X-ray Point Sources and Radio Galaxies in Clusters of Galaxies
aa r X i v : . [ a s t r o - ph . C O ] A ug Draft version October 16, 2018
Preprint typeset using L A TEX style emulateapj v. 08/22/09
X-RAY POINT SOURCES AND RADIO GALAXIES IN CLUSTERS OF GALAXIES
Quyen N. Hart, John T. Stocke, and Eric J. Hallman
Center for Astrophysics and Space Astronomy,Department of Astrophysical and Planetary Sciences,UCB-389, University of Colorado, Boulder, CO 80309
Draft version October 16, 2018
ABSTRACTUsing
Chandra imaging spectroscopy and VLA L-band maps, we have identified radio galaxies atP (1 . GHz ) ≥ × W Hz − and X-ray point sources (XPSs) at L (0 . − keV ) ≥ ergs s − in11 moderate redshift (0 . < z < .
4) clusters of galaxies. Each cluster is uniquely chosen to havea total mass similar to predicted progenitors of the present-day Coma Cluster. Within a projectedradius of 1 Mpc we detect 20 radio galaxies and 8 XPSs (3 sources are detected in both X-ray andradio) confirmed to be cluster members above these limits. 75% of these are detected within 500 kpc(projected) of the cluster center. This result is inconsistent with a random selection from bright, redsequence ellipticals at the > jet ∝ L . r from Bˆırzan et al. (2008), radio sources weaker thanour luminosity limit probably contribute the majority of the heat to the ICM. Also, because theseheat sources move around the cluster, AGN heating is distributed rather evenly. If a majority of ICMheating is due to large numbers of low power radio sources, triggered into activity by the increasingICM density as they move inward, this may be the feedback mechanism necessary to stabilize coolingin cluster cores. Subject headings: galaxies: active – galaxies: clusters: general – radio continuum: galaxies – X-rays:galaxies: clusters – X-rays: galaxies INTRODUCTIONObservations of galaxy clusters have provided impor-tant clues about cosmology, structure formation andthe evolution of galaxies. As the largest gravitationallybound objects in the Universe, clusters are unique loca-tions to study the creation and evolution of AGN. Basedupon the observed correlation between supermassiveBlack Hole mass and galaxy bulge mass (Magorrian et al.1998), we expect that there should be numerous, lumi-nous AGN in the bright ellipticals in rich clusters whichcan affect their surroundings. Indeed, observationalevidence supports the notion that non-gravitationalprocesses have affected the entropy of the intraclus-ter medium (ICM), as suggested by the steepness ofthe L x -T x relationship compared to self-similar scalingexpectations (Edge & Stewart 1991; Markevitch 1998;Ruszkowski et al. 2004). Radio-loud AGN have been di-rectly implicated in cluster ICM heating because evacu-ated “bubbles” in the diffuse X-ray emitting gas havebeen observed that are spatially coincident with non-thermal radio emission (Fabian et al. 2002; Bˆırzan et al.2004). However, up until now only AGN in the bright- est cluster galaxies (BCGs) have been observed to spawnthese bubbles and so only the central AGN has been in-corporated into ICM heating models (e.g., Br¨uggen et al.2005). In this case, theoretical difficulties arise when at-tempting to distribute this heating throughout the cen-tral cluster regions (e.g., Vernaleo & Reynolds 2007). Anatural solution would be to have other cluster AGN in-jecting energy throughout the inner cluster region (e.g.,Nusser et al. 2006).Most cluster AGN studies have either employed clus-ters in a flux-limited survey (e.g., Stocke et al. 1999;Ruderman & Ebeling 2005; Branchesi et al. 2006) orsimply selected clusters based upon availability (e.g.,Martini et al. 2007). But since flux-limited surveys pro-duce cluster samples which contain more luminous andthus more massive objects at higher redshifts, this selec-tion naturally identifies more evolved structures at ear-lier times. This is clearly opposite to the type of selectionone would prefer to use to study cluster, galaxy or AGNevolution. Moreover, this mismatch can result in com-parisons that can obscure any true evolution, since moremassive clusters are placed at the beginning of the evo- Hart, Stocke & Hallmanlutionary sequence and less massive objects at the end.In this paper, we choose galaxy clusters by using anew selection method that avoids the difficulty just de-scribed. In this summary of first results, we use this sam-ple to investigate the nature of radio galaxies and X-raypoint sources (XPSs) in clusters at moderate- z (0.2–0.4).For now, we avoid using the term X-ray AGN until theAGN nature of the XPSs is proven (or disproven) by ourobservations presented below. Previous studies simplyassumed that these X-ray emitters are AGN (e.g., the“optically-dull X-ray AGN”; Martini et al. 2002, 2006;Eastman et al. 2007). The AGN nature of these XPSshas not been carefully scrutinized before and their X-rayand radio properties could have a significant impact onmodels for AGN heating in clusters. We address eachof these points in detail in this paper. We describe oursample selection in § §
3. In § § § § = 70 km/sec, Ω Λ = 0 .
70, andΩ M = 0 . THE “ROAD TO COMA” SAMPLEGuided by hydrodynamical simulations to track thegrowth of massive cluster halos (
M > M ⊙ ) from z ∼ . < z < . ± T X (z)/T X (z=0) for individual massive cluster halosin the simulation are normalized by the Coma Cluster’spresent temperature to illustrate the variance of clusterproperties. Figure 1 displays the median temperatureof these simulated Coma Cluster progenitors as a func-tion of redshift (solid line) and the 25th and 75th per-centiles about this distribution (dashed lines). At z=0.3,the ICM temperature of a Coma Cluster progenitor ispredicted to be 7.0 ± ± Chandra archives with adequate exposure time to detect X-raysources with L . − . keV ∼ × ergs s − . This lu-minosity is well above the expected emission from low-mass X-ray binaries in ellipticals (L X < × ergs s − ;Kim & Fabbiano 2004) and high-mass X-ray binaries instarburst galaxies (L X < × ergs s − ; Grimm et al. 2003). Also, for this redshift range, a 1 Mpc radius regionfalls on one ACIS chip and so has been selected consis-tently as our survey radius centered on the peak of thediffuse X-ray emission. We also require radio observa-tions to detect radio galaxies with P . GHz ≥ × W Hz − across the entire survey. Along with our well-defined cluster sample, our multi-wavelength approach toidentify cluster AGN differs from many previous studies(e.g., Ledlow & Owen 1995; Martini et al. 2007) whichuse X-ray or radio observations, but not both, to de-tect these objects in only one waveband. Table 1 listsour eleven z =0.2-0.4 “Road to Coma” clusters, their red-shifts, Chandra observational details (ObsID, ACIS aim-point, exposure time, ICM temperature, flux and lumi-nosity limits), as well as VLA 1.4 GHz radio details. Thefollowing section details the X-ray, radio and optical ob-servations and data analysis. MULTI-WAVELENGTH DATA ANALYSIS3.1.
