SALT Spectroscopic Observations of Galaxy Clusters Detected by ACT and a Type II Quasar Hosted by a Brightest Cluster Galaxy
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Mon. Not. R. Astron. Soc. 000, 1–11 (2015) Printed 29 July 2018 (MN LATEX style file v2.2) SALT spectroscopic observations of galaxy clusters detected by ACT and a Type II quasar hosted by a brightest cluster galaxy Brian Kirk,1;2? Matt Hilton,1;3† Catherine Cress,2;4 Steven M. Crawford,5 John P. Hughes,6 Nicholas Battaglia,7 J. Richard Bond,8 Claire Burke,1 Megan B. Gralla,9 Amir Hajian,8 Matthew Hasselfield,10 Adam D. Hincks,11 Leopoldo Infante,12 Arthur Kosowsky,13 Tobias A. Marriage,9 Felipe Menanteau,14 Kavilan Moodley,1 Michael D. Niemack,15 Jonathan L. Sievers,16 Cristóbal Sifón,17 Susan Wilson,1 Edward J. Wollack,18 and Caroline Zunckel16 1 Astrophysics & Cosmology Research Unit, School of Mathematics, Statistics & Computer Science, University of KwaZulu-Natal, Durban 4041, SA 2 Centre for High Performance Computing, CSIR Campus, 15 Lower Hope St. Rosebank, Cape Town, SA 3 Centre for Astronomy & Particle Theory, School of Physics and Astronomy, University of Nottingham, NG7 2RD, UK 4 Department of Physics, University of Western Cape, Bellville 7530, Cape Town, SA 5 South African Astronomical Observatory, P.O. Box 9, Observatory 7935, Cape Town, SA 6 Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghuysen Road, Piscataway, NJ USA 08854-8019 7 McWilliams Center for Cosmology, Carnegie Mellon University, Department of Physics, 5000 Forbes Ave., Pittsburgh PA, USA, 15213 8 Canadian Institute for Theoretical Astrophysics, 60 St George, Toronto ON, Canada, M5S 3H8 9 Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218 10 Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544, USA 11 Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada 12 Departamento de Astronomía y Astrofísica, Pontificía Universidad Católica, Casilla 306, Santiago 22, Chile 13 Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260 USA 14 National Center for Supercomputing Applications, University of Illinois at Urbana-Champaign, 1205 W. Clark St, Urbana, IL 61801 15 Department of Physics, Cornell University, Ithaca, NY 14853, USA 16 Astrophysics & Cosmology Research Unit, School of Chemistry & Physics, University of KwaZulu-Natal, Durban 4041, SA 17 Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, Netherlands 18 NASA/Goddard Space Flight Center, Greenbelt, MD, 20771, USA Accepted for publication in MNRAS ABSTRACT We present Southern African Large Telescope (SALT) follow-up observations of seven massive clusters detected by the Atacama Cosmology Telescope (ACT) on the celestial equa- tor using the Sunyaev-Zel’dovich (SZ) effect. We conducted multi-object spectroscopic ob- servations with the Robert Stobie Spectrograph in order to measure galaxy redshifts in each arXiv:1410.7887v2 [astro-ph.GA] 17 Mar 2015 cluster field, determine the cluster line-of-sight velocity dispersions, and infer the cluster dy- namical masses. We find that the clusters, which span the redshift range 0:3 < z < 0:55, range 14 in mass from (5−20)×10 M (M200c). Their masses, given their SZ signals, are similar to those of southern hemisphere ACT clusters previously observed using Gemini and the VLT. We note that the brightest cluster galaxy in one of the systems studied, ACT-CL J0320.4+0032 at z = 0:38, hosts a Type II quasar. Only a handful of such systems are currently known, and therefore ACT-CL J0320.4+0032 may be a rare example of a very massive halo in which quasar-mode feedback is actively taking place. Key words: cosmology: observations – galaxies: clusters: general – galaxies: clusters: indi- vidual: ACT-CL J0320.4+0032 – galaxies: quasars: general 1 INTRODUCTION ? E-mail: [email protected] Clusters of galaxies mark the highest density regions of the Uni- † E-mail: [email protected] verse at mega parsec scales. By charting the evolution of their num- c 2015 RAS 2 Kirk et al. ber density as a function of mass and redshift, one is able to obtain scope located in northern Chile that observes the sky in three fre- constraints on cosmological parameters, including the amount of quency bands (centred at 148, 218, and 277 GHz) simultaneously dark matter and dark energy in the Universe (e.g., Vikhlinin et al. with arcminute resolution. 