----- Submission 1 (Tue May 20 11:02:20 GMT 2003) -----
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view the electronic version of this submission. ----- Submission 1 (Fri Jan 24 14:26:50 HST 2003) -----
Assigned ID: 1191
Successfully submitted: Fri Jan 24 14:26:58 HST 2003
----- Submission 2 (Fri Jan 24 14:29:09 HST 2003) -----
Successfully submitted: Fri Jan 24 14:29:16 HST 2003
Life in the fast lane: The dark-matter distribution in the most massive galaxy clusters in the Universe at z>0.5We propose two-filter ACS observations of a complete sample of 12 very X-ray luminous galaxy clusters at 0.5<z<0.7 as a cornerstone of a comprehensive multi-wavelength study of the properties of the most massive clusters in the universe. Our sample includes the famous systems Cl0016+16 and MS0451-03; all other clusters are new discoveries from the MACS survey. Being the counterparts of the best-studied systems at lower and higher redshift and comprising ALL massive clusters at 0.5<z<0.7 observable from Mauna Kea this sample will become the ultimate reference for cluster studies at z>0.5. HST's unique capabilities will allow us to: 1) measure accurately the clusters' dark matter distribution on scales from tens to more than 500/h_50 kpc from observations of strong and weak gravitational lensing, 2) use galaxy-galaxy lensing to measure the shape, extent, and mass content of the dark-matter halos of both cluster and field galaxies, and 3) study the color morphology of mergers and the star formation history of galaxies in a high-density environment. The proposed observations are complemented by Chandra observations of all our targets (all 12 awarded, 11 executed to date) which provide independent constraints on the dark matter and gas distribution in the cluster cores, as well a by extensive groundbased observations of weak lensing on yet larger scales, galaxy dynamics, and the SZ effect.PROPOSED OBSERVATIONS
We propose moderately deep ACS observations in two passbands (two
orbit in each of F555W and F814W) of a complete subsample of the 12
most distant MACS clusters at 0.5<z<0.7 as a cornerstone of our
comprehensive, quantitative, multi-waveband study of the properties of
massive distant clusters. The depth of the proposed observations is
dictated by the requirement for an accurate measurement of the
gravitational shear field; resolved color information is critical for
secure identifications of all faint, multiply imaged background
objects and thus an unambiguously determined lens model, as well as
for the proposed detailed study of the cluster galaxy population and
its evolution.
We expect MACS to become the sample of choice for future studies of
massive clusters at intermediate redshift 0.3<z<0.7 and stress the
heritage character of the observations proposed here which will prove
useful to a wider community for cluster-related studies.
SUPPORTING OBSERVATIONS
X-ray
Thanks to an investment of several hundreds of ks of
Chandra GTO time by Leon Van Speybroeck all 12 proposed
clusters have been or will be observed with ACIS-I, yielding superb
X-ray data for this best-defined sample of massive distant clusters
ever (see Fig.~5).
Groundbased optical
The proposed HST observations are supported
and complemented by observations with Subaru, Gemini, and Keck. We are
obtaining multi-object spectroscopy (MOS) of the galaxies in MACS
clusters to assess the prevalence of velocity substructure. MOS data
for 9 of the 12 MACS clusters at z>0.5 are in hand, and yield
velocity dispersions ranging from 820 to 1700 km/s, time to
complete the sample has been awarded on Gemini/GMOS. In addition, we
are conducting a wide-field VRIz' imaging survey of MACS clusters with
Subaru/SuprimeCam to constrain the large-scale mass distribution
beyond the ACS field of view via weak lensing, and to obtain global
colours for galaxies in these regions. SuprimeCam data for eight of
our 12 most distant MACS clusters are in hand (e.g. Fig.~4); the full
sample will be completed using three additional SuprimeCam nights in
the coming semester.
Sub-mm and radio
We are collaborating with John Carlstrom's
group to secure detections of the Sunyaev-Zel`dovich effect in MACS
clusters with the OVRO and BIMA mm arrays. Strong detections have been
obtained for all 8 clusters at z>0.5 observed so far (LaRoque et
al. 2003).