X-ray Imaging Spectroscopy
We re-processed the
Chandra archival observations us-ing CIAO v3.3 and CALDB v2.2. Following the typicalACIS data preparation pipeline, event-1 files were repro-cessed to remove bad pixels, afterglow pixels, and streak-ing patterns. The newest calibration files for chargetransfer inefficiency and time-dependent gain were ap-plied, and then finally the event files were filtered onstatus and grade. Flaring events were identified by ex-tracting the 0.3-12 keV count rate on either the ACIS-S1or ACIS-I1 chips and we eliminated time periods withrates > σ above the observed mean.3.1.1. Cluster ICM Temperature
Cluster ICM spectra were extracted from within a 1Mpc projected radius after obvious point sources wereremoved (point source identification is described below).If a background area could not be extracted on the samechip as the cluster emission, we extracted backgroundspectra from re-projected blank sky observations. Obvi-ous cool-core regions were excised prior to spectral mod-eling. Source spectra were binned to a minimum of 20cts/bin, then modeled using
XSPEC (Arnaud 1996) andMEKAL models with foreground extinction held fixed atthe Galactic hydrogen column at the cluster coordinates(Dickey & Lockman 1990) and a constant 0.3 Solar metalabundance.The lower redshift limit of our cluster sample is lim-ited by the size of one ACIS chip, allowing a ∼ ∼ . However, our simulated clustertemperatures were extracted from within R . Basedon the similarity in the temperature profiles of manyclusters for r > , in both cool-core and non-coolcores (e.g., De Grandi & Molendi 2002; Vikhlinin et al.2005; Baldi et al. 2007; Pratt et al. 2007), we expect theICM temperature profile to be decreasing beyond 0.5R . Thus, our temperature estimates extracted withina 1 Mpc projected radius will slightly overestimate thefull cluster temperature by ∼ x values listed incolumn 6 of Table 1 need to be decreased by 10–20%to be accurately compared to the simulated Coma Clus-PSs and Radio Galaxies in Clusters of Galaxies 3ter progenitor temperatures mentioned in the previoussection. 3.1.2. X-ray Point Sources (XPSs)
XPSs were identified using the CIAO tool wavdetect with a threshold limit set to 10 − which corresponds toone false source detection per 10 pixels (roughly thenumber of pixels in an unbinned image of one ACIS chip).Each potential source above this threshold was scruti-nized individually. Broadband (0.3-8.0 keV) counts wereextracted within a 95% encircled energy radius estimatedfor a monochromatic energy source of 1.5 keV at the ob-served off-axis angle. Background annuli regions were2.0 times the extraction radius. This background sub-traction does not account for the slow change of T x withradius within a cluster. However, a more sophisticatedbackground subtraction using annuli at the XPS clus-ter radius show resulting differences less than the quotedphoton statisticsConversion from net counts to flux was estimated forthe XPSs using XSPEC assuming a power-law spec-trum with photon index of Γ=1.7 (N E ∝ E − Γ ). Toestimate the X-ray flux limit for each observation (Col-umn 7 of Table 1), we estimated the point source fluxfor a 3 σ detection above the background noise nearthe survey edge (R = 1 Mpc). Abell 2111 has a for-mal 3 σ detection limit of L . − . keV ∼ . × ergs s − , slightly greater than our stated survey thresholdof L . − . keV ≥ ergs s − . However, we feel com-fortable leaving this cluster in our sample because theCIAO wavdetect routine identified a point source with L . − . keV ∼ × ergs s − if the source is locatedat the cluster redshift. Poissonian errors were calculatedusing the Gehrels (1986) approximation. X-ray sourceswith SNR > L . − . keV ≥ × ergs s − are catalogued as po-tential cluster AGN candidates. We note that standarddetection methods described above may miss XPSs asso-ciated with the BCG due to misidentifying AGN emis-sion as unresolved “cool core” ICM emission; i.e., thecusp of the ICM emission can mask the presence of anAGN. Therefore, we can make no solid claims about X-ray AGN in the bulk of the individual BCGs (see § Radio Imaging
Several of our low- z clusters fields have been surveyedby the Faint Images of the Radio Sky at 20cm (FIRST;Becker et al. 1995) and/or the NRAO VLA Sky Survey(NVSS; Condon et al. 1998) to detect radio galaxieswith P . GHz ≥ × Wm − . For our redshift range,the resolution of the FIRST maps may underestimatethe total radio power of galaxies if any extended fluxis close to the noise limit of the maps. Conversely, tworadio sources with small angular separation may be cata-logued as one object by FIRST. To mitigate these issues,we obtained additional 1.4 GHz VLA maps (Program IDAS873) and re-analyzed 1.4 GHz VLA archival observa-tions obtained in its A-configuration.In September/December 2006 we obtained VLA con-tinuum observations at 1.4 GHz (50 MHz bandwidth)for four clusters (MS 0440.5+0204, Abell 2111, MS1455.0+2232, and Abell 1995). Abell 1995 was observed for 75 minutes in B-array, while the remaining three low-z clusters were observed for 30 minutes in C-array. Theduration of a typical target scan was 15 minutes, brack-eted by 1 minute scans of a nearby phase calibrator. Aflux calibrator (e.g. 3C286) was observed for 3 minutesat the beginning and end of each observing session.We used the NRAO Astronomical Imaging ProcessingSystem (AIPS), version 31DEC07, in the usual mannerto flag, calibrate, transform and clean the images, so thatradio sources could be detected manually. Source fluxdensities are estimated with the AIPS task TVWIN andIMEAN. The typical 3 σ limit of our observations andFIRST is 0.3–0.5 mJy and 0.4 mJy, respectively. ForAbell 963, the 3 σ limit of the FIRST image is higher( ∼ . GHz ∼ ν ∝ ν − α and α =0.7). These limits allow de-tection of many lower radio power FR 1 sources whileexcluding the lower luminosity radio sources due to starformation (L . GHz < . W Hz − ; Morrison et al.2003). Given the very low radio power limits in col-umn 11, Table 1, we do not expect that we have missedsources due to the presence of strong point sources in ornear the primary beam, partially resolving the source orto beam-smearing in the outer regions of the fields, ex-cepting MS1358.4+6245. Due to a strong, nearby radiosource in this field, Stocke et al. (1999) only detected theBCG at 1.4 GHz. This 1.4 GHz map has a larger radiopower limit (P . GHz ≥ . × W Hz − ) for potentialcluster radio galaxies than the other clusters in our sam-ple. However, in a 5 GHz map Stocke et al. (1999) didnot detect any additional sources within 5’ of the BCGdown to a 3 σ limit of 0.2 mJy, which corresponds to 1.4GHz flux limit of 0.5 mJy and P . GHz ≥ . × WHz − . Thus, we include MS1358.4+6245 in our clustersample with only one detected radio source in the BCG.3.3. Optical Datasets
Multi-band Imaging
Two-color images and photometry are publicly avail-able from the the Sloan Digital Sky Survey Data Re-lease 6 (SDSS DR6; Adelman-McCarthy et al. 2008), theCanadian Network for Observational Cosmology images(CNOC; Yee et al. 1996), and/or the
Chandra
Multi-wavelength Project (ChaMP; Green et al. 2004) for 10of our 11 clusters in this paper.For 6 of our 11 clusters, SDSS DR6 provides five colorimaging ( ugriz ) and photometry. The SDSS limitingmagnitude in ( g,r,i ) is (22.2,22.2,21.3), adequately deepto detect M > M ∗ r cluster galaxies at z=0.4. SDSS pho- Hart, Stocke & Hallmantometric uncertainty is 1% (Adelman-McCarthy et al.2008). The CNOC cluster redshift survey examined 16X-ray luminous clusters with 0.2 < z < < ∗ r +1 with anindividual source magnitude uncertainty of ∼ >
100 archival
Chandra observations.For MS 2137.3-2353, we use ChaMP’s multi-bandoptical images (Sloan g,r,i g,r,i ) images ofAbell 370 with SPIcam on the Astrophysical ResearchConsortium (ARC) 3.5m Telescope at Apache Point Ob-servatory (APO). The FOV of SPIcam is 4.8 arcmin ;therefore, we required multiple pointings of Abell 370to image the entire 1 Mpc radius region (FOV ∼ ). We used NOAO’s Image Reduction and Anal-ysis Facility (IRAF), v2.14.1, in the usual manner to biassubtract and flat field individual images. Images werealigned and stacked into a large mosaic image with theIRAF package MSCRED. The limiting r-band magni-tude ( ∼ ∗ r +1 with an individualsource magnitude uncertainty of ∼ Spectroscopy