2009; Mantz et al. 2010; Sehgal et al. 2011; Benson et al. 2013; ACT surveyed two regions of the sky, searching a total area of Hasselfield et al. 2013; Planck Collaboration 2013). However, clus- 959 deg2 for SZ galaxy clusters. During 2008, ACT observed a 455 ter mass – which is the property predicted by N-body simulations deg2 patch of the Southern sky, centred on d = −55 deg, detecting of cold dark matter – is not a directly measurable quantity, and a total of 23 massive clusters that were optically confirmed using must instead be inferred from the observable properties of the clus- 4 m class telescopes (Menanteau et al. 2010; Marriage et al. 2011). ters. This has led to many studies that derive mass–observable scal- From 2009–2010, ACT observed a 504 deg2 region centred on the ing relations using a wide variety of observables including, optical celestial equator, an area chosen due to its complete overlap with richness (e.g., Rozo et al. 2009); X-ray luminosity and temperature the deep (r ≈ 23.5 mag) optical data from the 270 deg2 Stripe 82 (e.g., Vikhlinin et al. 2006); and Sunyaev-Zel’dovich effect signal region (Annis et al. 2011) of the Sloan Digital Sky Survey (SDSS; (e.g., Sifón et al. 2013). Abazajian et al. 2009). Hasselfield et al. (2013) describes the con- The discovery of new clusters from large area surveys using struction of the SZ cluster sample from the 148 GHz maps (see the Sunyaev-Zel’dovich effect (SZ; Sunyaev & Zel’dovich 1970) Dünner et al. 2013 for a detailed description of the reduction of began only in recent years (e.g., Staniszewski et al. 2009; Van- the ACT data from timestreams to maps). Optical confirmation and derlinde et al. 2010; Marriage et al. 2011; Reichardt et al. 2013; redshifts for these clusters are reported in Menanteau et al. (2013), Planck Collaboration et al. 2013; Bleem et al. 2014). The SZ effect using data from SDSS and additional targeted optical and IR obser- is the inverse Compton scattering of cosmic microwave background vations obtained at Apache Point Observatory. All 68 clusters in the photons by hot (> 107 K) gas trapped within the deep gravitational sample have either photometric redshift estimates, or spectroscopic potential wells of massive galaxy clusters. It is almost redshift in- redshifts (largely derived from SDSS data). The sample spans the dependent, and in principle, it allows the discovery of all clusters redshift range 0:1 < z < 1:4, with median z = 0:5. in the Universe above a mass limit set by the noise properties of In this pilot study with SALT, we targeted seven of the equa- the SZ survey (see, e.g., Birkinshaw 1999; Carlstrom et al. 2002). torial ACT clusters detected with reasonably high signal-to-noise In addition, the SZ signal, usually denoted by the integrated Comp- (4:6 < S=N < 8:3) and at moderate redshift (z ≈ 0:4), in order to tonisation (Y) parameter, has been shown to correlate with cluster ensure that targeted galaxies would be bright enough for success- mass, with relatively small scatter (e.g., Planck Collaboration et al. ful absorption line redshift measurements given the capabilities of 2011; Hoekstra et al. 2012; Sifón et al. 2013). Despite this, mass– the RSS instrument at the time of the observations1. Sifón et al. calibration is the main contribution to the error budget of current (in prep.) will present observations of a further 21 ACT equatorial cosmological studies using SZ-selected cluster samples (e.g., Seh- clusters observed with the Gemini telescopes. gal et al. 2011; Hasselfield et al. 2013; Reichardt et al. 2013; Planck Collaboration 2013; Bocquet et al. 2014), and so further work in this area is clearly needed. 2.2 Spectroscopic observations In this paper, we present the results of a pilot follow-up study We conducted observations of the seven target ACT clusters with of SZ-selected clusters detected by the Atacama Cosmology Tele- RSS in multi-object spectroscopy (MOS) mode, which uses cus- scope (ACT; Swetz et al. 2011) conducted using the Robert Stobie tom designed slit masks. Given that SALT is located at Sutherland Spectrograph (RSS; Burgh et al. 2003) on the Southern African where the median seeing is 1:300 (Catala et al. 2013), we chose to Large Telescope (SALT; Buckley et al. 2006). The goals of this use slitlets with dimensions of 1.500 width and 1000 length. The lat- programme were to obtain spectroscopic redshifts and dynamical ter was chosen to ensure reasonably accurate sky subtraction given mass estimates through velocity dispersions for ACT clusters, with these seeing conditions. The RSS has an 80 diameter circular field the aim of increasing the sample of clusters for our calibration of of view, and with these slit dimensions we found we were able to Y the -mass relation (Sifón et al. 2013; see Hasselfield et al. 2013 target 19-26 galaxies in each cluster field per slit mask. We selected for joint constraints on the dynamical mass scaling relation and cos- 3-4 bright (15−17:5 magnitude in the r band) stars per cluster field mological parameters). for alignment of the slit masks during acquisition. The structure of this paper is as follows. We briefly describe The slit masks were designed using catalogues extracted from the ACT cluster sample and the design, execution, and reduction the 8th data release of the SDSS (SDSS; Aihara et al.