TECHNICAL JUSTIFICATION
Our primary goal is to map in detail the dark matter distribution in
these clusters. The ACS field extends to a median radius of 120'' or
~0.9/h50 Mpc at z=0.5-0.7. The high masses
expected for these X-ray luminous systems predict typical shears of
20% at this radius. Our analysis of our Subaru data imaging (see
Fig.~4) confirm that ground-based data can measure the radially
averaged shear on these scales, but can not provide the
high-resolution mass maps essential for comparing the dark matter and
intracluster gas on ~100 kpc scales in the cluster cores.
The limiting shear necessary per resolution element in our mass map
determines the required depth. The rms error of the shear
averaged over the shapes of galaxies in a patch of sky is
sigma_gamma ~ \sigma_epsilon/sqrt(N), where
sigma_epsilon ~ 0.25 is the rms intrinsic ellipticity
of field galaxies and N is the number per patch. A 3sigma
detection on 20'' (~150 kpc) scales of the shear at the edge of
our field thus requires a sensitivity of sigma_gamma ~ 7%
(the systematic errors due to the HST optics can be removed to a
negligible level compared to this, Rhodes et al. 2000; Hoekstra et
al. 1998). This equates to a galaxy surface density of 150/arcmin^2, or a limiting magnitude of I=25.5, for clusters at z~0.5-0.7, after accounting for 30% of field galaxies lying
in front of the clusters (Kneib et al. 1996).
Based on our previous lensing work with HST we estimate that we need
to measure the shapes of individual galaxies to better than 20%
accuracy to be dominated by their intrinsic ellipticity distribution.
This requires that 10 pixels across the galaxy are detected above a
S/N=1 per pixel threshold. With 4 WFC pixels averaged to produce a
0.1''x 0.1'' superpixel, and for galaxies at I=25.5, with
scale sizes of r_hl ~ 0.25'', this corresponds to mu_F814W=25.7 mag/arcsec^2. For a 1sigma detection (per
2x2 WFC pixel) of this isophote using F814W, we calculate an
exposure time of 5000 s using the WWW exposure tool. This exposure
time translate into 2 orbits using LOW-SKY (to reduce Earth-shine) and
hence a visibility of 2800 sec for a typical target. We will dither
between exposures to cover the gaps between the WFC chips and also to
reduce the effects of flatfield and hot pixel errors. Customized
software (ImCat) designed to take into account the HST PSF
(Kaiser 2000) is in place for a comprehensive analysis of the observed
shear field.
The F814W observations will probe the restframe morphology of the
cluster members longward of 4000A. Shortwards of 4000A we
complement these with observations in F555W to study the internal
color structure of the cluster members and most importantly to use
colors to confirm the identity of multiply imaged background sources
in the crowded cluster cores. To achieve the latter we require
photometry to 20% precision at the limit of our arclet catalog,
I=25.5, or V=26.5 using the typical color at this depth. For a
5-sigma detection of V=26.5 in F555W within a 1'' aperture,
this requires 5600 s and hence 2 orbits per cluster (again based on
LOW-SKY).COSMOLOGY0false0.0falsefalsefalsefalsefalseSUPPORTEDCLUSTER OF GALAXIESHIGH REDSHIFT CLUSTERRICH CLUSTERJ200000 00 0.0000+00 00 0.00J2000ACS/WFCImagingNoNoF555WACS/WFCImagingNoNoF814WACS/WFCImagingNoNoF555WACS/WFCImagingNoNoF814WACS/WFCImagingNoNoF555WACS/WFCImagingNoNoF814WACS/WFCImagingNoNoF555WACS/WFCImagingNoNoF555WACS/WFCImagingNoNoF814WACS/WFCImagingNoNoF555WACS/WFCImagingNoNoF814WACS/WFCImagingNoNoF555WACS/WFCImagingNoNoF814WACS/WFCImagingNoNoF555WACS/WFCImagingNoNoF814WACS/WFCImagingNoNoF555WACS/WFCImagingNoNoF814WACS/WFCImagingNoNoF555WACS/WFCImagingNoNoF814WACS/WFCImagingNoNoF555WACS/WFCImagingNoNoF814WACS/WFCImagingNoNoF555WACS/WFCImagingNoNoF814WPOS-TARGDITHERDITHER33.0116-position ditherACS-WFC-DITHER-LINEtruetrue0.138285.28POS-TARGLINE32.991true
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