We combine optical (HST) and UV (GALEX) imaging of two intermediate redshift galaxy clusters with spectroscopy of member galaxies, to study the relation between the formation history of cluster galaxies and the assembly history of the cluster structure itself. We identify key differences in the large-scale structure and intracluster medium properties of each cluster. In order to assess the importance of cluster substructure and the ICM in the evolution of cluster galaxies, we examine several key indicators of the recent star-formation and assembly history of cluster galaxies. We find that galaxies in cluster MS 0451 (z  = 0.54) exhibit a markedly lower incidence of recent star formation activity than galaxies in cluster Cl 0024 (z = 0.39), likely the result of starvation by the ICM. In addition, Cl 0024 members show evidence for kinematic disturbances that can be linked to the assembly of substructure

Ellis, Richard S.

Moran, Sean M.

Smith, Graham P.

Treu, Tommaso

arXiv

Caltech Authors - Main

COSMIC FRONTIERS
ASP Conference Series, Vol. 379, 2007
N. Metcalfe and T. Shanks, eds.
Reflections of Cluster Assembly in the Stellar Populations
and Dynamics of Member Galaxies
Sean M. Moran,1 Richard S. Ellis,1 Tommaso Treu,2 and Graham P.
Smith3
1Caltech, Dept. of Astronomy, MC 105-24, Pasadena, CA 91125
2Dept. of Physics, University of California, Santa Barbara, CA 93106
3University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
Abstract. We combine optical (HST) and UV (GALEX) imaging of two
intermediate redshift galaxy clusters with spectroscopy of member galaxies, to
study the relation between the formation history of cluster galaxies and the
assembly history of the cluster structure itself. We identify key differences in the
large-scale structure and intracluster medium properties of each cluster. In order
to assess the importance of cluster substructure and the ICM in the evolution of
cluster galaxies, we examine several key indicators of the recent star-formation
and assembly history of cluster galaxies. We find that galaxies in cluster MS 0451
(z = 0.54) exhibit a markedly lower incidence of recent star formation activity
than galaxies in cluster Cl 0024 (z = 0.39), likely the result of starvation by the
ICM. In addition, Cl 0024 members show evidence for kinematic disturbances
that can be linked to the assembly of substructure.
1. Introduction
In general, it is well-known that environmental processes play a significant role
in shaping the evolution of galaxies as they assemble onto clusters. With the aid
of Hubble Space Telescope (HST) imaging and deep optical spectroscopy, recent
studies have quantified this evolution in galaxy properties, painting a picture
where the fraction of early-type (elliptical and S0) galaxies and the fraction
of passive non-star-forming galaxies both grow with time, and at a rate that
seems to depend sensitively on the local density of galaxies (Smith et al. 2005b;
Postman et al. 2005; Poggianti et al. 2006).
Yet there are a wide variety of physical processes that may be responsible for
these evolutionary trends–including galaxy mergers, harassment, gas stripping
by the ICM, or tidal processes (e.g. Moore et al. 1999; Fujita 1998; Bekki,
Couch, & Shioya (2002)). Observationally, it has so far been impossible to fully
separate the effects of the various physical processes, in large part due to the
overlapping regimes of influence for each of the proposed mechanisms (see Treu
et al. 2003). Further complicating the picture, the large scale assembly states of
clusters show considerable variety (Smith et al. 2005a), such that the dominant
forces acting on galaxies are likely to vary from cluster to cluster, or over the
course of an individual cluster’s assembly history. But gaining an understanding
of the complex interplay between a variable ICM, the properties of assembling
galaxies, and the overall cluster dynamical state is crucial if we are to have
243
244 Moran et al.
a complete picture of the growth and evolution of galaxies in a hierarchical
universe.
In this contribution, we combine optical (HST) and UV (GALEX) imaging
of two z ∼ 0.5 galaxy clusters with ground-based (Keck) spectroscopy of member
galaxies, in an attempt to better characterize the relation between the formation
history of cluster galaxies and the assembly history of the cluster structure itself.
2. Observations
We make use of HST imaging of Cl 0024 and MS 0451 from the comprehensive
wide-field survey described in Treu et al. (2003) and Smith et al. (in prepa-
ration). In Cl 0024, HST coverage consists of a sparsely-sampled mosaic of 39
WFPC2 images taken in the F814W filter (∼ I band), providing coverage to
a projected radius > 5 Mpc. MS 0451 observations were taken with the ACS,
also in F814W, and provide contiguous coverage within a 10Mpc×10Mpc box
centered on the cluster. For both clusters, reliable morphological classification
is possible to rest frame absolute magnitude MV = −19.5. All galaxies brighter
than this limit are classified visually following Treu et al. (2003).
Cl 0024 and MS 0451 were respectively observed for 15ksec and 80ksec
with GALEX (Martin et al. 2005) in Oct. 2004 (GO-22; Cycle 1; PI Treu),
reaching comparable depths in rest frame FUV (observed NUV). Galaxy fluxes
were measured within 6′′ circular apertures, centered on the optical position,
and comparable to the measured NUV FWHM (5.′′5). Observations with the
DEIMOS spectrograph on Keck II from Oct. 2001 to Oct. 2005 secured spectra
for over 500 members of both Cl 0024 (0.373 < z < 0.402) and MS 0451 (0.520 <
