Herschel photometry of brightest cluster galaxies in cooling flow clusters
A. C. Edge, J. B. R. Oonk, R. Mittal, S. W. Allen, S. A. Baum, H. Boehringer, J. N. Bregman, M. N. Bremer, F. Combes, C. S. Crawford, M. Donahue, E. Egami, A. C. Fabian, G. J. Ferland, S. L. Hamer, N. A. Hatch, W. Jaffe, R. M. Johnstone, B. R. McNamara, C. P. O'Dea, P. Popesso, A. C. Quillen, P. Salome, C. L. Sarazin, G. M. Voit, R. J. Wilman, M. W. Wise
aa r X i v : . [ a s t r o - ph . C O ] M a y Astronomy&Astrophysicsmanuscript no. 14572 c (cid:13)
ESO 2018May 29, 2018 L etter to the E ditor Herschel photometry of brightest cluster galaxies in cooling flowclusters ⋆ A. C. Edge1, J. B. R. Oonk2, R. Mittal3, S. W. Allen4, S. A. Baum3, H. B ¨ohringer5, J. N. Bregman6, M. N. Bremer7,F. Combes8, C. S. Crawford9, M. Donahue10, E. Egami11, A. C. Fabian9, G. J. Ferland12, S. L. Hamer1, N. A.Hatch13, W. Ja ff e2, R. M. Johnstone9, B. R. McNamara14, C. P. O’Dea15, P. Popesso5, A. C. Quillen16, P. Salom´e8,C. L. Sarazin17, G. M. Voit10, R. J. Wilman18, and M. W. Wise19 Institute for Computational Cosmology, Department of Physics, Durham University, Durham, DH1 3LE, UK Leiden Observatory, Leiden University, P.B. 9513, Leiden 2300 RA, The Netherlands Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology, Rochester, NY 14623, USA Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305-4085, USA Max-Planck-Institut f¨ur extraterrestrische Physik, 85748 Garching, Germany University of Michigan, Dept. of Astronomy, Ann Arbor, MI 48109, USA H H Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK Observatoire de Paris, LERMA, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France Institute of Astronomy, Madingley Rd., Cambridge, CB3 0HA, UK Michigan State University, Physics and Astronomy Dept., East Lansing, MI 48824-2320, USA Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA Department of Physics, University of Kentucky, Lexington KY 40506 USA School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK Department of Physics & Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1 Department of Physics, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623-5603, USA Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325, USA School of Physics, University of Melbourne, Victoria 3010, Australia ASTRON, Netherlands Institute for Radio Astronomy,P.O. Box 2, 7990 AA Dwingeloo, The NetherlandsReceived 30 March 2010 / Accepted
ABSTRACT
The dust destruction timescales in the cores of clusters of galaxies are relatively short given their high central gas densities. However,substantial mid-infrared and sub-mm emission has been detected in many brightest cluster galaxies. In this letter we present
Herschel
PACS and SPIRE photometry of the brightest cluster galaxy in three strong cooling flow clusters, A1068, A2597 and Zw3146. Thisphotometry indicates that a substantial mass of cold dust is present ( > × M ⊙ ) at temperatures significantly lower (20–28 K) thanpreviously thought based on limited MIR and / or sub-mm results. The mass and temperature of the dust appear to match those of thecold gas traced by CO with a gas-to-dust ratio of 80–120. Key words.
