de Messières, G.E.

Donahue, M.

McNamara, B.R.

Nulsen, P.E.J.

O'Connell, R.W.

Voit, G.M.

Wise, M.W.

English

We have obtained mid-infrared spectra of nine cool-core galaxy clusters with the Infrared Spectrograph aboard the Spitzer Space Telescope. X-ray, ultraviolet and optical observations have demonstrated that each of these clusters hosts a cooling flow which seems to be fueling vigorous star formation in the brightest cluster galaxy. Our goal is to use the advantages of the mid-infrared band to improve estimates of star formation. Our spectra are characterized by diverse morphologies ranging from classic starbursts to flat spectra with surprisingly weak dust features. Although most of our sample are known from optical/UV data to be active star-formers, they lack the expected strong mid-infrared continuum. Star formation may be proceeding in unusually dust-deficient circum-galactic environments such as the interface between the cooling flow and the relativistic jets from the active galactic nucleus

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UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)Spitzer mid-infrared spectra of cool-core galaxy clustersde Messières, G.E.; O'Connell, R.W.; McNamara, B.R.; Donahue, M.; Nulsen, P.E.J.; Voit,G.M.; Wise, M.W.Publication date2010Document VersionSubmitted manuscriptPublished inGalaxy Wars: Star Formation and Stellar Populations in Interacting GalaxiesLink to publicationCitation for published version (APA):de Messières, G. E., O'Connell, R. W., McNamara, B. R., Donahue, M., Nulsen, P. E. J., Voit,G. M., & Wise, M. W. (2010). Spitzer mid-infrared spectra of cool-core galaxy clusters. In B.Smith, J. Higdon, S. Higdon, & N. Bastian (Eds.), Galaxy Wars: Star Formation and StellarPopulations in Interacting Galaxies: proceedings of a conference held at East TennesseeState University, Johnson City, Tennessee, USA, 19-22 July 2009 (pp. 105-108).(Astronomical Society of the Pacific conference series; Vol. 423). Astronomical Society of thePacific. http://aspbooks.org/custom/publications/paper/423-0105.htmlGeneral rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:10 Mar 2023arXiv:0908.3445v1  [astro-ph.CO]  24 Aug 2009**FULL TITLE**ASP Conference Series, Vol. **VOLUME**, c© **YEAR OF PUBLICATION****NAMES OF EDITORS**Spitzer Mid-Infrared Spectra of Cool-Core Galaxy ClustersG. E. de Messières1, R. W. O’Connell1, B. R. McNamara2, M.Donahue3, P. E. J. Nulsen4, G. M. Voit3, M. W. Wise5Abstract.We have obtained mid-infrared spectra of nine cool-core galaxy clusterswith the Infrared Spectrograph aboard the Spitzer Space Telescope. X-ray, ul-traviolet and optical observations have demonstrated that each of these clustershosts a cooling flow which seems to be fueling vigorous star formation in thebrightest cluster galaxy. Our goal is to use the advantages of the mid-infraredband to improve estimates of star formation. Our spectra are characterized bydiverse morphologies ranging from classic starbursts to flat spectra with surpris-ingly weak dust features. Although most of our sample are known from opti-cal/UV data to be active star-formers, they lack the expected strong mid-infraredcontinuum. Star formation may be proceeding in unusually dust-deficient cir-cumgalactic environments such as the interface between the cooling flow and therelativistic jets from the active galactic nucleus.1 IntroductionBrightest cluster galaxies (BCGs) have elliptical morphologies and generally donot host significant star formation. However, a subset of BCGs are marked bytheir cuspy X-ray emission (Crawford et al. 1999; Edwards et al. 2007) and ahigh incidence of strong nebular line emission, which indicates star formation ornuclear activity. The intracluster gas in these cool-core clusters is thought to becondensing in the form of a “cooling flow” (Fabian 1994). Many such clustershave blue continua