5 is real, and not an artifact ofsmoothing.As noted in § 4.1, the cool arm seen in the temperature

map extends beyond the outer edge of the arm in the sur-face brightness map. R6 and R7 were defined to evaluatethe significance of the temperature difference in these re-gions. Table 1 shows that the best-fitting temperaturein R7 is higher than that in R6 with high significance(greater than 6σ). This confirms that the emission nearthe edge of the FOV is hotter northwest of the AGN ascompared to the northeast, as suggested by the arm fea-ture in the temperature map, independent of smoothingeffects.

4.2.2. The Surface Brightness Edges

The surface brightness edges indicated in Figure 1are similar to features arising from contact discontinu-ities, or “cold fronts”, seen in simulations and observa-tions of galaxy clusters and groups (e.g., Ascasibar &Markevtich 2006; Dupke et al. 2007; Gastaldello et al.2009). We therefore extracted spectra across these edgesto search for temperature jumps, which are characteris-tic of cold fronts. Spectra were extracted from the samesemi-annular regions shown in Figure 10, but with fewer(larger) radial bins, such that each bin contained ∼ 500net counts. The resulting spectra were fit with a singletemperature APEC model, with the abundance allowedto vary. The resulting temperature profiles are shown inFigure 9. Each profile shows a clear jump at the locationof the apparent surface brightness edge, identifying thesefeatures as cold fronts. These features are discussed inmore detail in § 5.1. We note that, for the northeast-ern profile, the elevated temperature of the innermostbin (which overlaps with region R5 in Figure 8) and thelower temperatures at large radii (as compared to thesouthwestern profile) are consistent with what is shownthe temperature map (Figure 6).

4.2.3. The Central Source

We extracted a spectrum for the central source inNGC 5098a using an aperture with a radius of 1.8′′. Thebackground was determined locally from an annulus cen-tered on the source with inner and outer radii of 1.8′′

and 4.0′′, respectively. This gave 264 net counts in the0.6 – 7.0 keV band. The spectrum was fit with an ab-sorbed power-law, giving a best-fitting photon index of1.93±0.16, which is typical for a radio galaxy (e.g., Sam-bruna et al. 1999). The source has an unabsorbed fluxof 4.2 × 10−14 ergs cm−2 s−1 and X-ray luminosity of1.4 × 1041 ergs s−1 in the 0.6 – 7.0 keV energy band.We also fit the spectrum with the XSPEC intrinsic ab-sorption model ZWABS and find that the spectrum isconsistent with no internal absorption.

5. DISCUSSION

5.1. Structure of the Cold Fronts

The Chandra image (Figure 1) shows a brightness edgealmost completely encircling NGC 5098a. It is sharpestand closest in the west/southwest, continues on to thesouth, and spirals out to form the outer boundary of thesurface brightness arm in the northeast. Spectral analy-sis revealed temperature jumps across each edge, indicat-ing that these edges are cold fronts (§ 4.2.2). As such, we

N5098 FROM CHANDRA 5

expect there to be a density discontinuity located at eachedge. To measure the amplitude of these density jumps,we extracted the Chandra 0.6–5.0 keV surface brightnessprofile in two sectors, one to the southwest and anotherto the northeast. The regions were defined such that theradii of curvature matched those of the edges. In eachcase, the center of curvature was reasonably close to thepeak of the diffuse X-ray emission, near the central AGN.The extraction regions are shown in Figure 10.The resulting emission measure profiles are shown in

Figure 11 (distance is measured from the center of curva-ture of the apparent edge). These profiles were fit with aspherical gas density model consisting of two power laws.The free parameters were the normalization, the inner(α) and outer (β) slopes, the position of the density dis-continuity (rbreak), and the amplitude of the jump (A).The temperature and abundance for each bin were deter-mined from the fits to the projected emission from thecoarser bins shown in Figure 9, as there were insufficientcounts to perform deprojected spectral fits. The profilefrom the southwest sector has the typical shape expectedfor a contact discontinuity. Fitting our model, we findα = −1.15+0.07