Galaxies: clusters: intracluster medium, Galaxies: clusters: elliptical and lenticular, cD
1. Introduction
The cores of cluster of galaxies are very energetic regions witha high X-ray emissivity, particle density, cosmic ray flux, stellardensity and AGN radiation. In this very hostile environment anydust grains are unlikely to survive for more than a few millionyears due to the action of collisional sputtering (Dwek & Arendt1992) unless they are shielded (Fabian et al. 1994). It is there-fore somewhat surprising to find that dust continuum emissionfrom the brightest cluster galaxies in the most rapidly coolingclusters being detected at sub-mm and MIR wavelengths (Edgeet al. 1999, Egami et al. 2006, O’Dea et al. 2008). The presenceof cold molecular gas (Edge 2001, Salom´e & Combes 2003) anddust absorption in HST imaging (McNamara et al 1996) implies ⋆ Herschel is an ESA space observatory with science instrumentsprovided by European-led Principal Investigator consortia and with im-portant participation from NASA. that the dust continuum traces a substantial, cold component tothe ISM in these massive elliptical galaxies. However, the originof the dust and how it is shielded are still poorly understood.The limitations with the current observations of dust emis-sion make it di ffi cult to establish an unambiguous dust mass asthey do not sample over the peak of the dust emission in theFIR. The unprecedented sensitivity of Herschel (Pilbratt et al.2010) to FIR continuum o ff ers the opportunity to accurately con-strain the full FIR spectrum of the dust emission in cluster cores.The authors were awarded 140 hours of time in an Open TimeKey Project (PI Edge) to investigate the FIR line and continuumproperties of a sample of 11 brightest cluster galaxies (BCGs) inwell-studied cooling flow clusters selected on the basis of opticalemission line and X-ray properties. The full goals of the projectare to observe at least five atomic cooling lines for each objectthat cover a range in density and temperature behaviour and ob-tain a fully sampled FIR spectral energy distribution. In this pa-
1. C. Edge et al.:
Herschel photometry of brightest cluster galaxies in cooling flow clusters per we present the Photodetector Array Camera & Spectrometer(PACS, Poglitsch et al. 2010) and Spectral and PhotometricImaging REceiver (SPIRE, Gri ffi n et al. 2010) photometry forthe three targets observed in the Science Demonstration Phase(SDP), Abell 1068 ( z = . z = . z = . L ⊙ ) and exhibits some contri-bution from an AGN (Crawford et al. 1999, O’Dea et al. 2008).On the other hand, Abell 2597 is a relatively weak MIR source(Donahue et al. 2007) with a weak CO detection (Salome, priv.comm.) and a powerful central radio source (Sarazin et al. 1995).The implied FIR luminosity of A2597 is a factor of around 30below that of A1068 and, in addition, the fractional contributionfrom an AGN in the MIR is also lower in A2597.
2. Observations
We performed photometric imaging of A1068, A2597 andZw3146 with PACS and SPIRE. The data were reduced withthe
Herschel
Interactive Processing Environment (HIPE) soft-ware version 2.3.1436 (Ott 2010). We used for both PACS andSPIRE the o ffi cial scripts as presented by the PACS and SPIREICC teams during the Herschel
SDP data processing workshopin December 2009.
The PACS photometric observations were taken inLargeScanMapping mode in all three bands of the pho-tometer, BS (70 µ m), BL (100 µ m) and R (160 µ m) using themedium scan speed (20 ′′ s − ). The scan maps comprised 18 scanline legs of 4 ′ length and cross-scan step of 15 ′′ . Each obser-vation had a “scan” and an orthogonal “cross-scan” directionand we calibrated the corresponding data separately beforecombining them into a single map of 9 ′ × ′ . The resulting mapshave a resolution of 5.2 ′′ , 7.7 ′′ and 12 ′′ at 70, 100 and 160 µ m,respectively and are presented in the electronic version of thispaper. The PACS photometer performs dual-band imaging suchthat the BS and BL bands each have simultaneous observationsin the R band so we have two sets of scans in the R band.We adopted the PACS Data Reduction Guideline to processthe raw level-0 data to calibrated level 2 products and used theo ffi cial script for PACS ScanMapping mode but with particularattention to the high pass filtering to remove “1 / p f ” noise. Wechoose to use the HighPassFilter method with a filter of 20 read-outs which will remove structure on all scales above 82 ′′ . Thetarget BCG and other bright sources in the field were maskedprior to applying the