and star formation rates (SFRs) as high as ∼ 100M⊙ yr−1,suggesting that the star formation is fueled by the cooling flows.Recent X-ray spectroscopy has demonstrated that cooling flows are subjectto regulatory heating, probably associated with nuclear activity (McNamara & Nulsen2007). The estimated net rates at which gas cools below 1 keV are compara-ble with the estimated star formation rates (O’Dea et al. 2008; Rafferty et al.2006). Cool-core clusters present a prime opportunity to test models of the cool-1 P.O. Box 400325, University of Virginia, Charlottesville, VA 22904-43252 Department of Physics & Astronomy, University of Waterloo, 200 University Ave. W.,Waterloo, Ontario, Canada N2L 3G13 Physics and Astronomy Department, Michigan State University, East Lansing, Michigan48824-23204 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 021385 University of Amsterdam, Astronomical Inst Anton Pannekoek, Kruislann 403, NL 1098 SJAmsterdam, Netherlands12 de Messières et al.ing and energy feedback mechanisms that are thought to dominate early galaxyformation.Radio observations of CO emission and near-infrared detections of the ro-vibrational transitions of H2 have revealed large reservoirs of cool moleculargas in many cooling flows (Edge 2001; Donahue et al. 2000). Furthermore, far-infrared photometry with the Spitzer Space Telescope shows that some BCGsin cool-core clusters have spectral energy distributions consistent with emissionfrom cool dust (Td∼< 100) (Egami et al. 2006; O’Dea et al. 2008; Quillen et al.2008). There are similarities between classic starbursts (Brandl et al. 2006) andBCGs in cool-core clusters, but many questions about star formation in theBCGs remain.We use Spitzer to study nine BCGs in cool-core clusters which have beenextensively observed in X-ray, optical and ultraviolet light. With one exception,our targets are confirmed star-formers with optical and UV continuum-basedSFRs ranging up to 130M⊙ yr−1. The mid-infrared (MIR) band contains manydiagnostics of star formation, including emission by small warm dust grains andcomplex emission by polycyclic aromatic hydrocarbons (PAHs). It also con-tains many narrow emission features, including atomic emission associated withnuclear activity and emission lines from molecular hydrogen in pure rotationalstates. Our goal is to establish a better understanding of the cooling flow, feed-back and star formation processes in BCGs.2 ObservationsThe spectra in Figure 1 were taken with the four low-resolution modes ofSpitzer’s Infrared Spectrograph under Program ID’s 3384 and 20345. Our tar-gets are Abell 1068, Abell 1835, PKS0745-19, Hydra A, Abell 1795, Abell 2597,Abell 478, ZwCl 1370, and 2A0335+096. We obtained sparse spectral maps of allBCGs, but we restrict our current discussion to a central extraction that yieldedthe best signal-to-noise. After subtracting background light and correcting tothe restframe of each BCG, we renormalized each spectrum to the total lightfrom the target, using the light profile along the SL1 slit and assuming circularsymmetry.3 Discussion and ConclusionThe resulting spectra are compared to a typical starburst template spectrum(Brandl et al. 2006) in Figure 1. Despite the high SFRs derived from opticaland UV data, most of our BCGs differ from normal starbursts in one or more ofthe following characteristics: weaker long-wave continua, weaker PAH features,and strong H2 features.The pure rotational lines of H2 are abnormally strong in our spectra. Fromthe line strengths, we infer a range of gas masses at different temperatures, simi-lar to results discussed by Ferland et al. (2008). The strong H2 features probablyoriginate in cold gas reservoirs subject to shocks (Guillard & Boulanger 2008)and are probably unrelated to star formation. We will discuss the quantitativeinterpretation of the H2 and other features in an ensuing paper.Mid-IR