Spectroscopic Characterization of the Protocluster of Galaxies Around 7C 1756+6520 at Z ~
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A&A 618, A128 (2018) Astronomy https://doi.org/10.1051/0004-6361/201833052 & c ESO 2018 Astrophysics Spectroscopic characterization of the protocluster of galaxies around 7C 1756+6520 at z ∼ 1.4? V. Casasola1, L. Magrini1, F. Combes2, E. Sani3, J. Fritz4, G. Rodighiero5, B. Poggianti6, S. Bianchi1, and E. Liuzzo7 1 INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: [email protected] 2 LERMA, Observatoire de Paris, CNRS, PSL Univ., Sorbonne Univ. and Collège de France, Paris, France 3 European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago 19001, Chile 4 Instituto de Radioastronomía y Astrofísica, UNAM, Campus Morelia, AP 3-72, 58089, Mexico 5 Dipartimento di Fisica e Astronomia, Università di Padova, vicolo dell’Osservatorio 3, 35122 Padova, Italy 6 INAF – Astronomical Observatory of Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy 7 INAF – Istituto di Radioastronomia, Via Piero Gobetti 101, 40129 Bologna, Italy Received 19 March 2018 / Accepted 23 June 2018 ABSTRACT Aims. The aim of this paper is the spectroscopic study of 13 galaxies belonging to the field of the protocluster associated with the radio galaxy 7C 1756+6520 at z = 1:4156. In particular, we focus on the characterization of the nuclear activity. Methods. This analysis has been performed on rest-frame optical spectra taken with the Large Binocular Telescope, using the spec- trograph LUCI, which is operative in the near-infrared domain. The adopted spectral coverage allowed us to observe emission lines such as Hα,Hβ, [O iii]λ 5007 Å, and [N ii]λ 6583 Å at the redshift of the central radio galaxy. We observed the central part of the protocluster, which is suitable to include the radio galaxy, several spectroscopically confirmed active galactic nuclei (AGN) belonging to the protocluster, and other objects that might be members of the protocluster. Results. For four previously identified protocluster members, we derived the redshift by detecting emission lines that have never de- tected before for these galaxies. We identified a new protocluster member and eight new possible protocluster members. The stacked spectrum of the galaxies in which we detected the [O iii]λ 5007 Å emission line revealed the second line of the [O iii] doublet at 4959 Å and the Hβ line, which confirms that they belong to the protocluster. By collecting all members identified so far in this work and other members from the literature, we defined 31 galaxies, including the central radio galaxy, around the redshift 1:4152 ± 0:056. This corresponds to peculiar velocities .5000 km s−1 with respect to the radio galaxy. The position versus velocity phase-space diagram suggests that three AGN of the protocluster and the central radio galaxy might be a virialized population that has been coexisting for a long time in the densest core region of this forming structure. This protocluster is characterized by a high fraction of AGN (∼23%). For one of them, AGN1317, we produced two so-called Baldwin, Phillips & Terlevich (BPT) diagrams. The high fraction of AGN and their distribution within the protocluster seem to be consistent with predictions of some theoretical models on AGN growth and feedback. These models are based on galaxy interactions and ram pressure as triggers of AGN activity. Conclusions. The high fraction of AGN belonging to the protocluster suggests that they were likely triggered at the same time, maybe by the ongoing formation of the protocluster. Observations of AGN in this protocluster and in other distant clusters will help clarifying whether the resulting high fraction of AGN is unusual or typical for such structures at high redshift. Our next step will be analyses of previously acquired high-resolution radio data of the central radio galaxy to derive information on the nature of the radio galaxy and connect it with its cosmic evolution. Key words. galaxies: clusters: individual: 7C 1756+6520 – galaxies: evolution – galaxies: formation 1. Introduction (e.g., Voit 2005; Vikhlinin et al. 2009; Stern et al. 2010). Galaxy clusters allow us to study large populations of early-type galax- Galaxy clusters provide an efficient tool for deriving cosmolog- ies, which provide stringent tests on galaxy evolution models in ical parameters and for studying galaxy formation and evolu- the current hierarchical formation paradigm (Renzini 2006), be- tion. As the largest collapsed structures in the Universe with total cause they are the most massive galaxies with the oldest stellar masses of up to 1015 M (e.g., Arnaud 2009), the cosmic history ∼ of galaxy clusters is sensitive to key cosmological parameters populations (at least out to redshift z 2). Over the past three decades, considerable effort has been ? Based on data acquired using the