SDSS and/or CNOC spectroscopic data are availablefor several candidate cluster XPSs and radio galaxies.The SDSS spectral coverage is 3800-9200 ˚A with a res-olution of 1800-2200. The SDSS spectroscopic pipeline(spectro1D) cross-correlates individual wavelength andflux-calibrated spectra with various template spectra(stellar, quasar, and emission-line galaxies) to determinesource redshifts via emission lines and/or absorption fea-tures (see Stoughton et al. 2002 for details). The CNOCspectral coverage is 4650-6100 ˚A with a dispersion of ∼ ′′ slit width. The CNOC survey also incor-porates cross-correlation techniques to determine sourceredshifts that have velocity uncertainties between 100–130 km s − (see Yee et al. 1996 for details). In addi-tion to these two surveys, we searched the literature (viaNASA’s Astrophysics Data System Bibliographic Ser-vices) and extragalactic databases (e.g. NASA/IPACExtragalactic Database) for published redshifts and/orspectra of our potential cluster targets.For remaining cluster AGN candidates without pub-lished redshift and/or spectra, spectroscopic observationswere obtained for all objects with Sloan r < . ∼ > α ). Theradial velocity accuracy from a typical spectral line is ∼
300 km s − (or ∆z ∼ M r < − . M ∗ r ∼ − . α ,[O III]). We do not find any example of a starburst op-tical spectrum among either the radio galaxies or XPSsas determined by comparisons of strong emission-line lu-minosities (e.g. [O III] 5007 ˚A comparable to H β ; seeStocke et al. 1991). This method is indicative, and not asrobust as “BPT diagnostic plots” (Baldwin et al. 1981)that utilize several emission-line features to separate star-burst galaxies from AGN.3.3.3. Cluster Red Sequence Galaxies
The color-magnitude diagrams (CMDs) of clusters gen-erally display a tight color sequence for the cluster el-lipticals, referred to as the cluster red sequence (CRS).We estimate the number of CRS galaxies within 1 Mpcof the cluster center and with M r ≤ − . σ of the mean CRS color, we define the CRSmore precisely by fitting a line to the observed color andmagnitude of these sources and thus verifying the well-known ”tilt” of the CRS for cluster color-magnitude dia-grams (e.g., Visvanathan & Sandage 1977). For galaxieswith M r ≤ − .
8, the large majority of blue galaxy con-taminators are foreground galaxies and not the fainterblue cloud galaxies with M r > M ∗ r . The estimated num-ber of CRS galaxies will be used to compute AGN frac-tions described in § RESULTSWithin 1 Mpc of the cluster X-ray emission centroid,we find 8 XPSs with L . − . keV > × ergs s − ,confirmed to be cluster members within the 3 σ velocitydispersion of the cluster (from the literature) and M r < − .
8, with the exception of Abell 963 X2 with M r = − .
3, which we include despite being 0.5 mag fainter.PSs and Radio Galaxies in Clusters of Galaxies 5We also find 20 radio galaxies with P . GHz > × W Hz − and M r < − .
8, of which six are located in theBCG.Tables 2–3 lists the basic data for the XPSs and ra-dio galaxies, respectively, in our sample. The sourcesare identified by cluster name and a consecutive number-ing which includes foreground and background sources.In Table 2 the letter identifiers in parentheses areused in subsequent plots and discussions for easy cross-referencing. Both tables include the spectroscopic red-shift (mostly from this work), observed magnitude of thehost galaxies in which they are found, the projected ra-dial distance from the cluster center as defined by thediffuse ICM X-ray emission peak in h − Mpc (called ”ra-dius”), and observed broadband (0.3-8 keV) X-ray lumi-nosity or limit. Table 2 also includes the net broadband(0.3–8.0 keV) counts for each XPS.Table 3 also includes the radio source flux density at1.4 GHz and K-corrected 1.4 GHz radio luminosity. Hostgalaxy colors and cluster “red sequence” (CRS) colors(see § g-r ) for z < r-i ) for z =0.3–0.4 clusters (photometric cal-ibration errors are typically 2% in these colors). For the3 EMSS clusters, CNOC photometry provides Gunn (g-r) colors. We note that MS 0440.5+0204 R3 & R4 andAbell 370 R1 & R3 are examples of two distinct sourcesdetected in the higher resolution VLA maps, but wereidentified as a single source in FIRST images.For radio galaxies with detectable X-ray emission, theX-ray luminosities (Table 3, column 12) were determinedin the same manner as for the detected XPSs described in § σ above the ICM emission attheir locations. For Abell 1758, the ICM X-ray emis-sion is both diffuse, allowing a good estimate for theupper limit on the X-ray luminosity of the BCG, anddouble-peaked, indicative of a merging system. In thiscase we have determined the projected distance of theradio galaxies to be from the nearest diffuse ICM emis-sion center. Thus, A1758-R4 is identified with the BCGin the NW X-ray clump and A1758-R6 is 2/3 Mpc fromthe other clump center to the SE. Otherwise, there areno ambiguous cases in the projected distance (in Mpc)from the center of the ICM X-ray emission. The radioupper limits for the XPS locations are set at 3 σ abovethe background noise and so are conservative.Due to the difficulty of detecting XPS at the centerof the diffuse X-ray emission, we probably have under-estimated the number of cluster XPSs. We did detectone BCG in both radio and X-rays, Abell 1758 R4/X1 –listed in Tables 2–3, where the ICM emission is particu-larly diffuse. Since 6 of 11 BCGs were detected as radiogalaxies (refer to Table 3), we might expect that a simi-lar fraction of BCGs are XPSs. However, since our radioand X-ray detection limits allow the detection of onlyhalf the total number of XPSs as radio galaxies (refer toTable 2) we conservatively estimate that ∼ L . − . keV > ergs s − in our sample, oneof which we have already identified (A1758 R4/X1).Figure 2 displays a composite color-magnitude dia-gram for our eleven low- z clusters. All but one ra-dio galaxy lies on the CRS. The exception is the BCG in MS 1455.0+2232 for which extremely luminous, ex-tended [O II] 3727 ˚A emission is present in the g-band,making it appear to be somewhat bluer than the CRS(Donahue et al. 1992). Also excepting one case (Abell963 X1, object A in Figure 2), XPSs are hosted by early-type galaxies on or near the CRS. Abell 963 X1 hasan optical spectrum consistent with a Seyfert nucleus(with emission-line luminosity of [OIII] >> H β ). Giventhe presence of only one Seyfert projected within 1 Mpcof a cluster core and ∼
745 km s − relative radial veloc-ity for this Seyfert (Abell 963 cluster velocity dispersionof σ v =1350 km s − ; Struble & Rood 1999), this sourceis consistent with being a cluster member either fore-ground or background to the core at a physical radius > § AGN Fraction and Radial Distribution of ClusterRed Sequence (CRS) Radio Galaxies and X-rayPoint Sources
Table 4 lists our estimates for the number ofbackground-subtracted CRS galaxies for each cluster (see § ∼
665 in our entire cluster sample. The average AGNfraction for CRS galaxies in our sample is 3% for radio-selected sources and 1% for X-ray selected sources within1 Mpc of the cluster center.The cumulative radial distributions of the cluster radiogalaxies and XPSs, as shown in Figure 3, reveal thesesources to be more centrally-concentrated in the clus-ter relative to CRS galaxies as a whole. Eighty percentof radio galaxies and 60% of XPSs are located within500 kpc of the cluster centers and appear more cen-trally concentrated than the CRS galaxies even in thatregion. A luminosity weighting of the CRS distributionusing the bivariate radio luminosity function derived byAuriemma et al. (1977) is indistinguishable from the un-weighted CRS galaxy distribution; i.e., there is no evi-dence for significant mass segregation in these clusters.Thus, the significant difference between the radio galaxy+ XPS population projected radial distances and theweighted CRS galaxy radial distances remains as shownin Figure 3.A two-sided Kolmogorov-Smirnov (K-S) test betweenthe full X-ray + radio population and the CRS galaxypopulation as a whole are inconsistent with being drawnfrom the same parent population at > × − ). The K-S probability is even lower if we onlyexamine the cluster radio sources (K-S D-statistic = 0.44and probability = 4.5 × − ). The K-S probability is no-ticeably larger for just the cluster XPSs (K-S D-statistic= 0.26 and probability = 1.9 × − ), but this compari-son is still significant at the 99.8% confidence level. Toaccount for the likely presence of XPSs in BCGs whichare not detected due to the presence of an ICM “cool Hart, Stocke & Hallmancore” cusp in the diffuse X-ray emission, we add twoXPSs at r=0. The K-S probability for this augmentedXPS distribution (K-S D-statistic = 0.38 and probabil-ity = 7.1 × − ) is comparable to the original X-ray +radio population and only strengthens our conclusion ofa centrally concentrated population of XPSs.Within 500 kpc 16 radio galaxies in our sample makeup 6 ±
2% of the bright (L & L ∗ ) ∼
265 CRS galaxies(40% of 665), while the 4 XPSs (excluding Abell 963X1) make up 2 ± ±
2% of the bright (L & L ∗ ) ∼
265 CRSgalaxies. Our result strongly suggests that the triggeringof a radio-loud AGN or an XPS in these cluster galaxiesis due to some, as yet undetermined, mechanism relatedto the ICM density such as in the Bondi accretion modelof Allen et al. (2006).4.2.
