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24 March 2003

HUBBLE SPACE TELESCOPE

DAILY REPORT       # 3325

PERIOD COVERED: DOYs 80-82

Part 2 of 6

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 9395

Is Bulge Formation Still Going-On? An ACS Survey of Pseudo-Bulges

Pseudo-bulges, i.e., bulges with an exponential light profile, have
been unveiled in the centers of many intermediate-type disks. Their
structural similarity with the disks provides support to theoretical
scenarios in which bulges may form due to secular evolution processes
within the host disks. If at play, these processes would likely be
active throughout a large fraction of cosmic history down to our
days: `young' bulges should exist. Our previous HST WFPC2 and NICMOS
survey of ~100 spirals has provided V-H colors for 11 Sb-Sc
pseudo-bulges, and these could be interpreted as suggestive of
relatively young stellar ages. Furthermore, dense nuclei have been
discovered in these pseudo-bulges, and their V-H colors may imply
stellar masses sufficiently large to activate the formation of the
pseudo- bulge by means of dynamical dissolution of progenitor bars.
However, the V-H color, on its own, is fully degenerate towards
stellar ages, metallicities and masses, as well as dust content. We
therefore propose to use ACS to observe the 11 pseudo-bulges of our
combined WFPC2 and NICMOS sample in the F330W, F435W, and F814W
filters. Extending the wavelength baseline to the bluer passbands is
essential to break the mass-age-metallicity-dust degeneracy, and will
provide far more accurate estimates for the stellar population
properties of the pseudo-bulges and their nuclei. Proving the
existence of `young' bulges in the local Universe would have a big
impact in our understanding of the formation of the Hubble sequence.

ACS 9401

The ACS Virgo Cluster Survey

We propose the most comprehensive imaging survey to date of
low-redshift, early-type galaxies. Our goal is to exploit the
exceptional imaging capabilities of the ACS by acquiring deep
images --- in the SDSS g^ and z^ bandpasses --- for 163 E, S0, dE,
dE, N and dS0 galaxies in Virgo, the nearest rich cluster. This
extraordinary dataset would likely constitute one of the principal
legacies of HST, and would have widespread applications for many
diverse areas of astrophysics. Our immediate scientific objectives
are threefold: {1} measure metallicities, ages and radii for the many
thousands of globular clusters {GCs} in these galaxies, and use this
information to derive the protogalactic mass spectrum of each
galaxy; {2} measure the central luminosity and color profile of each
galaxy, and use this information to carry out a completely
independent test of the merging hierarchy inferred from the GCs, with
the aid of N-body codes that simulate the merger of galaxies
containing massive black holes; and {3} calibrate the z^ -band SBF
method, measure Virgo's 3-D structure, and carry out the definitive
study of the GC luminosity function's precision as a standard candle.
Our proposed Virgo Cluster Survey will yield a database of
unprecedented depth, precision and uniformity, and will enable us to
study the record of galaxy and cluster formation in a level of detail
which will never be possible with more distant systems.

ACS 9440

The Composition of Io's Pele Plume

We propose to determine the composition of Io's largest volcanic
plume, Pele, with unprecedented accuracy. This will give us new
constraints on the temperatures, pressures, and magma composition of
this volcano, and thus an improved window into Io's interior. We will
use the proven Jupiter transit spectroscopy technique, which resulted
in the discovery of S_2 gas in the Pele plume, but will use exposures
that are 4 times longer than in the discovery observations. This will
allow us to accurately measure plume SO_2 abundances, seen only with
low S/N in the discovery observations, and possibly SO, in addition
to S_2, and gives the chance to discover other, currently unknown,
plume components. We will also use ACS to obtain UV and visible
images of the Pele plume in reflected light prior to Jupiter transit,
to constrain the dust abundance and particle size in the plume. This
will allow refined estimates of plume dust/gas ratios and resurfacing
rates. We will repeat the observations four times to build up S/N to
even higher levels, and to look for time variability in both dust and
gas abundance and chemistry. 

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 >= 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 = 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 <= z <= 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.

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