filter. The size of the mask was chosen tobe less than the filter size so as to minimize any left-over low-frequency artefacts under the masks. We used masks with a ra-dius of 15 ′′ for our sources. We tried varying the size for thefilter from 10 to 30 readouts and the mask radius from 10–30 ′′ and found our results to not change significantly for these rangesin values. Finally the task ‘photProject’, was used to project thecalibrated data onto a map on the sky in units of Jy pixel − . The“scan” and “cross-scan” maps were then averaged to produce thefinal coadded map. The PACS and SPIRE images are included in the electronic version of the paper. The spatial flux distribu-tion and flux densities of our target sources were investigatedusing cumulative flux curves. The spatial flux distribution foreach of our three sources is consistent with that expected from apoint source. Flux densities in the BS, BL and R band were ex-tracted using a 33 ′′ by 33 ′′ aperture centered on the BCG. Smallaperture corrections were applied as outlined in the PACS ScanMap release note (PICC-ME-TN-035). Care was taken to cali-brate these derived flux densities to account for the known fluxoverestimation in the used HIPE version by factors 1.05, 1.09and 1.29 in BS, BL and R bands respectively. The absolute fluxaccuracy is within 10 % for BS and BL, and better than 20 %for R. These uncertainties are not believed to be correlated dueto the BS and BL bands being taken at di ff erent times and the Rband using a di ff erent detector. The SPIRE photometry was performed in the LargeScanMapmode with cross-linked scans in two orthogonal scan directions.The photometer has a field of view of 4 ′ × ′ , which is ob-served simultaneously in three spectral bands, PSW (250 µ m),PMW (350 µ m) and PLW (500 µ m) with a resolution of about18 ′′ , 25 ′′ and 36 ′′ , respectively. The resulting maps measure12 ′ × ′ in size and are presented in the electronic version ofthis paper.We used the standard HIPE pipeline for the LargeScanMapobserving mode and the na¨ıve map-maker. The pre-processedraw telemetry data were first subject to engineering conver-sion wherein the raw timeline data were converted to mean-ingful units, the SPIRE pointing product was created, deglitch-ing and temperature drift correction were performed, and mapswere created, the units of which were Jy beam − . Our targetsare unresolved at the spatial resolution of SPIRE. We derivedtheir flux densities by fitting the sources with the SPIRE pointsource response function. Care was taken to de-blend our tar-get from other nearby sources at the longer wavelengths, wherethe sources are most likely to be background to the cluster. Weaccount for the known flux calibration o ff set in the used ver-sion of HIPE by applying the following multiplicative calibra-tion factors 1.02, 1.05 and 0.94 to the derived flux densities inthe PSW, PMW and PLW bands respectively (see Gri ffi n et al.2010, Swinyard et al. 2010). We also performed aperture pho-tometry using the HIPE point-source extraction (PSE) tool butthis method gives accurate results only for isolated point sources.At 350 µ m and 500 µ m, the BCGs in A2597 and Zw3146 areclose to the detection limit and at the confusion limit of SPIREmaking the PSE method of determining the fluxes unsuccessful.A1068 has a relatively strong compact BCG in far infrared andso we performed the PSE to find that the flux estimates usingAIPS and HIPE agree with each other to better than 5%.
3. Results
In the PACS photometry, A1068, A2597 and Zw3146 have beendetected in all three bands. For A1068, 70 and 100 µ m values areslightly less than the IRAS
60 and 100 µ m measurements. Thiscould be due to nearby sources that cannot be separated from theBCG in the much lower resolution IRAS observations but no suf-ficiently bright source is visible in our PACS imaging. There is alarge di ff erence between the Spitzer
MIPS 70 µ m flux (Quillen etal. 2008) and our PACS 70 µ m flux, the PACS flux being a factor1.7 lower than the MIPS flux. In the case of Zw3146 the MIPSand PACS 70 µ m fluxes also di ff er with the PACS value being
2. C. Edge et al.:
Herschel photometry of brightest cluster galaxies in cooling flow clusters
Table 1.
Log of
Herschel
Observations.
Cluster z Instrument λ Obsid Flux( µ m) (mJy)A1068 0.1386 PACS 70 1342187051 542 ± ± ± ± ± ± ± ± Spitzer
24 74.5 ± Spitzer
70 941 ± IRAS
60 577 ± IRAS
100 958 ± ± ± ± ± ± < ± Spitzer
24 2.1 ± Spitzer
70 49 ± Spitzer
160 52 ± ± ± ± ± ± < < ± Spitzer
24 4.1 ± Spitzer
70 68 ± Spitzer
160 157 ± Notes.
