Casting X-Rays on a Blue Galaxy Cluster with a Red Spiral BCG

Casting X-rays on a blue galaxy cluster with a red spiral BCG PI: Y. Su 1. Abstract SDSS-C4 3028 is a cluster of galaxies identi?ed in the Sloan Digital Sky Survey. It stands out as a nearby (z = 0.06) cluster containing a surprisingly large fraction of star forming galaxies, which is not fully understood in the standard model of galaxy evolution and cluster formation. It is also very unusual that the brightest galaxy of this cluster is a passive red spiral galaxy. We propose an XMM-Newton pointing that covers the entire cluster to determine its dynamical state and to obtain a more robust cluster mass measurement. We will also verify whether this passive red spiral galaxy is the brightest cluster galaxy residing at the the bottom of the gravitational potential of this cluster. SDSS-C4 3028 provides a valuable opportunity to study the evolutionary link between galaxies and galaxy clusters. 2. Description of the proposed programme A) Scienti?c Rationale: An essential subject in modern astronomy is to understand how galaxies form and evolve. Galaxies are observed to display a rich variety of sizes, morphologies, colors, stellar masses, star formation rates, and in the contents of their interstellar medium (ISM). A uni?ed theory that explains all the observational properties and their relations is desirable but has yet to be found. It is widely accepted that most galaxies follow the tight color-morphology relation: elliptical galaxies are red and quiescent, while spiral galaxies are blue and star-forming. The mechanisms driving the morphological transformation and those quenching the star formation must be related. Red spiral galaxies (RSG), those with a late-type morphology but little star formation, present a rare class of galaxies and challenge our understanding of galaxy evolution. Presumably, the morphological change can be driven by mergers that randomize the orbits of individual stars, while the color change is caused by quenching. The formation of RSG can be explained without mergers if the star formation of a blue spiral galaxy is suppressed by internal processes such as AGN or supernova feedback. Hamabata et al. (2019) found that RSG preferentially reside in infalling objects in galaxy clusters. The authors suggest that RSG represent a temporary phase in the evolution of cluster galaxies: their SFR is suppressed during the infall before reaching the cluster center where their morphology is transformed. However, recent results from the Mapping Nearby Galaxies at APO (MaNGA) survey suggest that RSG may be remnants of gas-rich major mergers before z ∼ 1, allowing its disk to reform, rather than being the evolutionary remnants of blue spirals (Hao et al. 2019). The origin of the RSG could help us understand the tight color-morphology relation and provide insights into the larger picture of galaxy evolution. The cluster environment plays a key role in the SFR and appearance of galaxies (Dressler 1980; Kau?mann et al. 2003). Galaxy clusters with a substantial intracluster medium (ICM) and a high galaxy number density are ideal laboratories to study critical processes such as gas stripping, strangulation, galaxy harassment, and galaxy mergers. X-ray observations, complementing observations in other wavelengths, provide crucial constraints on ISM-ISM collisions and ISM-ICM interactions (Kenney et al. 2008; Sun et al. 2007; Su et al. 2014; Kraft et al. 2017; Randall et al. 2008). In nearby rich clusters, however, we see mostly the end results of these processes: elliptical galaxies that are devoid of star formation, instead of galaxies in transition with those processes at play. The number fraction of star forming cluster galaxies is known to increase with redshift (Butcher & Oemler 1984) and decrease with cluster mass (Bai et al. 2009; Koyama et al. 2010). In distant young clusters, the dense environmental is just starting to in?uence the properties of member galaxies (Dannerbauer et al. 2019; Lee et al. 2019; Noble et al. 2017; 2019). Proto-clusters contain a large fraction of galaxies in transition, providing more opportunities to observe the ongoing processes and allowing us to trace the progenitors of modern elliptical galaxies, including the most massive and luminous galaxies in the Universe: the brightest cluster galaxies (BCG). However, observations of these distant clusters (z ∼ 2) are limited by the ?nite spatial resolution and the lengthy exposure time. The presence of a nearby cluster that is similar to proto- clusters may pose a challenge to the standard model of the structure formation and galaxy evolution in the ΛCDM Universe (Hashimoto et al. 2019), although the tension may be alleviated with a more robust 1 mass measurement provided by X-ray observations. Nevertheless, an analog of proto-clusters in the nearby Universe provides an alternative and more feasible way to probe proto-clusters and galaxies in transition (Cairns et al. 2019). We propose an XMM-Newton observation of a nearby cluster, SDSS-C4 3028 with an unusually large fraction