spectra of cool-core galaxy clusters 3Figure 1. IRS spectra of the nine BCGs, grouped by gross spectral mor-phology, and a starburst template spectrum (Brandl et al. 2006). Spectra areplotted in units of intensity per unit frequency (mJy/sr), normalized so thatthe highest point blueward of 30µm is equal to 1, and additively offset. Unre-solved emission lines and PAH bands are labeled at the top. Note the strongemission lines, particularly [Ne II] and the pure rotational series of H2. PAHemission is detected in all cases, but is weaker than normal in most.4 de Messières et al.Of our nine galaxies, only A1835 has the MIR spectral morphology of aclassic starburst. While two others exhibit the expected strong red continuum,they also have anomalous emission feature spectra. The remaining six spectrahave weak red continua, despite a firm lower limit on star formation establishedfor five of them by optical and ultraviolet observations. The well-establishedcorrelations between IR output and star formation (Rieke et al. 2009) breakdown. We have established that if the normal MIR continuum signatures of starformation were present in these galaxies, we would have easily detected them;instead, in most of our targets, the MIR continuum is subluminous by factorsof 10-70x.The abnormally weak MIR continua indicate anomalies in the amount ofdust, in its grain size or composition, or in its spatial or temperature distribu-tion. Our results may indicate that star formation in BCGs in cool-core clustersis progressing in an unusual environment. Rather than occurring in a dense,disk-like volume, as in normal starbursts, star formation may be concentratedin extended regions outside the main body of the galaxy, such as at the interfacebetween relativistic jets from the AGN and the inner cooling flow. One exam-ple is NGC 1275, where star formation occurs in a huge filamentary network(Conselice et al. 2001; Fabian et al. 2008; McNamara et al 1996). In this un-usual environment, the dust grain population is expected to differ considerablyfrom the dust in normal starbursts and spiral disks.Acknowledgments. This work is based on observations made with theSpitzer Space Telescope, which is operated by the Jet Propulsion Laboratory,California Institute of Technology under a contract with NASA. Support for thiswork was provided by NASA through two awards issued by JPL/Caltech.ReferencesBrandl, B. R., et al. 2006, ApJ, 653, 1129Conselice, C. J., Gallagher III, J. S., & Wyse, R. F. G. 2001, AJ, 122, 2281Crawford, C. S., Allen, S. W., Ebeling, H., Edge, A. C., & Fabian, A. C. 1999, MNRAS,306, 857Donahue, M., Mack, J., Voit, G. M., Sparks, W., Elston, R., & Maloney, P. R. 2000,ApJ, 545, 670Edge, A. C. 2001, MNRAS, 328, 762Edwards, L. O. V., Hudson, M. J., Balogh, M. L., & Smith, R. J. 2007, MNRAS, 379,100Egami, E., Rieke, G. H., Fadda, D., & Hines, D. C. 2006, ApJ, 652, L21Fabian, A. C. 1994, ARA&A, 32, 277Fabian, A. C., Johnstone, R. M., Sanders, J. S., Conselice, C. J., Crawford, C. S., ,J. S. G. I., & Zweibel, E. 2008, Nat, 454, 968Ferland, G. J., Fabian, A. C., Hatch, N. A., Johnstone, R. M., Porter, R. L., van Hoof,P. A. M., & Williams, R. J. R. 2008, MNRAS, 386, L72Guillard, P., & Boulanger, F. 2008, SF2A-2008, 371McNamara, B. R., O’Connell, R. W., & Sarazin, C. L. 1996, AJ, 112, 91McNamara, B. R., & Nulsen, P. E. J. 2007, ARA&A, 45, 117O’Dea, C. P., et al. 2008, ApJ, 681, 1035Quillen, A. C., et al. 2008, ApJS, 176, 39Rafferty, D. A., McNamara, B. R., Nulsen, P. E. J., & Wise, M. W.Rieke, G. H., Alonso-Herrero, A., Weiner, B. J., Pérez-González, P. G., Blaylock, M.,Donley, J. L., & Marcillac, D. 2009, ApJ, 692, 556

<i>Spitzer</i> mid-infrared spectra of cool-core galaxy clusters

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<i>Spitzer</i> mid-infrared spectra of cool-core galaxy clusters

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