The
Spitzer data are from Quillen et al. (2008), Donahue et al.(2007, priv. comm.) and Egami et al. (2006). The SCUBA data are fromEdge (priv. comm.), Zemcov et al. (2007) and Chapman et al. (2002). a factor 1.4 larger than the MIPS value (Egami et al. 2006). ForA2597 the PACS fluxes di ff er from the Spitzer
70 and 160 µ mfluxes reported by Donahue et al. (2007). Part of this di ff erencewas resolved when the MIPS 70 µ m data were re-analysed andfound to be a factor of two too high (Donahue, priv. comm.).The di ff erences observed between the PACS and Spitzer fluxesrequire further investigation. In the SPIRE photometry, A1068is detected in all three SPIRE bands. A2597 and Zw3146, whileclearly detected in PSW and PMW bands, have a 1–2 σ detec-tion in the PLW band. Table 1 gives the photometric results forthe three galaxies, with 2 σ upper-limit for A2597 and Zw3146 inPLW. Figure 1 presents the radio to optical spectral energy dis-tributions (SEDs) for the three targets. These plots show the sig-nificant variation in the relative radio-FIR-optical contributionsfor each of our galaxies. Here we focus on the sub-mm / MIR dustemission as sampled by PACS and SPIRE photometry, comple-mented by published
Spitzer and
IRAS measurements.We fit the SEDs of the dust emission using black bodiesmodified with a dust emissivity index, β . The FIR-MIR slopesof our sources require the presence of at least two dust com-ponents. Previous studies of star-forming galaxies have indeedestablished that a single modified black body (MBB) is inade-quate to account for the observed dust emission (Wiklind 2003).Hence, our model for the SEDs consists of two MBBs with the dust emissivity index for each fixed to β = ffi cient, κ d ν , of 2.5 m kg − at 100 µ m.For A1068 we fit the 24–850 µ m emission. For A2597 andZw3146 the SCUBA 850 µ m detections have been removed andwe fit only the 24–350 µ m range. In the case of A2597, this isdue to the unknown amount of radio contamination at 850 µ m. Inthe case of Zw3146 the BCG is blended with strong backgroundsource at 850 µ m (Chapman et al. 2002). The data are weightedin the fit inversely to the square of their error. The resulting fitsare shown in Figure 1. The derived dust temperatures and totalFIR luminosities for each source are listed in Table 2.The results in Table 2 indicate that at least two dust compo-nents, one at 20–25 K and one at 50–60 K, are present in all threesources. The FIR emission is much stronger relative to the opti-cal in A1068 and Zw3146 as compared to A2597. The SEDs ofA1068 and Zw3146 resemble those of strongly star-forming sys-tems and, based on the total FIR luminosity derived here, we findstar formation rates (SFR) of 60 and 44 M ⊙ yr − in these two sys-tems using the Kennicutt (1998) conversion factor. For A2597 amuch more modest SFR of 2 M ⊙ yr − is inferred. These valuesare comparable to SFRs derived from H α line and / or UV contin-uum emission given the uncertainties of these tracers. However,the SFR values derived from Spitzer data are higher for A1068and Zw3146. The di ff erence for A1068 is the most pronouncedand can be directly attributed to the stronger AGN contributionin this object (Quillen et al. 2008) which boosts the 24 µ fluxcompared other comparable sources. Therefore, when the to-tal FIR luminosity is derived from the 15 µ m flux infered from Spitzer it will be overestimated. The value for Zw3146 fromEgami et al. (2006) is higher than ours as their fit includes theSCUBA 850 µ m point from Chapman et al. (2002) which appearsto be overestimated on the basis of our SPIRE data.The gas to dust ratio is found to be between 80 and 140 (seeTable 2). Gas temperatures can be inferred from CO measure-ments (Edge 2001, Salome & Combes 2003). These estimatesinfer gas temperatures of 25–40 K thus implying that the gasand dust share a common environment and are potentially co-located in the denser regions of cold, molecular gas clouds. Wehave attempted to determine how much extended emission ispresent from our highest spatial resolution PACS 70 µ m imagebut we find no evidence for more than 10% additional flux be-yond a point source. Clearly these limits will improve with abetter characterisation of the instrument but we believe that wecan conclude that the dust emission in our targets has an extentcomparable to that the bulk of the CO emitting gas and opticalemission lines ( < ′′ or 5–20 kpc).
4. Discussion and conclusions
Our initial
Herschel results confirm the presence of the strikingdust emission peak expected from the observations at sub-mm(Edge et al. 1999, Chapman et al. 2002) and MIR (Egami et al.2006, O’Dea et al. 2008).The star formation rates derived from the full-sampled FIRSED are comparable to those derived from
Spitzer µ m fluxesapart from A1068, which has the strongest contribution froman AGN so hot dust dominates to the 24 µ m flux. However, inthe sub-mm the contribution from the radio continuum from anactive nucleus must be correctly accounted for before any dustmass can be estimated from the 850 µ m flux. In the case of A2597here and A2390 in Edge et al. (1999), the presence of a powerfulradio source appears to contribute to the SCUBA 850 µ m flux.While it is di ffi cult to draw any general conclusions from justthree BCGs, we note with interest that the ratio of dust mass to
3. C. Edge et al.:
Herschel photometry of brightest cluster galaxies in cooling flow clusters
Table 2.
Summary of results and other cluster properties.