−0.06, β = −0.88+0.06−0.06, rbreak = 31+2

−1 kpc, and

A = 2.13+0.21−0.17. In the northeast, the density jump is

less pronounced, but still significant within the errors:α = −0.70+0.11

−0.12, β = −1.47+0.16−0.15, rbreak = 56+3

−5 kpc , and

A = 1.45+0.22−0.17. We note that the presence of the long

trail of cool gas seen in the northeast in Figure 6 couldcontaminate the temperature profile shown in Figure 9(right), and therefore affect our fits to the density jumpin this region. However, the emissivity is only weaklydependent on the temperature (which does not stronglyvary in this region), so the integrated emissivity profilewe fit to is dominated by surface brightness variations.Furthermore, if we were able to remove any contributionfrom the cool arm beyond the surface brightness edge wewould raise the overall temperature in this region, whichwould increase the size of the temperature jump in Fig-ure 9 (right) and only strengthen our conclusion that thissurface brightness edge corresponds to a cold front. Forboth the northeastern and southwestern edges, the twopower-law model was a much better fit to the data thana single power-law or beta model. The best fit modelsare plotted in Figure 11.

5.2. The Dynamical State of NGC 5098

Based on temperature profiles and fits to the surfacebrightness profiles (see § 5.1 and § 4.2.2), the edges shownin Figure 1 are identified as cold fronts. Such contact dis-continuities are expected to arise during a merger, eitherat the leading edge of a remnant gas core, or from thesloshing of gas around the potential minimum after itis perturbed by an off-axis encounter with a secondaryobject (see Markevitch & Vikhlinin 2007 for a review).There are several reasons to identify these features assloshing fronts. The centers of curvature of the featuresare roughly centered on the X-ray brightness peak, as ex-pected for sloshing fronts. The density jumps are modest,in the 1.1–2.1 range, in contrast to the larger jumps seenin remnant core cold fronts (e.g., Markevitch et al. 2002;Randall et al. 2009). The spiral structure, seen in boththe X-ray surface brightness and temperature maps (Fig-ures 1 & 6), is characteristic of sloshing fronts in existing

observations and simulations (Ascasibar & Markevitch2006; Dupke et al. 2007; Gastaldello et al. 2009). Theapparently nearby galaxy NGC 5098b provides a likelyinteraction candidate to initiate sloshing in the core ofNGC 5098a. Finally, and most compellingly, the tem-perature map (Figure 7) clearly shows an offset betweenthe cool central gas and the central AGN (and the cen-troid of the optical emission), which presumably lies atthe local minimum of the potential. The structure isvery similar to that observed in simulations of gas slosh-ing (e.g., Figure 7 in Ascasibar & Markevitch 2006)As noted in § 4.1, one potential problem with this

picture is that the cool arm seen in the temperaturemap (Figure 6) extends well beyond the apparent outerboundary of the surface brightness arm. Detailed spec-tral fits, described in § 4.2, show that the arm is likely areal feature, and not an artifact of the smoothing inher-ent in the temperature map. While it is possible that thearm represents sloshed gas from the core of NGC 5098a,the scales involved make this explanation seem unlikely.To explain this feature, we note that NGC 5098b doesnot show any associated emission in X-rays, implyingthat this system has been stripped of its gas. The armseen in the temperature map may be gas stripped fromNGC 5098b as it fell into the group from the north-east and interacted with the ambient ICM. The orienta-tion of the arm is consistent with the sloshing interpre-tation, as the direction of angular momentum impliedby the winding direction of the surface brightness spi-ral arm morphology is consistent with a perturber thathas approached from the northeast and passed east ofNGC 5098a (in projection). In this scenario, the ap-parent connection between the outer temperature maparm and the inner surface brightness arm is a projec-tion effect, which may explain the less than perfect cor-respondence between these features near the tip of thesurface brightness arm. Furthermore, it is possible thatNGC 5098b, after being ram pressure stripped and pass-ing east of NGC 5098a, circled around NGC 5098a (notnecessarily in the plane of the sky) and is now movingroughly to the east, creating a subtle conical wake in thediffuse emission, similar to those seen in the first and lastpanels of Figure 21 in Ascasibar & Markevitch (2006).The surface brightness arm could be a manifestation ofthe wake, rather than cool gas sloshed from the core ofNGC 5098a, which would explain the lack of correlationwith the temperature map. Unfortunately, we do nothave enough net counts to test in detail either the inter-pretation of the extended cool arm in the temperaturemap as stripped gas from NGC 5098b, or the possibilitythat the surface brightness arm is a conical wake gener-ated by the return passage of NGC 5098b. We note onlythat the current data are consistent with either interpre-tation.As shown in Figure 4, the central bubbles are swept

back to the east of the central AGN in projection. Thiscould be the result of ram pressure due to relative mo-tion between the host galaxy and the ICM either due tothe galaxy’s motion within the group or bulk motion ofthe gas. Alternatively, the jets may have encounteredand been deflected by inhomogeneities in the ICM. Wenote that, in simulations, sloshing fronts are not staticfeatures. Rather, gas flows along the spiral arm, fromthe outer regions into the center (Figure 7 in Ascasibar