z < 0.560). Details are provided in Moran et al. (2005) and Moran et al. (in
preparation). Briefly, we observe with 1′′ × 8′′ slits, with a typical velocity
resolution of 50 km s−1, covering rest frame wavelengths from ∼ 3500A˚ to ∼
6700A˚. Exposure times totaled 2.5hrs in Cl 0024 and 4hrs in MS 0451. DEIMOS
data were reduced using the DEEP2 data reduction pipeline (Davis et al. 2003).
3. Results
3.1. Global Cluster Properties
Similar in their total mass, the two clusters were chosen for study primarily due
to their complementary X-ray properties (See Table 1). While MS 0451 is one
of the most X-ray luminous clusters known (Donahue et al. 2003), Cl 0024 is
Table 1. Basic Properties of the Clusters
Name RV IR M200 z LX
(Mpc) (M⊙) (L⊙)
Cl 0024 1.7(1) 8.7× 1014 (2) 0.395 7.6× 1010 (3)
MS 0451 2.6 1.4× 1015 (4) 0.540 3.8× 1011 (4)
(1) Treu et al. (2003), (2) Kneib et al. (2003), (3) Zhang et al. (2005), (4) Donahue et al. (2003).
Reflections of Cluster Assembly 245
Figure 1. Distribution of galaxies in the field of Cl 0024 (left) and MS 0451
(right). Galaxies showing spectroscopic signs of recent evolution are marked.
The distributions of passive spirals are quite different between the two clusters.
somewhat under-luminous (Zhang et al. 2005). This implies a large difference
in the density and radial extent of the intracluster medium (ICM) between
the two clusters, such that ICM-related physical processes are expected to be
significantly more important in the evolution of MS 0451 galaxies. In addition,
marked differences in the spatial and redshift distributions of galaxies between
the two clusters indicate that the clusters have quite dissimilar recent assembly
histories. While the distribution of galaxies in MS 0451 is largely smooth and
seemingly well-virialized, Cl 0024 shows obvious signatures of infalling groups
(Treu et al. 2003) and, indeed, a recent cluster-subcluster merger (Czoske et al.
2001). In the following, we examine how such differences in the overall cluster
environment lead to striking differences in the galaxies themselves.
3.2. Passive Spirals
Passive spirals belong to a class of galaxies that show spiral morphology in
HST images, but reveal weak or no [OII] emission in their spectra, suggesting
a lack of current star formation. Through GALEX UV imaging, we find that
passive spirals in Cl 0024 exhibit UV emission nearly as strong as regular star-
forming spirals, implying the presence of young stars (Moran et al. 2006). Their
unusual combination of UV emission with weak Hδ strength supports a picture
where passive spirals have experienced a rapid decline and eventual cessation of
star formation over the last ∼ 1 Gyr. The timescale of this decline implicates
“starvation” by the ICM as the likely cause (Bekki et al. 2002) – a process where
diffuse gas is stripped from a galaxy’s halo, thereby halting any further accretion
of cold gas onto the galactic disk.
In MS 0451, over one third of all spiral galaxies fall into the ‘passive’ cate-
gory, compared to ∼ 24% in Cl 0024. Of these, 1/3 are detected in the UV (c.f.
2/3 in Cl 0024), suggesting that the typical passive spiral in MS 0451 has fewer
young stars than those in Cl 0024. Both effects are likely due to the denser,
more extended ICM in MS 0451, which should boost the efficiency of starvation,
246 Moran et al.
and increase rate of passive spiral creation compared to that of Cl 0024. In fact,
the spatial distribution of passive spirals across both clusters strongly supports
this scenario (Fig. 1). Passive spirals in MS 0451 are spread across a wide area,
as expected in the presence of a dense ICM, while Cl 0024 passive spirals are
largely concentrated near the cluster core, where its ICM is densest.
3.3. Kinematic Disturbances
By constructing the Fundamental Plane (FP) of Cl 0024, we observe that el-
liptical and S0 galaxies (E+S0s) exhibit a high scatter in their FP residuals,
equivalent to a spread of 40% in mass to light ratio (M/LV ) (Moran et al.
2005). Upon closer inspection, this high scatter appears to occur only among
galaxies within 1 Mpc of the cluster core. Outside of this radius, cluster E+S0s
follow a tight FP. In contrast, ellipticals in MS 0451 do not exhibit such a high
scatter, and no radial dependence is seen. It is therefore likely that the recent
merger with a subcluster (Czoske et al. 2001) has disturbed galaxies in the core
of Cl 0024, such that they show significant deviations from the FP.
Similarly, in a recent study of spiral galaxies across both clusters (Moran et
al. 2007), a large scatter about the Tully-Fisher relation is measured, compared
to the field at the same redshift. In Moran et al. (2007), we argue that the large
scatter is likely driven by galaxy–galaxy interactions during cluster infall.
4. Discussion
Taken together, our results paint a picture where the fates of infalling cluster
galaxies depend heavily on the pre-existing assembly state and ICM density of
the cluster. Cluster–subcluster mergers, as seen in Cl 0024, seem to induce
departures from the FP, and cluster spirals show signs of similar disturbances.
Importantly, passive spirals appear to be surprisingly abundant, suggesting that
starvation by the ICM causes a steady decline in star formation rate, driving
the eventual buildup of the cluster red sequence.
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Donahue, M. et al. 2003, ApJ, 598, 190
Fujita, Y. 1998, ApJ, 509, 587
Kneib, J-P. et al. 2003, ApJ, 598, 804
Martin, D. C. et al. 2005, ApJ, 619, L1
Moore, B. et al. 1999, MNRAS, 304, 465
Moran, S. M. et al. 2005, ApJ, 634, 977
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