X-ray Properties
Of the 8 detected XPSs (Table 2), we detect 7 XPSswith 25 −
115 net broadband (0.3-8.0 keV) counts andone XPS (Abell 963 X1) with ∼ r ∼ -22.8), blue (almost 1 magni-tude bluer than the CRS) galaxy in Figure 2 (Object A)and possesses a typical Seyfert optical spectrum. Using XSPEC , the spectrum of Abell 963 X1 is well-modeled bya power-law with spectral index Γ = 2 . ± .
1, emissionfrom the 6.4 keV Fe K- α line with an equivalent width of1.7 keV, and no intrinsic absorption. But this spectrumalso can be fit by a Raymond-Smith thermal model withkT = 3.4 +0 . − . keV, but at a higher reduced- χ value (1.18vs. 1.03). Given the clear Seyfert-like optical spectrumand likely power-law X-ray spectrum we identify Abell963 X1 as a clear AGN.But due to low photon counts in the other 7 XPSs,we instead compute X-ray colors to estimate their spec-tral slope and amount of intrinsic absorption. Figure 4displays the ratio of two X-ray colors. The three bands(S1, S2, and H) are defined for similar energy rangesas in Kim et al. (2004) and Martini et al. (2006). Over-laid are the expected colors for a power-law spectrumwith spectral slopes of Γ = 1 . − . n H = 0 , , , cm − . Each individual sourceis displayed and labeled as in Figure 2 and in Table 2;the composite X-ray color (filled square) is also displayedusing all the XPSs without detectable radio emission andthe XPSs with detectable radio emission (filled triangle),excepting Abell 963 X1 (Object A). With the exceptionof MS0440 X8 (Object G) and RXJ0952 X6 (Object H),the XPSs are consistent with a typical AGN photon in-dex of Γ ∼ .
75 (Tozzi et al. 2006). MS0440 X8 hasrelatively few hard (2.5-8.0 keV) counts and has consis-tent colors for source with Γ ∼
3, but as with the others,little evidence for obscuration in the X-rays. RXJ0952X6 has approximately equal counts in the soft (0.5-2.5keV) and hard (2.5-8.0 keV) bands and has consistentcolors for a source with Γ ∼ . ± . unobscured AGN. There is no evidence forX-ray spectral differences for XPSs with and without ra-dio detections, and, although the statistics are modest, this evidence also weighs in favor of all of these XPSsbeing AGN of some sort.4.3.
X-ray to Radio Luminosity Ratios
We find that XPSs and radio galaxies rarely overlapin our sample. There are only 3 XPSs which are radio-detected, suggesting two populations. Are we really see-ing two populations of sources or is this just a functionof our flux limits? To investigate this further, we deter-mined the X-ray to Radio Luminosity ratios for our clus-ter sources (Figure 5) and compared them to optical el-liptical “core” galaxies in Balmaverde & Capetti (2006),more luminous 3C/FR 1 radio galaxies in Donato et al.(2004) and Seyferts hosted by early-type galaxies inCapetti & Balmaverde (2007). A loose linear relation-ship exists in Figure 5 for the low (optical “core”galaxies) and higher power (FR 1s) ellipticals of theBalmaverde & Capetti (2006) and Donato et al. (2004)samples, respectively. The X-ray limits on the radiogalaxies in our sample are consistent with other FR 1s;i.e., we would require perhaps a factor of ∼ if they were similar to FR 1radio galaxies; indeed, they should have been detected atpower levels comparable to or significantly higher thanthe radio galaxy detections we did make. So, we can ruleout that the radio galaxies and the XPSs are drawn fromthe same parent population. But the XPSs do have X-ray luminosities and radio luminosity limits that placethem among the Seyferts in Figure 5. If these XPSsare some sort of Seyfert galaxy, an order of magnitudeimprovement in our radio flux limit probably would berequired to detect all of them. Indeed, if the XPSs are allSeyferts, it is somewhat surprising that we did not detectsome of them already given the distribution of Seyfertsin Figure 5.Are the XPSs simply radio-quiet AGN, similar toSeyfert galaxies? While they possess X-ray luminositiesof Seyferts, they do not possess the emission-line spectraof Seyferts nor is there unambiguous evidence for ob-scuration. Even if we use the X-ray to H α flux ratios(mean value = 7.3) of much lower luminosity AGN likein the nearby sample of Ho (2008), we would expect H α and other emission line fluxes for our sample of XPSs atL Hα > ergs s − , which we easily exclude using our3.5m spectra. We have also gone through the exercise ofdetermining if the Seyfert spectrum (i.e., blue power-lawand luminous emission lines of [ OIII ] and H α ) of Abell963 X1 would be detectable if scaled down by the dif-ference in X-ray luminosity between this sources and theothers (using standard scaling relations observed for X-ray emitting Seyferts). We find that a Seyfert-like AGNof power 10% of Abell 963 X1 would be detected eas-ily in our optical spectra and a 3% power-level probablywould be detected as well. But it might be possible to“hide” a Seyfert 2 type spectrum (line luminosities canbe an order of magnitude lower than many Seyfert 1s),which leads to the suggestion that these could be highly-obscured AGN. However, as we showed above, there isno strong evidence for obscuration in these XPSs basedon their X-ray spectra. Nor are their optical broad-bandcolors significantly redder than the CRS although a lowPSs and Radio Galaxies in Clusters of Galaxies 7luminosity AGN with M r ∼ −
18 could be hidden in theseluminous galaxies without affecting the broad-band col-ors within our photometric errors. We conclude that theXPSs are unlikely to be similar to Seyferts or that theyare highly-obscured AGN of any description.4.4.