6 RANDALL ET AL.

& Markevitch 2006). In this system, we therefore expectgas to be flowing inwards from the east, around the thesouth, and approaching the central AGN from the west.It is therefore possible that the bulk motions in the gaswhich sweep the central bubbles to the east are due tothe velocity field set up in the sloshing front, though wecannot distinguish this possibility from the others previ-ously mentioned.

5.3. The X-ray Cavities

The X-ray surface brightness (Figure 1) and unsharp-masked (Figure 3) images show several statistically sig-nificant cavities in the central region of NGC 5098a. Inparticular, there are two distinct bubbles (seen as X-raydeficits surrounded by rims of bright emission) just northand southeast of the central AGN. The bubbles are theonly cavities which correlate with existing radio obser-vations (see § 3.1 and Figure 4), and we focus on thesefeatures first.The bubbles are morphologically similar to features

seen in X-ray observations of other galaxies, groups, andclusters. These structures are formed when AGN jetspush into the local ICM, evacuating cavities, and of-ten creating bright rims of X-ray emission from the dis-placed gas. In more evolved remnant cavities the rimstend to be cooler and more dense than the nearby am-bient ICM, as in Abell 2052 (Blanton et al. 2003) andPerseus (Fabian et al. 2006), whereas in more recentoutbursts they often show higher temperatures associ-ated with shocks, as in NGC 4552 (Machacek et al.2006), Hercules A (Nulsen et al. 2005), and Centaurus A(Croston et al. 2009). To test for a temperature differ-ence in the rims, we extracted spectra from the northern(brighter) bubble rim and a similar region just outsidethe rim, subtracted off spectra from local backgroundregions, and fit each with an absorbed APEC modelwith the abundance fixed at 30% solar. We find best-fitting temperatures of kTrim = 0.978+0.085

−0.096 for the rim

and kToutside−rim = 1.085+0.085−0.198 just outside the rim. Al-

though the best-fitting temperatures indicate that therim is somewhat cooler, the difference is not statisticallysignificant.The proximity of the bubbles to the central AGN, and

their relatively small physical size, suggest that they arecurrently forming as the cavities are inflated by the AGN.If this is the case, one might expect the total pressurein the cavities to be on the order of that in the rimsand the surrounding ICM, possibly larger if the cavi-ties are driving shocks. We can estimate the pressure inthe X-ray emitting gas in the northern rim using the fitto the spectrum to estimate the temperature and den-sity of the gas. Assuming an edge-on oblate spheroidalgeometry for the bubble (with semi-major and minoraxes of 3 kpc and 1.6 kpc, respectively), we find anelectron density of ne ≈ 0.03 cm−3 and a pressure ofPrim ≈ 4.5× 10−11 dyne cm−2. To estimate the pressureinside the bubble, ideally one would like flux measure-ments from radio observations at multiple frequencies todetermine the spectral index of the relativistic particlepopulation. Under the assumption of equipartition, theradio pressure at minimum energy is given by

Prad =B2

min

8π+

4Emin

7φV, (1)

where the magnetic field at minimum total energy

Bmin =[

6π(1 + k) c12(α, ν1, ν2)Lφ−1 V −1]2/7

, (2)

and the minimum total energy

Emin =7

4

[

1

6πφV (1 + k)4/3(c12(α, ν1, ν2)L)

4/3

]3/7

.

