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HUBBLE SPACE TELESCOPE

DAILY REPORT        # 3155

PERIOD COVERED: DOYs 122-124

Part 2 of 5

ACS 9476

Galaxy Evolution in the Richest Clusters at z=3D0.8: the EDisCS
Cluster Sample 

The study of distant cluster galaxies requires two key ingredients:
{1} deep high-resolution imaging, to constrain galaxy structure; and
{2} 8m-class spectroscopy, to measure stellar content, star-formation
rates, dynamics, and cluster membership. We will reach both
conditions with the addition of HST/ACS imaging to our suite of VLT
{36 nights} and NTT {20 nights} observations of 10 confirmed clusters
at z~0.8, drawn from the ESO Distant Cluster Survey {EDisCS}. The
proposed HST/ACS data will complement our existing optical/IR
imaging and spectroscopy with quantitative measures of cluster galaxy
morphologies {i.e. sizes and shapes, bulge-disk decompositions,
asymmetry parameters}, and with measurements of cluster masses via
weak lensing. Major advantages unique to the EDisCS project include:
{i} uniform selection of clusters; {ii} large enough sample sizes to
characterize the substantial cluster-to-cluster variation in galaxy
populations; {iii} large quantities of high quality data from 8m ]
telescopes; {iv} uniform measurements of morphologies, spectroscopic
and photometric redshifts, SEDs, star-formation/AGN activities, and
internal kinematics; {v} optical selection of clusters to complement
the X-ray selection of almost all high-z clusters in the ACS GTO
programs; {vi} forefront numerical simulations designed specifically
to allow physical interpretation of observed differences between the
high-z and local clusters. 

ACS 9656

Stability of the ACS CCD: geometry, flat fielding, photometry

A moderately crowded stellar field, located ~6' West of the centre of
the cluster 47 Tuc, is observed repeatedly {every three weeks with
the WFC, every other month with the HRC} in various filters, spending
1 orbit per epoch. Different filters will be used every time, so that
over the course of the year all filters will have been employed at
least twice. The most common filters will be checked more frequently.
The same field has been observed in the course of the SMOV phase and
the positions and magnitudes of the most prominent stars have been
accurately measured. Although the field is neither a proper
astrometric nor a proper photometric standard one, the positions and
magnitudes of the objects in it can be used to monitor any local and
large scale variations in the platescale and sensitivity of the
detectors. It should be noted that for the filters which have already
been used during the SMOV phase it will be sufficient to take one
single image, without CR-SPLIT, since the exposure time is always
short {20-30 sec} and there will be so many stars that the few of
them which are affected by cosmic rays can be discarded as outliers
in the photometry. For narrow and medium band filters not exercised
on this target in the SMOV phase, however, a baseline will have to be
set. This expenditure of time will apply to the current cycle only.
At variance with the approach used in SMOV, there is no need for
large telescope slews to place the same objects on opposite sides of
the detectors, thence allowing the programme to remain compact and
efficient. All exposure level parameters are set to their default
values, except for the amplifier gain of the WFC exposures in the
F606W band, which will be collected with the gain value of 2 for the
WFC for compatibility with the SMOV observations. The exposure time
is typically 30 seconds for the WFC, 60 sec for the HRC. No attempt
will be made to attain a predefined or the same orientation on the
sky amongst different epochs. Typically, for the WFC, five exposures
will be accommodated in one orbit. For the HRC, about 10 exposures
can be fitted within one orbit

ACS 9352

The Deceleration Test from Treasury Type Ia Supernovae at Redshifts
1.2 to 1.6

Type Ia supernovae {SNe Ia} provide the only direct evidence for an
accelerating universe, an extraordinary result that needs a rigorous
test. The case for cosmic acceleration rests on the observation that
SNe Ia at z ~ 0.5 are ~ 0.25 mag fainter than they would be in a
universe without acceleration. A powerful and straightforward way to
assess the reliability of the SN Ia measurement and the conceptual
framework of its interpretation is to look for cosmic deceleration at
z >=3D 1. This would be a clear signature of a mixed dark-matter and
dark-energy universe. Systematic errors in the SN Ia result
attributed to grey dust or cosmic evolution of the SN Ia peak
luminosity would not show this change of sign. We have demonstrated
proof of this concept with a single SN Ia, SN 1997ff at z =3D 1.7,
found and followed by HST. The results suggest an early epoch of
deceleration, but this is too important a conclusion to rest on just
one object. Here we propose to use HST for observations of six SNe
Ia in the range 1.2 <=3D z <=3D 1.6, that will be discovered as a
byproduct from proposed Treasury programs for high-latitude ACS
surveys. Six objects will provide a much firmer foundation for a
conclusion that touches on important questions of fundamental
physics.

ACS/NICMOS 9483

Origin and Evolution of IR Luminous Galaxies: Are z>=3D1 Dusty
Starbursts and z=3D0

ULIRGs the Same?

Interactions and mergers involving gas-rich galaxies are the main
driving mechanism behind the luminous IR galaxy phenomenon. However
it is dangerous to extrapolate this model directly to high redshifts
because massive spiral progenitors may have been relatively uncommon
at earlier epochs. Mergers and interactions involving less massive
but more gas-rich progenitors may have occurred instead. We propose
to test this hypothesis directly by imaging 12 FIR-selected, dusty
starbursts at z~1 at sub-kpc resolution afforded by HST in the rest
frame B and I {observed I and H} bands using ACS and NICMOS. While
studying higher redshift systems is clearly desirable, band-shifting
and surface brightness dimming makes the investigations of tidal
features and the nature of progenitors possible only out to z~ 1
{Hibbard & Vacca 1997}. From the morphologies, surface brightnesses,
and color distribution, we will determine the physical status of the
starburst hosts, the history of tidal interactions/mergers, and the
nature of the progenitors. We will also test for the presence of
hidden AGNs and for enhanced galaxy number density. Our 12 target
galaxies form a complete sample of known ultraluminous and
hyperluminous galaxies at 0.7 * Origin: SpaceBase(tm) Pt 1 -14.4- Van BC Canada 604-473-9358 (1:153/719.1)

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