Comparison to Other Cluster AGN Studies
How do our results compare to other cluster AGNstudies? Cluster radio galaxies have been known to re-side in passive galaxies (Morrison et al. 2003), thus ourfindings on their host galaxies are not surprising. Sta-tistical studies of cluster radio galaxies have also re-vealed enhanced concentration towards cluster centers(e.g., Ledlow & Owen 1995; Lin & Mohr 2007) similarto what we have found.Ledlow & Owen (1995) surveyed ∼
300 low-z (z < ∼ . GHZ ) > ∼ ∼ . GHZ ) > W Hz − ) within 5r to be morecentrally concentrated than Two Micron All-Sky Sur-vey (2MASS; Jarrett et al. 2000) K-band galaxies withM K < -24. They estimate the radio-active fraction (RAF)of cluster and field galaxies with P . GHZ > W Hz − and M K < -24 to be 4.9 ± ± ∗ r ∼ − . ∼ ∗ K ∼ − .
7. These authors surveyed a larger regionaround clusters (R ∼ r ) that could account for thesmaller cluster galaxy RAF compared to our value. Also,there is both a luminosity and a Hubble type bias to thisdifference since many field galaxies are late-type systemswhich do not harbor radio galaxies.Studies of X-ray point sources in the fields of smallsamples of galaxy clusters (e.g., Cappi et al. 2001;Branchesi et al. 2007) revealed a slight excess ∼ σ inthe expected surface density compared to cluster-freeregions. Ruderman & Ebeling (2005) surveyed 51 clus-ters with 0.3 < z < . − keV > ergs s − andfound a large excess in the composite surface densityof XPSs within 3.5 Mpc of the cluster center com-pared to their 20 control fields. In relaxed clusters,i.e. clusters with smooth, symmetric X-ray emissionprofiles, Ruderman & Ebeling (2005) find a prominentexcess within 0.5 Mpc, whereas clusters with disturbedX-ray morphologies appear to have their XPS excess dis-tributed more uniformly within the larger 3.5 Mpc sur-vey region. Gilmour et al. (2009) analyzed 148 galaxyclusters with 0.1 < z < σ excess in XPS counts compared to 44control fields, resulting in ∼ < z < ∗ R , their R-band images show that most of theirX-ray point sources are hosted by galaxies with colors(see Martini et al. 2007, Fig. 3) consistent with (B-R)=1.57 ± X > and located within 1 Mpc ofthe cluster cores), their numbers are small. If we assumethat the XPSs in our sample are all AGN, then our X-ray AGN fraction of ∼
2% is consistent with Martini et al.(2006) for L . − . keV > ergs s − . It is interestingto note that although we have a more robust selectionmethod for our low- z cluster sample, the radio galaxyAGN fraction of Ledlow & Owen (1995) and XPS AGNfraction of Martini et al. (2007) are similar to our values. THE NATURE OF CLUSTER X-RAY POINTSOURCES (XPSS)As stated in the previous section, our cluster XPSsdo not appear to have optical nor X-ray signatures typ-ical of Seyferts. However, the XPSs have many of theproperties of low luminosity BL Lac Objects of theclass now called “High-energy-Peak” BL Lacs (HBL;Padovani & Giommi 1995; first called X-ray selected BLLac Objects; Stocke et al. 1985). With X-ray luminosi-ties ∼ ergs s − an HBL would be expected to havea radio luminosity of 10 − W Hz − based upon HBLsat higher total luminosities (e.g., Perlman et al. 1996),at or below our radio detection limits for this sample.Indeed, 3 of these XPSs are radio-detected. Most HBLspreviously studied have X-ray and radio luminosities 2–3orders of magnitude more luminous than our detectionsand still only sometimes exhibit the featureless power-law optical spectrum of classical BL Lacs (Rector et al.1999). Mostly, HBLs have optical spectra of passive ellip-tical galaxies with emission line luminosities < ergss − , consistent with what we find for the cluster XPSs.Rector et al. (1999) investigated the general proper-ties of low-luminosity HBLs, finding that their propertiesmerge with normal ellipticals, with normal (i.e., ∼ r = -22 to-25) elliptical galaxies (Wurtz et al. 1997; Falomo et al.1999); and in rich groups to moderately rich clustersat similar redshifts to our sample (Wurtz et al. 1997).HBLs show no evidence for internal X-ray obscuration Hart, Stocke & Hallmanand some show quite soft X-ray spectra like MS0440 X8(Perlman et al. 1996).The detailed optical properties of Abell 963 X2 (sourceD in Figures 2, 4, & 5) also support the HBL classifica-tion of the XPSs. Figure 2 shows that this XPS hostgalaxy is ∼ ± x =10 . ergs s − (althoughtheoretical luminosity functions of beamed sources allhave a flattening at low power levels; Urry & Shafer1984) and so numerous lower power HBLs would be ex-pected in bright cluster ellipticals. In Figure 5, except-ing the one Seyfert (Object A), the other XPSs are 1.5–2orders of magnitude more luminous in X-rays than thesequence of FR 1s and compact core galaxies. Thus,the approximate “excess L x ” of the XPSs in our sampleabove the FR 1 sequence suggests a Doppler-boosting ofX-rays by factors of 30–100. Taking the simplified modelof HBLs described in Urry & Padovani (1995), our line-of-sight is located well off the radio beaming axis (thusno beamed radio emission is observed) but within the γ ≥ δ ∼ γ and theratio of observed-to-intrinsic X-ray luminosity is δ p wherep=3+ α for discrete emitting “blobs” and p=2+ α for acontinuously emitting structure (where α is the intrin-sic spectral index in energy units for the source; here weconservatively assume α = 1). In the simplifying assump-tions of the HBL model we obtain a maximum luminosityboosting of 200-1300 depending upon the physical struc-ture of the emitting region (or even higher if Γ is greaterthan as estimated from the inferred opening angles of25–60 degrees based upon source counts; Rector et al.2000). Since the most luminous HBLs have L x ∼ − ergs s − , the unbeamed X-ray emission is estimated tobe 10 − ergs s − , or even lower depending upon how these quantities scale with source luminosity. Thus, ourestimate above that the XPSs are boosted in their X-rayemission by 30–100 times seems plausible.The most luminous HBLs are found in large-solid-angle “serendipitous” X-ray surveys and so are quite rare( ∼ − sources Mpc − at L x ≥ ergs s − fromthe Rector et al. (2000) XLF). A power-law extrapola-tion of that XLF yields ∼ − BL Lacs per Mpc − at L x ≥ ergs s − . Since the elliptical galaxy LFfinds ∼ × − Mpc − for L ≥ L ∗ (Marzke et al. 1994;Im et al. 1996), the expectation from observed space den-sities is ∼ ≥ L ∗ elliptical galaxies. Inthis survey we have observed ∼
665 bright cluster el-lipticals and found 7 XPSs (not include Abell 963 X1)which translates to ∼ Chandra serendipitous sourcesurveys; Barger, Cowie, Mushotzky, & Richards 2001b).A final known category of X-ray emitter into which theXPSs might be placed is an accreted “cool core” thermalcorona in galaxy-sized halos (Sun et al. 2007). ThermalX-ray coronae of early type galaxies have been knownsince the X-ray observations of
Einstein (Forman et al.1985). Examples of this phenomenon have been seenin nearby clusters (e.g., Yamasaki et al. 2002; Sun et al.2005), and not necessarily in the BCG. We expected tospatially resolve this type of emitting region since thenearest XPSs in our sample are at a distances wherethe 1 arcsec