of star forming galaxies. The brightest galaxy in SDSS-C4 3028 is a passive red spiral galaxy (Figure 1). SDSS-C4 3028 could represent a critical but short (therefore rare) phase in both galaxy evolution and cluster evolution. Figure 1: The SDSS-C4 3028 galaxy cluster identi?ed in SDSS DR7. The member galaxies classi?ed as blue and star forming are marked by blue circles, while red quiescent galaxies are marked by red circles. We assume its virial radius is the same as that of the Virgo cluster of 1 Mpc, which is shown with a black dashed circle. The proposed XMM-Newton pointing is marked in the magenta pn ?eld-of-view. The brightest galaxy in SDSS-C4 3028 is a red spiral galaxy with an r-band magnitude of 14.57. We also identi?ed an ongoing collision between two member galaxies in SDSS-C4 3028. B) Immediate Objective: SDSS-C4 3028 is a cluster of galaxies at z=0.061 ?rst identi?ed in the spectroscopic sample of the Second Data Release (DR2) of the Sloan Digital Sky Survey (SDSS) (Miller et al. 2005). Its velocity dispersion of 510 km/s (Hashimoto et al. 2019) is similar to that of the Virgo Cluster (Mei et al. 2007), corresponding to a dark matter halo mass of 2.0 × 1014 M . Hashimoto et al. (2019) studied a volume-limited galaxy sample from the SDSS DR7 with redshifts between 0.02 and 0.082. The authors found that SDSS-C4 3028 contains 12 star forming galaxies among 21 spectroscopically con?rmed member galaxies, which is in sharp contrast to all other clusters in their sample and unexpected from semi-analytical models (Figures 1 and 2a,b). We note that the brightest galaxy in SDSS-C4 3028 is a RSG near the cluster center (Figure 1), which is also extremely atypical. X-ray observations are the best way to probe the mass and dynamical state of a cluster. They would also provide a vital test of whether this RSG is the true BCG of the cluster. We propose an XMM-Newton observation of SDSS-C4 3028 to unveil this cluster full of mysteries. The proposed observation would achieve the following scienti?c goals: 1) Testing the origins of red spiral galaxies and brightest cluster galaxies We will determine whether the giant RSG as shown in Figure 1 is the legitimate BCG of SDSS-C4 3028. For instance, M49, a massive elliptical galaxy falling into Virgo from its southern ourskirts, is brighter than 2 Figure 2: (a) The fraction of the blue star-forming galaxies as a function of the cluster velocity dispersion taken from Hashimoto et al. (2019). SDSS-C4 3028, with a surprisingly large fraction of star forming galaxies is denoted by the encircled star. (b) The same as the left panel but for galaxies from the semi-analytic model of GALACTICUS at z = 0.05 (taken from Hashimoto et al. 2019). The number fraction of star forming galaxies in SDSS-C4 3028 deviates from the theoretical models by 4.7σ. (c) Projected relative velocity distribution of the member galaxies in SDSS-C4 3028. Blue, red, and black histograms show velocity distributions of blue star-forming, red quiescent, and all member galaxies, respectively. The relative velocity of the brightest cluster galaxy – a red spiral galaxy, is marked by the red arrow. M87 in the optical but M49 is obviously not the BCG of Virgo (Su et al. 2019). The real BCG, formed by accretion and mergers with other satellite galaxies falling into the gravitational potential of the cluster, should reside at or near the cluster center (Lin et al. 2004, Bogdan et al. 2018). To this end, we will determine whether this RSG is associated with any X-ray peak. We note that the second brightest galaxy in SDSS-C4 3028 is an elliptical galaxy 3 (206 kpc) away from this RSG. If the X-ray peak is at this elliptical galaxy it would strongly suggest that the galaxy is the true BCG, while this RSG is simply the brightest galaxy in this cluster. It would favor the scenario that RSGs are in a short window between being quenched and being morphologically transformed (Hamabata et al. 2019). If this RSG is the true BCG, it would support that RSG can form from gas rich mergers allowing the disk to reform (Hao et al. 2019). Gas rich galaxies are prevalent in young clusters like SDSS-C4 3028 and its star formation can be quenched later on by AGN feedback. It also implies that a RSG can be the progenitor of a BCG, providing constraints on the formation and evolution of BCG. 2) Studying formation and evolution of galaxies and clusters of galaxies The proposed observation would reveal the basic gas properties and dynamical state of this cluster. It is unclear whether SDSS-C4 3028 is a relaxed cluster or a merging cluster from the radial velocity distribution of its member galaxies as shown in Figure 2c. Although the Dressler-Shectman test (Dressler & Shectman 1988) implies that SDSS-C4 3028 is dynamically relaxed (Hashimoto et al. 2019), clusters with enhanced star formation are more likely to be merging clusters (Stroe et al. 2015; 2017; Cava et al. 2017). The dynamical state of clusters hosting a large fraction of star forming galaxies is critical to understanding the evolutionary links between clusters and galaxies. The