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04 Apr 2003

HUBBLE SPACE TELESCOPE

DAILY REPORT        # 3332

PERIOD COVERED: DOY 91

Part 2 of 4
ACS 9470

Deep Lyman alpha images of starburst galaxies

We propose a pilot study to obtain deep ACS Ly-alpha images of a
carefully selected sample of local starburst galaxies. Ly-alpha
imaging of such objects has become feasible with ACS. These
observations will bring unprecedented insight into the processes
regulating the luminosity of the cosmologically important Ly-alpha
line. Our targets cover the full range of observed Ly-alpha
properties. They have been chosen to investigate the effects of dust,
the starburst luminosity, and outflows within the ISM. The sample is
optimized for a most favorable trade-off between the relevant
parameter space and the minimum number of orbits. We can build on
this pilot study for a much larger follow-up survey in a later cycle.
Deep H Alpha and H Beta high-resolution images from the ground will
allow us to quantify the Ly-alpha emission {or its lack} and its
attenuation as a function of the local dust content. The H Alpha
emission at a given location will constrain the intrinsic intensity
of the expected Ly-alpha emission. A non-detection will allow us to
quantify the fraction of Ly- alpha photons destroyed within the
neutral gas, and to correlate this fraction with the properties of
the neutral gas and dust. Since Ly-alpha emitters are used to
identify and study galaxies at redshift 3-6, our study will document
the circumstances when a star--forming galaxy shows Ly-alpha
emission. This will be an important first step towards an empirical
calibration of the relation between Ly-alpha and the star--formation
rate. 

ACS 9476

Galaxy Evolution in the Richest Clusters at z=0.8: 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 9480

Cosmic Shear With ACS Pure Parallels

Small distortions in the shapes of background galaxies by foreground
mass provide a powerful method of directly measuring the amount and
distribution of dark matter. Several groups have recently detected
this weak lensing by large-scale structure, also called cosmic shear.
The high resolution and sensitivity of HST/ACS provide a unique
opportunity to measure cosmic shear accurately on small scales. Using
260 parallel orbits in Sloan textiti {F775W} we will measure for the
first time: beginlistosetlength sep0cm setlengthemsep0cm setlength
opsep0cm em the cosmic shear variance on scales <0.7 arcmin, em the
skewness of the shear distribution, and em the magnification effect.
endlist Our measurements will determine the amplitude of the mass
power spectrum sigma_8Omega_m^0.5, with signal-to-noise {s/n} ~ 20,
and the mass density Omega_m with s/n=4. They will be done at small
angular scales where non-linear effects dominate the power spectrum,
providing a test of the gravitational instability paradigm for
structure formation. Measurements on these scales are not possible
from the ground, because of the systematic effects induced by PSF
smearing from seeing. Having many independent lines of sight reduces
the uncertainty due to cosmic variance, making parallel observations
ideal. 

ACS 9482

ACS Pure Parallel Lyman-Alpha Emission Survey {APPLES}

Ly-alpha line emission is an efficient tool for identifying young
galaxies at high redshift, because it is strong in galaxies with
young stars and little or no dust --- properties expected in galaxies
undergoing their first burst of star- formation. Slitless
spectroscopy with the ACS Wide-Field Camera and G800L grism allows an
unmatched search efficiency for such objects over the uninterrupted
range 4 <~ z <~ 7. We propose the ACS Pure Parallel Ly-alpha Emission
Survey {``APPLES''}, to exploit this unique HST capability and so
obtain the largest and most uniform sample of high redshift Ly-alpha
emitters yet. Parallel observations will allow this survey to be
conducted with minimal impact on HST resources, and we will place
reduced images and extracted spectra in the public domain within
three months of observation. We aim to find ~ 1000 Ly-alpha emitters,
5 times the biggest current sample of Ly-alpha emitters. This
unprecedented sample will provide robust statistics on the
populations and evolution of Ly-alpha emitters between redshifts
4--7; a robust measurement of the reionization redshift completely
independent of the Gunn-Peterson trough; spatial clustering
information for Ly-alpha emitters which would let us probe their bias
function and hence halo mass as a function of redshift; many galaxies
at redshift exceeding 6; and lower redshift serendipitous
discoveries.

NICMOS 9485

Completing A Near-Infrared Search for Very Low Mass Companions to
Stars within 10 pc of the Sun

Most stars are fainter and less massive than the Sun. Nevertheless,
our knowledge of very low mass {VLM} red dwarfs and their brown dwarf
cousins is quite limited. Unknown are the true luminosity function
{LF}, multiplicity fraction, mass function, and mass-luminosity
relation for red and brown dwarfs, though they dominate the Galaxy in
both numbers and total mass. The best way to constrain these
relations is a search for faint companions to nearby stars. Such a
search has several advantages over field surveys, including greater
sensitivity to VLM objects and the availability of precise parallaxes
from which luminosities and masses can be derived. We propose to
complete our four-filter NICMOS snapshot search for companions to
stars within 10 pc. With a 10 sigma detection limit of M_J ~ 20 at 10
pc, we can detect companions between 10 and 100 AU that are at least
9 mag fainter than the empirical end of the main sequence and at
least 6.5 mag fainter than the brown dwarf Gl 229B. When completed,
our search will be the largest, most sensitive, volume-limited search
for VLM companions ever undertaken. Our four-filter search will
permit unambiguous identification of VLM-companion candidates for
follow-up observation. Together with IR speckle and deep imaging
surveys, our program will firmly establish the LF for VLM companions
at separations of 1-1000 AU and the multiplicity fraction of all
stars within 10 pc. 
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