Cluster A1068 A2597 Zw3146Dust Temperatures 24 ±
4K 21 ±
6K 23 ± + − K 48 + − K 53 + − KCold Dust Mass 5.1 × M ⊙ × M ⊙ × M ⊙ Warm Dust Mass 3.9 × M ⊙ × M ⊙ × M ⊙ Total FIR Luminosity 3.5 × L ⊙ × L ⊙ × L ⊙ Star Formation Rate 60 ±
20 M ⊙ yr − ± ⊙ yr − ±
14 M ⊙ yr − SFR
Spitzer
188 M ⊙ yr − ⊙ yr − ±
14 M ⊙ yr − SFR optical / UV ⊙ yr − ⊙ yr − ± ⊙ yr − CO gas mass 4.1 × M ⊙ × M ⊙ × M ⊙ H α Slit Luminosity 8 × erg s − × erg s − × erg s − Notes.
The
Spitzer
SFR values are from O’Dea et al. (2008), Donahueet al. (2007) and Egami et al. (2006). The Optical / UV SFR values arefrom McNamara et al. (2004), Donahue et al. (2007) and Egami et al.(2006). The CO gas masses are from Edge (2001) and Salom´e (priv.comm.) and the H α slit luminosities are from Crawford et al. (1999). CO-derived gas mass is constistent for all three within a factorof five. If the dust were mostly generated through dust ejectionfrom evolved stars then the dust mass should closely correlatewith the total stellar mass. However, our three galaxies have verysimilar optical / NIR absolute magnitudes. So, unless the ejecteddust were “captured” by the cold gas clouds protecting it fromX-ray sputtering, this suggests that the apparent correlation be-tween the molecular gas and dust masses arises from a directconnection between the gas reservoir and star formation.These results are a limited example of those to come in thevery near future from
Herschel as there are two other Open TimeKey Projects (PI Egami and Smith) that are targetting a total of70 clusters that cover a broad range of BCG properties so thewider context of these initial observations can be determined. Inparticular, the amount of dust present in more quiescent BCGsand other massive cluster ellipticals will be important in assess-ing how much of the dust seen in cool core BCGs orginates fromthe underlying stellar population.
Acknowledgements.
We would like to thank the
Herschel
Observatory and in-strument teams for the extraordinary dedication they have shown to deliver sucha powerful telescope. We would like to thank the HSC and NHSC consortium forhelp with data reduction pipelines. J.B.R.O. thanks HSC, the
Herschel
Helpdeskand the PACS group at MPE for useful discussions. R. M. thanks the NHSC forthe HIPE tutorials.
References
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4. C. Edge et al.:
Herschel photometry of brightest cluster galaxies in cooling flow clusters
Fig. 1.
Spectral energy distributions for A1068 (top),A2597 (middle) and Zw3146 (bottom) including
Herschel
PACS / SPIRE (blue / red symbols), Spitzer , Radio, NIR photom-etry from 2MASS and optical photometry from SDSS. To ac-count for absolute flux uncertainties we have set the followingerrors on the fluxes derived from the various instruments (unlessthe quoted error is larger than this); PACS BS / BL 10%, PACSR 20%, SPIRE 15%,
Spitzer µ m 30% andSCUBA 850 µ m 20%. The model fit to the sub-mm / FIR / MIRdata is shown by the black solid line. Only filled symbols havebeen used in the fit. The two modified blackbodies making up themodel are shown by the black long dash and dash-dot lines. ForA2597 we also show two VLBI measurements (black crosses)of the BCG core at 1.3 and 5 GHz (Taylor et al. 1999). Thesepoints show that the BCG has a strong, inverted radio core.
5. C. Edge et al.:
Herschel photometry of brightest cluster galaxies in cooling flow clusters
Fig. 2.
Colour images from the three PACS bands (BS, BL and R in the blue, green and red channels) for the three clusters withinradius of 2.5 ′ of the BCG The top row are images combined in their original resolution and the bottom row are the images combinedwith a common smoothing of 12 ′′ to match resolution.
6. C. Edge et al.:
Herschel photometry of brightest cluster galaxies in cooling flow clusters
Fig. 3.
Colour images from the three SPIRE bands (PSW, PMW and PLW in the blue, green and red channels) for full field coveredfor the three clusters covering approximately 12 ′ × ′ . The top row are images combined in their original resolution and the bottomrow are the images combined with a common smoothing of 36 ′′ to match resolution and clipped to remove areas of low exposure.The BCG is at the centre of the image and in A2597 and Zw3146 is the bluest object present (see text).to match resolution and clipped to remove areas of low exposure.The BCG is at the centre of the image and in A2597 and Zw3146 is the bluest object present (see text).