Chandra resolution is ∼ x less thanthe surrounding medium, a power-law fit is better in allindividual cases and composites (see § ≥ ergs s − is a remnant core of agroup BCG that is in the process of falling into the clus-ter. But none of the XPSs we detect (except one clusterBCG which also has radio emission) is in a very brightCRS galaxy. We conclude that it is unlikely that thesesources are remnant cool cores and that the best modelfor XPSs is HBL-type BL Lac Objects.PSs and Radio Galaxies in Clusters of Galaxies 9 DISCUSSIONBased upon the observed properties of the 20 radiogalaxies and the 7 XPSs (total of 24 AGN countingoverlaps and excluding Abell 963 X1) in these 11 low- z clusters, we conclude that all these sources are radio-loud AGN. The radio galaxies have properties consistentwith high luminosity FR 1 sources while the XPSs with-out radio emission are most consistent with being lowpower FR 1s with X-ray emission that has been Doppler-boosted above our detection threshold. Thus, there is ob-servational evidence that both AGN classes (X-ray andradio selected) possess relativistic jets that can heat thesurrounding ICM through jet interaction.Since both classes have luminosity function determina-tions from previous work, we can extrapolate downwardin both X-ray and radio flux below our threshold usingprevious data. We have normalized these extrapolationsusing our own sample of ∼
265 L > L ∗ cluster ellipticalsat projected radial distances <
500 kpc from the clusterICM X-ray emission centroid, of which 6% (16 objects)are radio galaxies at L . GHz > × W Hz − and 1%(3, excluding A963 X1) are XPSs at L x > ergs s − .Based on Figure 4 of Machalski & Godlowski (2000),we integrate the AGN radio luminosity function (RLF)for log(P) > < log(P) < ∼
210 low power radio sources). Based upon thesevalues, we predict that ∼
85% of all bright CRS galax-ies within 500 kpc of cluster center possess radio sourceswith L . GHz > . W Hz − . The XLF of HBLs pre-dicts even a higher number, all (100%) bright clusterellipticals should possess an XPS at L x ≥ ergs s − .But the XLF of Morris et al. (1991) and Rector et al.(2000) extends at constant logarithmic slope down onlyto 10 . ergs s − . Since flattening of this function is ex-pected at or below 10 ergs s − (Urry & Shafer 1984),the number of predicted HBLs would be somewhat lessthan 100% and so in good agreement with the extrapo-lation of the RLF. Thus, the two extrapolations roughlyagree in predicting that almost all bright CRS galaxies( ≥ L ∗ ) at projected radial distance <
500 kpc containradio-loud and X-ray loud AGN at L . GHz > . WHz − and L x ≥ ergs s − , respectively (see Fig. 5).This agreement is additional evidence that our identifi-cation of the XPSs as low luminosity BL Lacs is correct.This result suggests that deeper X-ray surveys of nearbyclusters will discover large numbers of XPSs in bright el-lipticals and very deep Chandra imaging of the PerseusCluster appears to show just that (Santra et al. 2007).What impact do all of these AGN have on cluster heat-ing models? They certainly can provide a distributedsource of heat which moves about the inner 0.5 Mpc ormore of these clusters on a characteristic crossing timeof a few × yrs. But is their input miniscule com-pared with the AGN in the BCG? To evaluate the heatingby these AGN we have assumed results from two recentstudies of the relationship between radio luminosity andjet power by Bicknell (1995) and by Bˆırzan et al. (2004,2008). First we assume that the magnetic field strengthin the inner regions of these clusters is relatively constantso that the ICM heating rate is proportional only to theAGN radio luminosity (i.e., a constant efficiency factor for heating the ICM; Bicknell 1995). This assumptionallows us to use the observed RLF to predict that allcluster AGN below L . GHz ≤ × W Hz − accountfor ≤
10% of cluster heating; whereas the brightest twoor three sources (including the BCG) contribute ≥ jet ∝ L . − . r . The largest uncertainty in these calculationsappears to be due to source aging; the correlation forthe youngest sources is at the steep end of the range(Bˆırzan et al. 2008). Therefore, statistical results ob-tained from the RLF is more robust than a “snapshot” ofa single cluster’s AGN population at any given time. Forexample, while our modest-sized sample provides a meanradio galaxy census of ∼ . GHz ≥ × W Hz − , the spread in relative radio luminositiesand thus ICM heating in these clusters is large. For ex-ample, 5 of these clusters have their total radio luminos-ity dominated by a BCG radio galaxy, 5 do not and onehas no radio galaxy above our flux limit at all. Neverthe-less, if the Bˆırzan et al. (2008) scaling is correct, weakerradio galaxies are proportionally more important thanpreviously believed in heating the ICM. If P jet ∝ L . r ,sources weaker than 3 × W Hz − account for morethan half ( ∼ ∼ CONCLUSIONWe have presented a summary of first results from alarge survey of X-ray and radio-selected AGN in clustersof galaxies. In the 11 clusters we chose as potential pro-genitors to the present-day Coma cluster, we find 20 ra-dio galaxies and 8 X-ray point sources (XPSs) with 75%of these AGN centrally concentrated within 500 kpc pro-jected radius. This central concentration of the AGN issignificantly different ( > > L ∗ ) cluster redsequence (CRS) galaxies. This extreme central concen-tration strongly suggests that an AGN triggering mecha-nism similar to the Bondi accretion model of Allen et al.0 Hart, Stocke & Hallman(2006) is operating in these clusters. However, since mostof these AGN are moving supersonically with respect tothe cluster ICM, Hoyle-Littleton accretion must be op-erable, making any gas accretion onto a supermassiveBlack Hole extremely inefficient. Therefore, other mod-els for triggering and powering these radio-loud AGNshould be considered (e.g., extracting Black Hole spin;Blandford & Znajek 1977).Except for 3 X-ray/radio sources, cluster radio galax-ies and XPSs generally are not coincident within our ob-served luminosity limits of L . GHz ≤ × W Hz − and L . − . keV ≥ ergs s − , respectively. While ra-dio galaxies are certainly associated with AGN activity,we questioned prima facie whether cluster XPSs in CRSellipticals are truly AGN. These X-ray sources show nostrong evidence for obscuration and are hosted by lumi-nous red galaxies with no evidence for typical Seyfert-likeemission signatures. But non-AGN models for clusterXPSs (e.g., “cool core” emission in luminous galaxy nu-clei) seem quite implausible, with these XPSs being 1–2orders of magnitude brighter than expected in simula-tions and as observed in very nearby clusters like Virgo(Sun et al. 2007).These XPSs are not spatially resolved in the Chandra images and have X-ray spectra which individually or incomposite are better fit as power-law sources, not ther-mal sources. We conclude that the most viable modelfor the XPSs is that they are the beamed X-ray emis-sion from low-luminosity BL Lac Objects, similar to thehigh-energy-peaked BL Lacs (HBLs) studied in detailby Stocke et al. (1985), Padovani & Giommi (1995), andRector et al. (1999). HBLs are found in luminous, pas-sive ellipticals, have steep X-ray spectra with no evi-dence for internal absorption, optical spectra which inmany cases are indistinguishable from normal giant el-lipticals and are found in rich groups and moderatelyrich clusters at similar redshifts to those observed here(see Urry & Padovani 1995 for a review of their proper-ties). The presence of excess blue light (blueward of theCa II break) when compared to CRS elliptical galaxiesin one XPS (Abell 963 X2) supports the HBL classifica-tion of these sources; a B -band polarization detection inthis object would solidify this assertion. One other XPSswith a smaller amount of excess blue light also is presentin our sample.X-ray sources with similar properties have been termedX-ray emitting, passive elliptical galaxies or “optically-dull AGN” in other contexts (e.g., Barger et al. 2001b),and so we suggest based on our current observations thatthese sources are low luminosity HBLs as well. Moreaccurate colors and calibrated spectra can be used totest this hypothesis by searching for the excess blue lightwe have found in at least one of our sample members.Therefore, both the XPSs and the radio sources are mostreadily identified as radio-loud AGN with jets which cantransfer heat into the surrounding ICM. Extrapolationsof the radio galaxy population in these clusters using theobserved RLF of AGN (Machalski & Godlowski 2000)and the XPS population using the observed XLF of HBLs(Rector et al. 2000) converge to predict that 85-100% ofall bright ellipticals within projected radii of 500 kpcfrom rich cluster centers will be detected by deep radiomaps (L . GHz ≥ . W Hz − ) and X-ray imaging (detection limits of L . − . keV ≥ ergs s − ). Thereis already a hint that this prediction is correct in thatSantra et al. (2007) have found that all bright ellipti-cals in the very core of the Perseus Cluster have XPSs(L . − . keV = 10 − ergs s − ) based on a very deepX-ray image of the core of that cluster.These results have an impact on models for ICM heat-ing. Based upon the RLF for AGN and the jet power vs.radio luminosity scaling law of Bicknell (1995), we expectthat the few brightest radio galaxies (including the ra-dio source associated with the BCG) contribute ≥
90% ofthe heating while the remainder at L . GHz ≤ × WHz − contribute ≤ ≥
55% of the heat input. Due to their relative motion, thenon-BCG AGN heat the ICM more uniformly and wouldnot be expected to possess obvious X-ray “bubbles” inthe ICM associated with their jets due to their peculiarvelocities. It seems likely that these ideas can be used totest whether AGN are responsible for heating the clus-ter ICM as has been proposed (Ruszkowski et al. 2004).Putting together two of our results, (1) radio galaxiesare more centrally-concentrated than CRS galaxies as awhole and (2) large numbers of low power radio galax-ies are responsible for most of the ICM heating, suggeststhat a feedback mechanism by which the density of theICM triggers the heat sources to offset cooling may beoperable in rich clusters of galaxies. We might also ex-pect that those clusters whose total radio luminositiesare dominated by radio sources in the BCG might pos-sess a different temperature profile or detailed tempera-ture/density signatures than clusters dominated by non-BCG radio galaxies. The existence of these signaturesand what these signatures might be will be addressed ina future publication (Hallman et al. 2009).QNH acknowledges the support from the
Chandra
Chandra
Data Archive and software provided by the
Chandra
X-ray Center (CXC) in the application packagesCIAO, ChIPS, and Sherpa.This work made use of images and/or data prod-ucts provided by the
Chandra
Multi-wavelength Project(ChaMP; Green et al. 2004) supported by NASA. Op-tical data for ChaMP are obtained in part through thePSs and Radio Galaxies in Clusters of Galaxies 11National Optical Astronomy Observatory (NOAO), op-erated by the Association of Universities for Research inAstronomy, Inc. (AURA), under cooperative agreementwith the National Science Foundation.This research has made use of the NASA/IPAC Extra-galactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technol-ogy, under contract with the National Aeronautics andSpace Administration, as well as NASA’s AstrophysicsData System.
Facilities:
CXO, ARC (3.5m), VLA, Sloan
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Fig. 1.—
Predicted X-ray temperature (keV) versus redshift for Coma Cluster progenitors. The model curves show results from detailedhydrodynamical simulations (similar to Evrard 1988 and Bialek et al. 2001) of the evolution of clusters which evolve to masses similar to the ComaCluster. Measures of T X (z)/T X (z=0) from the simulations are normalized by the Coma Cluster’s present temperature to illustrate the varianceof cluster properties. The solid line shows the median value for this distribution and the dashed line shows the 25th and 75th percentiles. Elevenclusters are chosen to be a representative sample of Coma Cluster progenitors at moderate redshifts (0.2 < z < § PSs and Radio Galaxies in Clusters of Galaxies 13 -25 -24 -23 -22 -21 -20 -19Rest Frame Magnitude, M r -1.0-0.50.00.5 C o l o r D i ff e r en c e GA DHEF BC
Optical SourcesBCGCluster Radio GalaxyCluster XPS
Fig. 2.—
Composite color-magnitude diagram for eleven cluster fields. For each cluster, the color difference is computed relative to the averagecolor of the cluster red sequence galaxies (see § ∗ = -20.8 is marked by a verticaldashed line. The horizontal dotted lines are representative of the 3 σ spread ( ∼ ∼ -0.03 based on SDSS (g,r) photometry of clusters in our sample, with more luminous CRSgalaxies having redder colors. C u m u l a t i v e F r a c t i on Cluster Red SequenceCluster Radio Galaxies (RG)Cluster XPSsCombined Cluster RG + XPSs
Fig. 3.—
The cumulative fraction of cluster red sequence (CRS) galaxies ( dot-dash line ), cluster radio galaxies ( dotted line ), cluster X-ray pointsources (XPSs) ( dashed line ), and combined cluster radio galaxies and XPSs ( solid line ). The population of cluster radio galaxies and XPSs aremore centrally concentrated than cluster red sequence galaxies with ∼
75% of them located within a 500 kpc projected radius from the cluster X-raycentroid. A Kolmogorov-Smirnov test rules out at the 99.999% level that the radio galaxies and XPSs are drawn from the CRS parent population,suggesting that AGN triggering is connected to the ICM environment.
PSs and Radio Galaxies in Clusters of Galaxies 15 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0C21 = log(S1/S2)-0.50.00.51.0 C = l og ( S / H ) Γ=1.0 Γ=2.0 Γ=3.0 Γ=4.0 x x . x x ADC BGH FE
Cluster XPSComposite XPSs withradio non-detectionComposite XPSs withradio detection S1 = 0.5-0.9 keVS2 = 0.9-2.5 keV H = 2.5-8.0 keV
Fig. 4.—
Rest-frame X-ray colors of cluster X-ray point sources. The colors are defined as follows: S1=0.5-0.9 keV, S2=0.9-2.5 keV, andH=2.5-8.0 keV. Overlaid is a grid of the expected color ratios in this color system for Γ=1-4 and n H =0,0.1,1.0,10 × cm − . The open circles areour detected cluster XPSs. Letter identifiers correspond to the X-ray point sources listed in Table 2. The filled square is the composite color ofthe stacked XPS spectra without radio emission above our limits, excluding the one XPS (Abell 963 X1, Object A) with a typical Seyfert opticalemission line spectrum. The filled triangle is the composite color of the stacked XPS spectra with radio emission above our limits. The X-ray colorsof the stacked XPS spectra are consistent with an unobscured AGN with Γ = 1 .