(3)In these relations, k is the ratio of proton to electron en-ergies, V is the volume of the emitting region, φ ≈ 1 isthe volume filling factor, L is the radio luminosity at agiven frequency, and c12(α, ν1, ν2) is a parameter (tab-ulated in Pacholczyk 1970) that depends on the spec-tral index α and the lower and upper cut-off frequencies,which we take to be ν1 = 10 MHz and ν2 = 10 GHz.We assume k ≈ 1, which is expected for a young sourcesince it has not yet had time to entrain material from theICM. Useful expressions from these relations are givenby Govoni & Feretti (2004). Morganti et al. (1997) give6 cm fluxes of 8.7, 10.3, and 7.1 mJy for the core, thenorthern lobe, and the southeastern lobe, respectively.Unfortunately, from the literature we were only able tofind radio flux measurements for the resolved AGN andlobes at a single frequency, so the value of α inside thelobes is unknown. As a rough estimate, we assume thatα = −1.6, consistent with results from similar X-raybubbles in Abell 2052 (Zhao et al. 1993; see Bırzan etal. 2008 for indicies for several sources, though they aregiven for the total source, not just the lobes, and henceare expected to be steeper). We find an equipartitionmagnetic field strength of Beq ∼ 40 µG and a minimumradio pressure of Prad = 1.5 × 10−10 dyne cm−2, morethan three times the X-ray pressure in the rims. How-ever, this result is sensitive to the assumed value of α: ifwe instead take α = −1, which is closer to the value forthe bubbles in the Perseus cluster (Pedlar et al. 1990),we find Prad = 2.6× 10−11 dyne cm−2, less than the X-ray pressure in the rims. Morganti et al. (1997) find thatthe ratio of flux from the northern radio lobe to the to-tal flux from both lobes and the central core is f ≈ 0.43at 6 cm (5 GHz). The NRAO/VLA Sky Survey (NVSS,Condon et al. 1998) gives a total flux of 82.9 mJy at1.4 GHz. Assuming that f ≈ 0.43 at 1.4 GHz, we findα ≈ −1. We note that this is an upper limit on α, sincethe core is expected to be brighter at higher frequencies(e.g., the lobes are not seen in 8.49 GHz X-band imagesfrom the VLA archive, whereas the AGN is clearly visi-ble), so that f is a decreasing function of frequency. Wetherefore conclude that Prad > 2.6× 10−11 dyne cm−2 isa hard lower limit, and the radio pressure in the northernbubble is on the order of the pressure in the surroundingX-ray emitting rims. The fact that pressures are roughlyequal under the assumption that k = 1, and that thebubbles are physically close to the AGN, suggest thatthe bubbles are young features, possibly currently beinginflated by the central AGN. Follow-up high-resolutionradio observations, along with deep X-ray observations,would be of interest for a better comparison of the rela-tive pressures in the rims and bubbles, and of the X-raytemperature in the rims and the ambient ICM.The unsharp-masked image (Figure 3) shows several

surface brightness depressions beyond the inner bubbles.All are within the surface brightness edge shown in Fig-

N5098 FROM CHANDRA 7

ure 1. Although these depressions show no correlationwith the existing radio data, some of them are more sta-tistically significant than the depressions associated withthe bubbles, which are clearly real and seen in radio ob-servations. We therefore make the assumption that thesefeatures are “ghost cavities”, leftover from previous AGNoutbursts, devoid of X-ray emitting gas but no longerstrongly emitting in the radio, as seen in other systems(e.g., Jetha et al. 2008). We estimate the total volumeoccupied by the cavities by extracting spectra from thecentral 30′′ (22 kpc), with and without including the re-gions of the cavities, and fitting with an absorbed APECmodel. Comparing the volume emissivities (calculatedfrom the area-normalized normalizations of the fits) di-rectly gives an estimate for the fraction of the total vol-ume occupied by the cavities. We find a total cavity vol-ume of 1240 kpc3 for 14 cavities within the central 22 kpc(i.e., a filling-factor of about 3%), giving an average cav-ity volume of 31 kpc3 (note that not all of these cavitiesare visible in Figure 3, as some are within the large darkfeature to the east that corresponds to an edge in the cen-tral surface brightness, and are not visible at this scalingand contrast). For spherical cavities, this gives an aver-age radius of ∼ 2 kpc, fully consistent with the averageprojected cavity radius (even though several cavities areclearly non-spherical) and with our assumption that thecavities contain no X-ray emitting gas. Assuming thatthe bubbles rise buoyantly at the sound speed in the gas,cs ≈ 460km s−1 for kT = 0.8 keV, the average distanceof the bubbles from the AGN davg = 9 kpc gives an av-erage cavity age of tage ≈ 18 Myr.The mechanical energy input required to inflate the

