NASA Hubble Space Telescope Daily Report # 3370 (part 1)

HUBBLE SPACE TELESCOPE

DAILY REPORT # 3370

PERIOD COVERED: DOYs 143-146

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 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 9400

ARE THERE YOUNG GALAXIES IN THE LOCAL UNIVERSE: THE AGE OF THE BLUE
COMPACT
DWARF GALAXY I ZW 18

The question of whether there exists young galaxies in the local universe
is
important for cosmology. Cold Dark matter models predict that low-mass
galaxies
could still be forming at the present epoch. In the hierarchical model of
galaxy
formation, large galaxies result from the merging of smaller structures.
These
primordial building-block galaxies are too faint and small to be studied
at
high
redshifts, while we stand a much better chance of understanding them if
we can
find some local examples. One of the best candidates for being a young
nearby
galaxy forming stars for the first time at the present epoch, is the blue
compact dwarf {BCD} galaxy I Zw 18 because of its extremely low heavy
element
content {2% that of the Sun}. We propose to obtain deep $V$ and $I$ ACS
images
of I Zw 18. Our goal is to detect or put limits on the red giant branch
{RGB}
stellar population in this galaxy. If RGB stars are not detected, then we
can
set an upper limit for the age of I Zw 18 to be less than 1 Gyr. If they
are
detected, I Zw 18 is not young, and the RGB tip can be used to derive its
distance and set limits on the metallicity of the pregalactic gas.

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.

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 9420

Intensive Coverage of the Eta Carinae Event in 2003

For a variety of reasons, HST can provide a very special and unique data
set
when Eta Car experiences its next spectroscopic event in mid-2003.
Explaining
the phenomenon is only part of the motivation. This star and its ejecta
have
unique characteristics that make them important for several branches of
astrophysics; and when a spectroscopic event occurs, it’s like varying
the
parameters in an experiment {or rather, set of experiments}. The 2003
event
will
be the last chance in the forseeable future to obtain such a data set.
Eta
Carinae has extreme parameters; it is mysterious in surprisingly basic
ways; and
HST/STIS can gather useful data on it at a terrific rate. As we explain
below,
the proposed data set will be valuable in several independent ways: It
will
help
solve a specific set of current problems, it will constitute a large and
unique
archival data base for both stellar and nebular astrophysics, and it will
be
well-suited for educational uses.

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 >= 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.