presence of such a massive star forming system in the nearby Universe is in tension with the semi- analytical model of structure formation and galaxy evolution as shown in Figure 2b (taken from Hashimoto et al. 2019). The current mass estimate of SDSS-C4 3028 is based on its velocity dispersion, which may be an overestimate for merging systems (Frenk et al. 1996). We can determine if SDSS-C4 3028 is a merging cluster with the proposed observation. Furthermore, we can obtain a more robust mass estimate through its X-ray luminosity, gas mass, and temperature. Since the fraction of star forming galaxies also depends on cluster mass, the discrepancy between SDSS-C4 3028 and the theoretical prediction may be alleviated if its actual mass is smaller than the current estimate. SDSS-C4 3028 may be at a similar evolutionary stage to proto-clusters. Its gas properties revealed by XMM-Newton can provide insights into distant young clusters: do they have a regular X-ray morphology? 3 do they have ample ICM? SDSS-C4 3028 also provides an ideal laboratory to study ongoing galaxy trans- formation processes (Figure 1). We will look for X-ray signatures of shock heated gas from merging galaxies or any stripped tails adjacent to the visible part of a galaxy. 3. Justi?cation of requested observing time, feasibility and visibility SDSS-C4 3028 (z = 0.06) resides at a suitable distance for XMM-Newton with 1 arcmin=68.5 kpc. SDSS-C4 3028 and the Virgo cluster have similar velocity dispersions. We therefore assume the virial radius of SDSS- C4 3028 is the same as that of the Virgo cluster of 1 Mpc (Simionescu et al. 2017). One EPIC pointing covers the entire cluster near the virial radius (∼ 14 ) in all directions. Wang et al. (2014) measured an X-ray ?ux of 6.63 × 10−13 erg/s in the energy band of 0.5–2.0 keV for SDSS-C4 3028 with ROSAT. We assume that the ICM has a temperature of kT = 2 keV and a metallicity of 0.2 Z . Using PIMMS, we expect to obtain a count rate of 0.726 cts/s with EPIC (MOS1,2 and pn) over the entire cluster. Our requested exposure time of 36(×1.4 = 50) ksec1 is driven by the need to obtain at least 30 net counts per arcmin2 near the cluster center to determine the X-ray peak of the cluster, identify infalling substructure, and ?nd features associated with the interactions between member galaxies and the ICM. The same exposure time provides at least 5000 counts from within its half virial radius to measure basic gas properties such as accurate X-ray luminosity, gas mass, temperature, and metallicity and to determine whether it is a relaxed cluster or it is undergoing a merger. The short exposures and the low angular resolution of RASS are inadequate for our proposed study. An XMM-Newton observation of SDSS-C4 3028 is imperative. According to Viewing, our proposed pointing has multiple XMM-Newton orbits with a window duration of 130 ksec per orbit in the coming cycle. 4. Report on the last use of XMM data XMM observations of the M49 group Su et al. 2019, AJ 158, 6 XMM observations of the Fornax cluster Su et al. 2017, ApJ 851, 69 XMM observations of the NGC 1407 group Su et al. 2014, ApJ 786, 152 XMM observations of early-type galaxies Su & Irwin 2013, ApJ 766, 61 5. Most relevant applicant’s publications [1] Su, et al. 2019, AJ 158, 6: “Extended X-ray study of M49: the frontier of the Virgo cluster” [2] Su, et al. 2016, ApJ 821, 40:“Chandra Observation of Abell 1142: A Cool-core Cluster Lacking a Central Brightest Cluster Galaxy?” [3] Stroe et al. 2015, MNRAS, 450, 646: “The rise and fall of star formation in z ∼ 0.2 merging galaxy clusters” [4] Stroe et al. 2017, MNRAS, 465, 2916: “A large Hα survey of star formation in relaxed and merging galaxy cluster environments at z ∼ 0.15 − 0.3” [5] Yan 2018, MNRAS, 481, 467: “Nitrogen-to-Oxygen abundance ratio variation in quiescent galaxies” REFERENCES • Bai et al. 2009 ApJS, 182, 543 • Bogdan et al. 2018 ApJ, 869, 105 • Butcher & Oemler 1984 ApJ, 285, 426 • Cairns et al. 2019 ApJ, 882, 132 • Cava et al. 2017 A&A 606, 108 • Dannerbauer et al. 2019 • Dressler & Shectman 1988 AJ, 95, 985 • Dressler 1980 ApJ, 236, 351 • Frenk et al. 1996 ApJ, 472, 460 • Hamabata et al. 2019 MNRAS, 488, 4117 • Hao et al. 2019 ApJ, 883, 36 • Hashimoto et al. 2019 MNRAS.tmp.2130 (https://arxiv.org/pdf/1908.01666.pdf) • Kau?mann et al. 2003 MNRAS, 341, 33 • Kenney et al. 2008 ApJ, 687, 69 • Koyama et al. 2010 MNRAS, 403, 1611 • Kraft et al. 2017 ApJ, 848, 27 • Mei et al. 2007 ApJ, 655, 144 • Miller et al. 2005 AJ, 130, 968 • Noble et al. 2017 ApJL, 842, L21 • Noble et al. 2019 ApJ, 870, 56 • Lee et al. 2019 ApJ, 883, 92 • Lin et al. 2004, ApJ, 617, 879 • Randall et al. 2008, ApJ, 688, 208 • Stroe et al. 2015 MNRAS, 450, 646 • Stroe et al. 2017 MNRAS, 465, 2916 • Simionescu et al. 2017 MNRAS, 469, 1476 • Su et al. 2014 ApJ 786, 152 • Su et al. 2019 AJ 158, 6 • Sun et al. 2007 ApJ, 671, 190 • Wang et al. 2014 MNRAS, 439, 611 1We follow the standard practice of increasing the exposures by 1.4 to mitigate possible contamination from high particle background periods for faint di?use source. 4