First Results from an HST/ACS Snapshot Survey of Intermediate Redshift, Intermediate X-ray Luminosity Clusters of Galaxies: Early Type Galaxies and Weak Lensing

1
First Results from an HST/ACS Snapshot Survey of
Intermediate Redshift, Intermediate X-ray
Luminosity Clusters of Galaxies Early Type
Galaxies and Weak Lensing
Authors Trombley, Christine (Michigan State
University, East Lansing, MI), Donahue, Megan
(MSU), Bruch, Seth (MSU), Conselice, Christopher
(Nottingham, UK), McNamara, Brian (Waterloo),
Hoekstra, Henk (Univ. Victoria, BC)
Abstract We present first results from our
HST/ACS Snapshot survey of clusters randomly
selected from a complete X-ray survey, the 160
Square Degree Survey (Vikhlinin et al. 1998 ApJ
502, 558 Mullis et al. 2003, ApJ, 594, 514). We
have catalogued and classified over 5,000
galaxies in 25 fields around X-ray clusters of
low to moderate X-ray luminosities (Lx
1043-1044 erg s-1) and spectroscopic redshifts
between 0.3-0.6. We present the morphological
properties of the early-type galaxies, with
particular emphasis on the concentration index.
We compare the concentration parameter of the
early-type galaxies with those of early-type
galaxies in the field at similar redshift, and
with those of early-type galaxies in more massive
and luminous clusters, also at similar redshifts.
We find that early type galaxies in clusters may
be more concentrated than early type galaxies in
the field. We discuss preliminary results in our
comparisons with broader samples. We also present
the first estimates of shear-based mass estimates
for these clusters, derived entirely from the ACS
images.
By-Eye Galaxy Classification
Early Types
Late Types
Solid line shows the histogram of concentration
values for early-type galaxies within a projected
radius of 500 kpc of the brightest cluster
galaxy. The dotted line shows the distribution of
concentration values for galaxies in the same ACS
field (lt 1 virial diameter across) as the
cluster. The dashed line shows the distribution
of concentration values for field early-type
galaxies from 5 high galactic latitude ACS fields
and the Hubble Deep Field (with mi lt 24). The
distributions within a virial radius of the BCG
are indistinguishable from each other, while the
field and cluster galaxies differ statistically
at the 97 confidence level, such that the early-
type cluster galaxies are MORE centrally
concentrated in their light profiles than early
type field galaxies at similar redshift and
magnitude.
Elliptical E/S0 Lenticular
S0/Spiral Spiral Spiral/Irr
Irregular Irr/Disk Disk
Late type galaxies show signs of internal
structure, such as spiral arms or barred features
in the disk. Irregular galaxies do not show
signs of consistent internal structure, whereas
the structure within spiral galaxies is very
easily identifiable as winding arms. Disk type
galaxies could be either spiral or lenticular
galaxies seen edge on. Unless the disk showed
signs of warping, no further classification was
made.
Early type galaxies show very few signs of
internal structure. Lenticular and elliptical
galaxies are hard to distinguish when observed
face-on. Lenticular galaxies are very flat with
a concentrated center, unlike ellipticals, which
are rounded from all observation points.
Try it yourself What do you think?
irregular
S0/spiral
Concentration index C vs. the asymmetry index A
(Bershady, Jangren, Conselice 2000, AJ, 119,
2645 Conselice 2003, ApJS, 147, 1) for galaxies
in three of the clusters in our HST snapshot
survey and for galaxies in the Hubble Deep
Fields. The red circles are the ellipticals in
the 3 clusters, while the black triangles are the
spirals from those same clusters. The blue
crosses are Hubble Deep Field early type
galaxies. Potential merger candidates have A gt
0.35, denoted by the vertical line. This figure
shows that the galaxies in clusters are clearly
more concentrated than galaxies in the field.
elliptical
S0
S0
disk
Plot of the cluster velocity dispersion (as
inferred from the HST weak lensing signal) as a
function of the X-ray luminosity. These
measurements are the results from a preliminary
weak lensing analysis of the 24 clusters observed
in our 160SD ACS snapshot program. The cluster
velocity dispersion was determined by fitting a
singular isothermal sphere model to the observed
tangential shear profile. The solid points
correspond to the HST/ACS measurements of X-ray
selected 160SD clusters. The small open points
are from the Canadian Cluster Comparison Project
(which will have observed 60 clusters, once
complete.) The HST observations extend the range
in X-ray luminosity probed by a almost a factor
10 (most of the CCCP clusters are Lx gt 8 1044
ergs s-1.), and demonstrate a clear relation.
Definitions Asymmetry index is a measure of the
difference between the galaxys image and a
rotated version of that image. Concentration is 5
log (r(80)/r(20)), where the radii are defined
by the percentage of peak surface brightness. For
reference, a r1/4 law C5.2, exponential C2.7,
and Gaussian C2.1.
E/S0
star with spiral
Low Lx (160SD) Cluster z0.05
High Lx (MACS) clusterz0.05
Field Example