Status Report

HST Daily Report # 3283 (Part 1)

By SpaceRef Editor
January 21, 2003
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HUBBLE SPACE TELESCOPE

DAILY REPORT        # 3283

PERIOD COVERED: DOYs 17-20

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 9468

ACS Grism Parallel Survey of Emission- line Galaxies at Redshift z pl 7

We propose an ACS grism parallel survey to search for emission-line
galaxies
toward 50 random lines of sight over the redshift interval 0 < z pl 7. We request ACS parallel observations of duration more than one orbit at high galactic latitude to identify ~ 300 HAlpha emission-line galaxies at 0.2 pl z pl 0.5, ~ 720 O IILambda3727 emission-line galaxies at 0.3 pl z pl 1.68, and pg 1000 Ly-alpha emission-line galaxies at 3 pl z pl 7 with total emission line flux f pg 2* 10^-17 ergs s^-1 cm^-2 over 578 arcmin^2. We will obtain direct images with the F814W and F606W filters and dispersed images with the WFC/G800L grism at each position. The direct images will serve to provide a zeroth order model both for wavelength calibration of the extracted 1D spectra and for determining extraction apertures of the corresponding dispersed images. The primary scientific objectives are as follows: {1} We will establish a uniform sample of HAlpha and O II emission-line galaxies at z<1.7 in order to obtain accurate measurements of co-moving star formation rate density versus redshift over this redshift range. {2} We will study the spatial and statistical distribution of star formation rate intensity in individual galaxies using the spatially resolved emission-line morphology in the grism images. And {3} we will study high-redshift universe using Ly-alpha emitting galaxies identified at z pl 7 in the survey. The data will be available to the community immediately as they are obtained.

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 9673

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 9578

Cosmic Shear With ACS Pure Parallels. Targeted Portion.

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 i {F775W} we will measure for the first time: the cosmic shear variance on scales <0.7 arcmin, the skewness of the shear distribution, and the magnification effect. Our measurements will determine the amplitude of the mass power spectrum sigma_8 Omega_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 9427

Globular Cluster Systems in Supergiant E Galaxies

he most populous globular cluster systems are found in the giant ellipticals at the centers of galaxy clusters {Brightest Cluster Galaxies}. It is among this BCG class that the characteristics of globular cluster systems display their widest range in metallicity distribution and specific frequency and thus confront galaxy formation models with their most varied challenges. Were these systems built primarily by early in situ conversion of gas clouds, by later mergers, or by ongoing accretions of dwarfs? Their extensive halo cluster systems contain many clues to these relic events. BCGs are the biggest and rarest type of galaxy, and to date the globular clusters in only two BCGs {M87, NGC 1399} are well studied. The ACS camera now brings many more such systems within reach. We propose to image the globular cluster systems in 13 BCGs in the distance regime cz ~eq 2000 – 5000 km s^-1. With deep {B, I} exposures we will measure the globular cluster metallicity distribution functions, specific frequency, radial distributions, and luminosity distributions, as well as correlations among these quantities. This work will be the first comprehensive, homogeneous deep survey of globular clusters in BCGs. The superior area and sensitivity of ACS will yield a gain of a factor of eight over previous WFPC2 studies for sample size and metallicity discrimination.

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 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 9450

The lensing galaxy of JVAS B0218+357: determination of H_0

Much effort has been devoted to estimating Hubble’s constant H_0 using observations of very nearby objects. Gravitational lensing time delays offer potentially the most accurate method for determining H_0 using observations on cosmological scales; it is a very clean method in that little complicated astrophysics is involved, and it is a single–step method compared to the traditional multi–step distance ladder. The major problem with most such determinations in the past has been systematic errors due to uncertainties in the lens mass model, leading to 20 Einstein-ring lens system, is the one system for which these systematic uncertainties can be reduced very substantially, and in particular is unique in that the modeling systematics can be reduced to the level of the uncertainties in the measurement of the time delay. The only requirement left is to be able accurately to locate the center of the lensing galaxy. We propose an extremely deep ACS image in I-band of this system for this purpose; the prize is a robust 5 {lens mass model}. We have conducted simulations to estimate the necessary S:N ratio in an ACS observation in order to be able to achieve a successful deconvolution of the lens galaxy and lensed images with the required accuracy.

ACS 9454

The Nature of the UV Continuum in LINERs: A Variability Test

LINERs may be the most common AGNs, and the signposts of accretion onto the massive black holes present in most galaxies. However, the LINER spectrum is the result of UV excitation, and, in at least some LINERs, a nuclear cluster of hot stars, rather than an AGN, dominates the energetics in the UV. Thus, it is still unknown if the UV continuum, or the optical emission lines it excites, have anything to do with an AGN. The demographics and accretion physics of low-luminosity AGNs hinge on this question. We propose to search for variability in a sample of 17 LINERs with compact UV nuclei. Variability can reveal an AGN component in the UV continuum, even when its light is not dominant. We will test systematically the handful of non-definitive reports of UV variability, and potentially quantify the AGN contribution to the UV emission. Variability in all or most objects will be strong evidence that LINERs mark dormant AGNs in most galaxies. Alternatively, a general null detection of variability will suggest that, even in LINERs with additional AGN signatures, the UV continuum is stellar in origin. Contemporaneous monitoring with the VLA/VLBA of 11 objects which have radio cores {five of which we already know are radio-variable} will reveal the relations between UV and radio variations. The UV-variable objects will be targeted for future, better-sampled, monitoring.

SpaceRef staff editor.