Large Binocular Telescope (LBT). devoted to discovering ever more distant galaxy clusters us- The LBT is an international collaboration among institutions in the ing different observational methods (see Rosati et al. 2002, for United States, Italy and Germany. LBT Corporation partners are: The a review). X-ray selection of galaxy clusters has been central University of Arizona on behalf of the Arizona university system; Is- in several studies, as it naturally provides gravitationally bound tituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, systems with a relatively simple selection function. However, Germany, representing the Max-Planck Society, the Astrophysical In- stitute Potsdam, and Heidelberg University; The Ohio State University, only few clusters are currently confirmed at z > 1: Exam- and The Research Corporation, on behalf of The University of Notre ples of distant, confirmed galaxy clusters identified through Dame, University of Minnesota and University of Virginia. the extended X-ray emission of the intracluster medium Article published by EDP Sciences A128, page 1 of 16 A&A 618, A128 (2018) are XMMU J2235.3-2557 at z = 1:39 (Mullis et al. 2005; already partly virialized and a core of older, massive galaxies Lidman et al. 2008; Rosati et al. 2009), XMMXCS J2215.9- is in place. 1738 at z = 1:46 (Stanford et al. 2006; Hilton et al. 2007), Although the method of studying the immediate surround- 2XMM J083026.2+524133 at z = 0:99 (Lamer et al. 2008), ings of HzRGs has successfully found overdensities of red XDCP J0044.0-2033 at z = 1:58 (Fassbender et al. 2014), and galaxies at z > 2, it has been challenging to spectroscopically con- XMMU J2235-2557 at z = 1:39 (Chan et al. 2016). However, firm their association with the HzRGs (e.g., Venemans et al. 2005; the X-ray identification of candidate clusters is very difficult at Kodama et al. 2007; Doherty et al. 2010). A few studies have also z > 1 since the surface brightness of the extended X-ray emission applied related methods to HzRGs with slightly lower redshift fades as (1 + z)4. (e.g., Best et al. 2003; Stern et al. 2003; Galametz et al. 2009a, Another method for finding galaxy clusters relies upon 2010; Franck et al. 2015; Cooke et al. 2016) and found overden- the observation that all rich clusters, at all redshifts observed sities of extremely red galaxies in the environments of HzRGs so far, appear to have a red sequence of early-type galaxies at z ∼ 1:5. Overdensities of galaxy candidates at z > 1:5 were (Gladders & Yee 2000). The colors of such galaxies are quite in particular discovered in the vicinity of HzRGs using Spitzer/ distinct as a result of the strong 4000 Å break in their spec- IRAC colour selection techniques. A number of these overden- tra. However, this break shifts into the near-infrared (NIR) at sities were recently spectroscopically confirmed by the Clusters z > 1:5 and the colors can become degenerate. The fundamental Around Radio Loud AGNs survey (CARLA; Wylezalek et al. advantage of this approach, the so-called red sequence method, 2013, 2014) via a 40-orbit HST/WFC3 grism follow-up campaign is that with appropriate filters, cluster elliptical galaxies at a (Noirot et al. 2016, 2018). At such high redshifts, these overden- given redshift are redder than all normal galaxies at lower red- sities are suspected to still be forming and to be not yet (com- shifts. We stress that the red sequence method could be biased at pletely) bound, and the term “protoclusters” is commonly used to high redshift since galaxies tend to become bluer and younger at describe such systems. This last selection technique, based on the high-z. Despite its biases, this technique has given notable re- surroundings of HzRGs, has permitted the discovery of the galaxy sults using Spitzer Space Telescope IRAC bands, and several protocluster on which this paper is focused. galaxy clusters found in this way have been spectroscopically An important aspect in the study of clusters is the demo- confirmed at z > 1:3 (e.g., Eisenhardt et al. 2008; Papovich 2008; graphics and distribution of AGN within clusters and their evo- Wilson et al. 2009). More recently, similar results have been ob- lution with redshift. The AGN population of a cluster has in- tained using the IR photometry in the UKIDSS Ultra-Deep Sur- deed important implications for the AGN fueling processes and vey field (Lawrence et al. 2007) and Hubble spectroscopy (e.g., how tightly black holes (BHs) at the centers of cluster galax- Lee-Brown et al. 2017). ies and galaxies coevolve. Numerous lines of evidence sug- During the past two decades, a new technique for detect- gest that there is coevolution between the growth of the BHs ing galaxy clusters at z > 1 has been to study the immediate and the formation of stars in galaxies. Perhaps the most strik- surroundings of high-redshift radio galaxies (hereafter HzRGs).