35 36 37 38 39 40 41 42Log( ν L ) [ergs s -1 ]3839404142434445 Log ( L . - . k e V ) [ e r g s s - ]
19 20 21 22 23 24 25Log(P ) [W Hz -1 ] G EFAB C DHH
Cluster Radio Galaxies (RG)Cluster XPSsCluster X-ray/Radio SourcesFRI (3C/B2)Core GalaxiesPower-Law GalaxiesSeyferts
Fig. 5.—
X-ray (0.3-8.0keV) versus radio (1.4 GHz) luminosity for cluster radio galaxies and X-ray point sources. Cluster sources are com-pared to typical FR 1s (Donato et al. 2004), ellipticals classified as ”core” and ”power-law” galaxies (Balmaverde & Capetti 2006), and Seyferts(Capetti & Balmaverde 2007). The vertical and horizontal dashed lines represent our radio power limit of P . GHz > × W Hz − and X-rayluminosity limit (0.3-8.0 keV) of L X > ergs s − . Cluster radio galaxies are identified with filled triangles (radio-loud BCGs are additionallyidentified with a black circle around the triangle), while cluster X-ray point sources are identified with filled circles . Letter identifiers correspond toX-ray point sources listed in Table 2. Filled squares are sources with both detectable X-ray and radio emission within our defined luminosity limits(with BCGs identified as above). Notice that our X-ray point sources with radio non-detections and our radio galaxies with X-ray non-detectionsdo not overlap in this plot, suggesting two different populations of objects. P S s a nd R a d i o G a l a x i e s i n C l u s t e r s o f G a l a x i e s TABLE 1“Road to Coma” Cluster Sample for z < . z Chandra ACIS Exp. Time T F
X,limit L X,limit
VLA S . GHz,limit P . GHz,limit
CommentsName ObsID chip (ksec) (keV) × − × × (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)MS 0440.5+0204 0.197 4196 S3 38.2 6.1 +0 . − . +0 . − . +0 . − . +0 . − . ∗ AS873 0.3 0.5 FIRSTMS 1455.0+2232 0.260 4192 I3 75.6 4.5 +0 . − . +0 . − . +0 . − . +0 . − . +0 . − . +0 . − . ∗∗ ∗∗ Stocke et al. (1999)Abell 370 0.373 515 S3 59.3 8.5 +0 . − . Note . — Columns: (1) cluster name (2) cluster redshift (3)
Chandra observation ID (4) ACIS aimpoint (5) Exposure time of
Chandra observation after filtering for flaring events(6) ICM temperatures determined by XSPEC modeling of spectra extracted with a 1 Mpc (projected) radius and with cooling cores excised in some clusters. See § − s − for a point source near the edge of our survey region (R = 1 Mpc). See § − for the flux limits in Column 7 (9) VLA program or Reference; AS873 Observations were obtained in B or C-array configuration (10-11) 1.4 GHz Radio flux density in mJyand power limits in W Hz − for a 3 σ detection at the edge of our survey region (12) Additional VLA programs used to identify radio sources * The X-ray luminosity limit in Abell 2111 is slightly greater than our minimum threshold of L X > × ergs s − . See § ** These limits are extrapolated from 5 GHz observations assuming F ν ∝ ν − α and α =0.7. See § H a r t , S t o c k e & H a ll m a n TABLE 2Cluster X-ray Point Sources with L . − . keV > ergs s − Object α δ z r Radius Net Counts F X,observed × − L X,rest × Comments(J2000) (J2000) mag (h − Mpc) (0.3-8.0 keV) (ergs cm − s − ) (ergs s − )(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)(A) A963-X1 10:17:00.8 +39:04:33.0 0.209 a ± ± ± α emission lines(B) MS1008-X3 10:10:35.3 -12:40:22.1 0.3095 b ± ± ± ac ± ± ± a ± ± ± b ± ± ± a ± ± ± b ∗ ± ± ± a ± ± ± Note . — Columns: (1) Cluster X-ray point source IDs, with same letter ID as displayed in Figs. 2, 4 & 5. (2-3) (J2000) RA & DEC of X-ray point source (4) redshift (5) observed Sloanr-band magnitude, except for MS0440-X8 which is Gunn r-band magnitude (6) projected radial distance (Mpc) from the peak in the cluster X-ray emission (7) net counts in 0.3-8 keV bandpass(8) 0.3-8.0 keV Flux (9) 0.3-8.0 keV Rest-frame Luminosity assuming a power-law spectrum with a photon index of 1.7 (N E ∝ E − Γ ). * Gunn r-magnitude a This work b CNOC; Yee et al. (1998) c Barger et al. (2001a) P S s a nd R a d i o G a l a x i e s i n C l u s t e r s o f G a l a x i e s TABLE 3Cluster Radio Galaxies with P . GHz > × W Hz − α δ z r (g-r) (r-i) CRS Radius S . GHz P . GHz × L X × Object (J2000) (J2000) mag color color color Mpc (mJy) (W Hz − ) ergs s − (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)MS0440-R3* 04:43:10.0 +02:10:18.4 0.199 a ∗∗ ± ± ± < a ∗∗ ± ± ± < a ∗∗ ± ± ± ± c ± ± ± < d ± ± ± < c ± ± ± < j ± ± ± ± j ± ± ± < c ± ± ± < c ± ± ± ± d ± ± ± < ce ∗∗ ± ± ± < c ∗∗ ± ± ± < f ∗∗ ± ± ± < c ± ± ± < g ± ± ± < b ± ± ± < ch ± ± ± < c ± ± ± < h ± ± ± < Note . — Columns: (1) Radio Galaxy ID with brightest cluster galaxies (BCGs) denoted by an asterisk. For Abell 2111, no radio galaxy was detectedabove our radio power limit. (2-3) J2000 Coordinates (4) object redshifts (see reference below) (5) apparent Sloan r-band magnitude except for those withdouble asterisks which are Gunn r-band magnitudes (6) Observed color (g-r) or (7) (r-i) (8) Mean color of the cluster red sequence. The mean Sloan colorand 3 σ spread of the cluster red sequence for M ∗ r < − .
8. (9) Projected radial distance from the X-ray peak emission. Note that the X-ray image ofAbell 1758 is a double-peaked in appearance, indicative of a merging system. Therefore, the projected radii values are relative to the NW and SE X-rayemission clumps. (10) observed flux density of the source (11) observed radio power of the source (12) Rest frame X-ray Luminosity (0.3-8 keV) assuminga power-law spectrum with Γ=1.7 (N E ∝ E − Γ ). The BCG X-ray flux limits are calculated assuming a minimum of 3 σ above the diffuse cluster emission.For the remaining radio sources, X-ray flux limits were determined using net counts (observed minus local background) plus 3 σ above the background noise.References for source redshifts: (a)Gioia et al. (1998) (b) CNOC; Yee et al. (1998) (c) This work (d) SDSS DR6 (e) Shectman et al. (1996) (f) Martini et al.(2007) (g) Patel et al. (2000) (h) Mellier et al. (1988) (i) Stocke et al. (1999) (j) CNOC; communication from E. Ellingson * Radio source is hosted in the Brightest Cluster Galaxy ** Gunn r-band magnitude ———————————————-0 Hart, Stocke & Hallman
TABLE 4AGN Fraction for Cluster Red Sequence (CRS) Galaxies
Cluster N CRS N r f R N X f X Color Used Survey(1) (2) (3) (4) (5) (6) (7) (8)MS 0440.5+0204 18 3 a a a a a a Note . — Columns: (1) Cluster name (2) number of CRS galaxies with M ∗ r < − . < § M ∗ r < − . . GHz > × WHz − (4) radio galaxy fraction within our limits (5) number of X-ray point sources with M ∗ r < − . . − keV > ergs s − (6) X-ray point source fraction within ourlimits (7) observed color used for the CRS galaxy estimation (8) source of the imagingdata a The brightest cluster galaxy (BCG) is a radio source.
TABLE 5Comparison of Observed vs. Expected Optical Colors for 2 “Blue” XPSs
Object Alternative ID Color Observed CRS a CommentsAbell 963 X2 Object D (u-g) 0.70 ± ± ± ± > ± ± ± a The expected mean color (SDSS DR6) of a cluster red sequence (CRS) galaxy ofsimilar luminosity to the object and its associated 1 σσ