Grants and Contracts Details
Description
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
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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
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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?
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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”
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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.
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Status | Finished |
---|---|
Effective start/end date | 4/4/22 → 4/1/25 |
Funding
- National Aeronautics and Space Administration: $63,451.00
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