cavities, Emech, can be estimated from the X-ray gaspressure and the total volume occupied by the cavities.Using the density and temperature from the spectral fitto the central 22 kpc, we find an average pressure of∼ 1.8×10−11 dyne cm−2, which gives a total mechanicalenergy input of Emech ≈ 7 × 1056 ergs, consistent withwhat is found in other galaxy groups (McNamara 2004).We obtain an estimate for the cooling rate of the centralgas from the 0.3–12 keV X-ray luminosity in the sameregion, which we find from our fitted spectral model tobe LX = 1.5 × 1042 ergs s−1. The mechanical energy inthe cavities is therefore sufficient to offset cooling in thediffuse gas for 15 Myr, very similar to what was foundfor the Virgo galaxy NGC 4552 (Machacek et al. 2006),and similar to the average cavity age calculated from therise time above. This is on the order of central AGN cy-cle times inferred for other galaxy clusters (e.g., Blantonet al. 2009; Clarke et al. 2009) of a few tens of Myrs.We therefore conclude that the current average mechan-ical luminosity of the AGN is nearly if not completely

sufficient to balance radiative cooling of the gas in thecentral region (we also note that the total energy in thebubbles may be up to a factor of 2–4 times more thanthe mechanical energy alone, see McNamara 2004).

6. SUMMARY

We have analyzed Chandra ACIS-S3 observations ofthe NGC 5098 galaxy group. X-ray images reveal a spiralarmmorphology extending to the northeast, a sharp edgein the diffuse emission surrounding much of NGC 5098aand connecting to the outer boundary of the arm, aswell as bubbles and other X-ray surface brightness de-pressions in the core. Temperature and density profilesacross the edges indicate that they are cold fronts. Thestructure of the cold fronts (which have relatively mod-est density jumps, and radii of curvature centered on thepeak of the diffuse emission), the spiral structure seenin the X-ray surface brightness and temperature maps,and the offset between the central clump of cool gas seenin the temperature map from the central AGN and op-tical center of the galaxy all point to gas sloshing inthe core, as seen in simulations and other observations.The obvious candidate perturber is the nearby galaxyNGC 5098b, which has apparently been stripped of itsX-ray emitting gas. We have suggested that the longouter arm of cool gas seen in the temperature map mayin part be the stripped tail of NGC 5098b, formed as itpassed through the group ICM and initiated sloshing inNGC 5098a. The winding direction of the inner spiralarm is consistent with the perturber approaching fromthe northeast to the southeast, which matches the trajec-tory implied for NGC 5098b if we interpret the extendedtemperature map arm as its stripped tail. The two bub-bles in the core of NGC 5098a, which are seen as X-raycavities surrounded by bright rims of emission, correlatewith radio observations. A comparison of the radio pres-sure in the bubbles to the X-ray pressure in the rimsshows that they are about equal, consistent with thesebeing young features that are currently being inflated byjets from the central AGN. We make the assumption thatthe other X-ray cavities seen in the core are ghost cavi-ties, left over from previous AGN outbursts. An estimateof the mechanical energy required to inflate these cavi-ties shows that the energy output of the central AGN issufficient to balance radiative cooling of the gas in thisregion.

The financial support for this work was partially pro-vided for by the Chandra X-ray Center through NASAcontract NAS8-03060, and the Smithsonian Institution.We thank the anonymous referee for useful comments.

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416, 51

N5098 FROM CHANDRA 9

TABLE 1Spectral Fits

Region # kT Abund. χ2/dof Net Cnts.(keV) (solar)

R1 0.934+0.014

−0.0140.30+0.04

−0.0350.2/50=1.00 3743

R2 0.768+0.045

−0.0580.17+0.11

−0.073.6/7=0.82 554

R3 0.813+0.043

−0.0510.20+0.13

−0.082.8/7=0.90 484

R4 0.993+0.030

−0.0330.26+0.11

−0.084.7/7=0.69 793

R5 1.223+0.046

−0.0540.58+0.28

−0.186.55/7=0.48 687

R5 1.147+0.056

−0.060(0.26) 11.1/8=1.38 687

R6 1.056+0.066

−0.0420.13+0.04

−0.0327.8/32=0.68 791

R7 1.298+0.062

−0.0760.20+0.06

−0.0825.5/18=1.42 546

Fig. 1.— Left Panel: Exposure corrected, background subtracted 0.6–5 keV Chandra ACIS-S3 observation of NGC 5098. The image hasbeen smoothed with an 3′′ radius gaussian. Regions with less than 10% of the total exposure were omitted. The image shows a spiral armstructure to the northeast, a sharp surface brightness edge beginning in the west and connecting around the south to the outer boundaryof the arm, and central bubbles surrounded by bright rims of emission. The blue cross marks the optical position of NGC 5098b. RightPanel: DSS image of the same field. NGC 5098a is the western galaxy of the central bright galaxy pair while NGC 5098b is to the east.

