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12 May 2003

HUBBLE SPACE TELESCOPE

DAILY REPORT        # 3360

PERIOD COVERED: DOYs 129-131

Part 1 of 5

OBSERVATIONS SCHEDULED

ACS 9472

A Snapshot Survey for Gravitational Lenses among z >= 4.0 Quasars

Over the last few years, the Sloan Digital Sky Survey has
revolutionized the study of high-redshift quasars by discovering over
200 objects with redshift greater than 4.0, more than doubling the
number known in this redshift interval. The sample includes eight of
the ten highest redshift quasars known. We propose a snapshot imaging
survey of a well-defined sample of 250 z > 4.0 quasars in order to
find objects which are gravitationally lensed. Lensing models
including magnification bias predict that at least 4% of quasars in a
flux-limited sample at z > 4 will be multiply lensed. Therefore this
survey should find of order 10 lensed quasars at high redshift; only
one gravitationally lensed quasar is currently known at z > 4. This
survey will provide by far the best sample to date of high-redshift
gravitational lenses. The observed fraction of lenses can put strong
constraints on cosmological models, in particular on the cosmological
constant Lambda. In addition, magnification bias can significantly
bias estimates of the luminosity function of quasars and the
evolution thereof; this work will constrain how important an effect
this is, and thereby give us a better understanding of the evolution
of quasars and black holes at early epochs, as well as constrain
models for black hole formation. 

ACS 9658

ACS Earth Flats

This program will obtain sequences of flat field images by observing
the bright Earth. Several UV filters from the interim calibration
program {9564} require additional exposures to obtain the required
illumination. A few UV filters from this program will be repeated to
monitor for changes in the flat fields and to verify the interim
results. Since no streaks are observed in the UV, the wavelength
coverage is extended to longer wavelengths in order to explore the
severity of streaks in the flats from clouds in the FOV. We have
added exposures for the HRC in the visible filters to verify the
results derived from the L-flat campaign and to explore the severity
of streaks. We have also added exposures on WFC using the minimum
exposure time and using filters which will not saturate the brightest
WFC pixel by more than 10 times the full well. 

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. 

ACS 9463

Are OH/IR stars the youngest post-AGB stars? An ACS SNAPshot imaging
survey

Essentially all well-characterized preplanetary nebulae {PPNs}--
objects in transition between the AGB and planetary nebula
evolutionary phases - are bipolar, whereas the mass-loss envelopes of
AGB stars are strikingly spherical. In order to understand the
processes leading to bipolar mass-ejection, we need to know at what
stage of stellar evolution does bipolarity in the mass-loss first
manifest itself? We have recently hypothesized that most OH/IR stars
{evolved mass- losing stars with OH maser emission} are very young
PPNe. We propose an ACS/SNAPshot imaging survey of a large,
morphologically unbiased sample of these objects, selected using
their IRAS 12-to-25micron colors. Our ground-based imaging study of
OH/IR stars has revealed a few compact bipolar objects, supporting
our hypothesis. However since most objects remain unresolved, HST
observations are needed to determine how and when the bipolar
geometry asserts itself. Our complementary program of interferometric
mapping of the OH maser emission in our sources is yielding kinematic
information with spatial resolution comparable to that in the HST
images. The HST/radio data will provide crucial input for theories of
post-AGB stellar evolution. In addition, these data will also
indicate whether the multiple concentric rings, ``searchlight
beams'', and truncated equatorial disks recently discovered with HST
in a few PPNs, are common or rare phenomena.

ACS 9674

CCD Daily Monitor

This program consists of basic tests to monitor, the read noise, the 
development of hot pixels and test for any source of noise in ACS CCD
detectors. This programme will be executed once a day for the entire
lifetime of ACS. 

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. 

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