10 RANDALL ET AL.

Fig. 2.— The 0.6–5.0 keV Chandra image, smoothed with a 12′′ radius gaussian to better show faint, diffuse emission at large radii.Point sources have been removed, as described in the text (see § 3). There is a sharp, linear edge in the emission to the south. Diffuseemission from the group member NGC 5096 is seen to the southwest.

N5098 FROM CHANDRA 11

20.0 18.0 16.0 14.0 12.0 13:20:10.0

30.0

33:09:00.0

30.0

08:00.0

Arm

Edge

S. Bubble

N. Bubble

Fig. 3.— An unsharp-masked 0.3–5 keV image of the central region of NGC 5098a. X-ray cavities, or bubbles, encompassed by brightrings of emission are clearly seen to the north and southwest of the AGN. The bubbles are filled with radio-emitting plasma (see Figure 4).The unsharp-masking reveals other fine structure and surface brightness depressions, particularly in the southwest (all within the surfacebrightness edge shown in Figure 1).

12 RANDALL ET AL.

17.0 16.0 15.0 13:20:14.0 13.0 12.0

33:09:00.0

50.0

40.0

30.0

20.0

Fig. 4.— A close-up of the bubbles shown in Figure 3. The logarithmically-spaced contours were generated from 1.45 GHz VLA L-bandimages taken from the VLA data archive. Radio emission fills the X-ray bubbles.

N5098 FROM CHANDRA 13

Fig. 5.— The 0.6-5.0 keV surface brightness profile in four sectors containing the northern bubble (red dotted), the southern bubble (blueshort-dashed), the region to the east (green long-dashed), and the region to the west (magenta dot-dashed) as compared to the azimuthalaverage (black solid). Error bars have been omitted for clarity. The peaks at ∼ 6 kpc correspond to the bright bubble rims, and the deficitsat 3-4 kpc to the bubbles themselves. The overall east-west asymmetry is also evident. Pixels are 0.98′′×0.98′′.

14 RANDALL ET AL.

Fig. 6.— Temperature map derived from the ACIS-S3 data, with Chandra X-ray logarithmic surface brightness contours overlaid. Thegreen cross indicates the optical position of NGC 5098b. The color-bar gives the temperature in keV.

N5098 FROM CHANDRA 15

Fig. 7.— A higher resolution temperature map of the central region of NGC 5098a, with the same X-ray surface brightness contoursoverlaid as in Figure 6. The green cross indicates the optical position of NGC 5098b. The elliptical cool region to the west is split intotwo cool spots, one to the north and one to the south. The holes in the temperature map indicate pixels that were completely containedwithin an excluded source region (e.g., at the central AGN).

16 RANDALL ET AL.

Fig. 8.— The regions fitted in Table 1 overlaid on the 0.6 – 5.0 keV Chandra image (left) and temperature map (right). The image hasbeen smoothed with a 6′′ radius gaussian, and the point sources have been removed by filling in source regions as described in the text(§ 4.2).

20 40 60 80 100

0.8

1

1.2

1.4

R (kpc)

20 40 60 80 100 1200.9

1

1.1

1.2

1.3

1.4

R (kpc)

Fig. 9.— Temperature profiles to the southwest (left) and northeast (right) for the semi-annular areas shown in Figure 10 (but withfewer, larger bins). The dashed lines mark the positions of the density jumps calculated in § 5.1.

N5098 FROM CHANDRA 17

Fig. 10.— Regions used to generate the projected emission measure profiles shown in Figure 11 overlaid on the background subtracted,exposure corrected, smoothed, 0.6 – 5.0 keV Chandra image (with point sources removed).

10

100

10 100

∫npn

edl (

1e+

60 c

m-6

kpc-2

)

R (kpc)

5

10

50

50 100

∫npn

edl (

1e+

60 c

m-6

kpc-2

)

R (kpc)

Fig. 11.— Left Panel: Integrated emission measure profile extracted from the southwestern region shown in Figure 10. The x-axis givesthe radius from the apparent center of curvature defined by the feature. The best fit two power law density jump model is given by thesolid line. The vertical dashed line shows the best fit location of the density discontinuity. Right Panel: Same for northeastern region inFigure 10.