Status Report

NASA Hubble Space Telescope Daily Report # 3486 (part 1)

By SpaceRef Editor
November 10, 2003
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ACS 9984

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.

WFPC2 9964

Dynamical Masses of White Dwarfs from Resolved Sirius-Like Binaries

In Cycle 8 we initiated a WFPC2 snapshot survey for resolved,
“Sirius-like” systems containing hot white-dwarf companions of
cooler main-sequence stars. Out of 17 systems observed to date, 8 have
been resolved with WFPC2 by using UV filters. Two of the resolved
systems—56 Persei and Zeta Cygni—have predicted or known orbital
periods short enough that dynamical masses can be determined for the
white dwarfs within reasonable times. These would thus add to the
extremely small number of white dwarfs presently having accurately and
directly measured masses. We propose to image them annually in the UV
with WFPC2. In addition, we will observe Zeta Cyg with FGS in order to
measure the absolute motion of the optical component, needed for the
mass solution. We also propose to observe Sirius itself with WFPC2
over the next 3 Cycles. The resulting astrometric data will not only
greatly improve the precision of the binary orbit and the dynamical
mass measurements for both the main-sequence and white-dwarf
components, but will also test definitively for the claimed presence
of a third body in this famous system.

ACS/WFC 9892

H-alpha Snapshots of Nearby Galaxies observed in F300W: Quantifying
Star Formation in a Dusty Universe

Previous studies of nearby galaxies show large discrepancies between
different star formation {SF} indicators on large {>100 pc, or even
global} scales: the strikingly complex interplay of young stars, dust
and ionized gas are the primary cause of this variance. The few
galaxies in the HST Archive with both WFPC2 H-alpha and mid-UV {F255W
or F300W} imaging show this complex geometry extending down to <10 pc scales. We propose a SNAPshot survey in the ACS/WFC H-alpha filter of 48 galaxies of all Hubble types, that are nearby but beyond the Local Group, and that were previously imaged with WFPC2 in the mid-UV and in F814W. We aim to provide a benchmark for understanding the SF processes in both normal and star-bursting galaxies, at spatial resolutions unattainable from the ground for a large and varied galaxy sample. These data can be applied to a wide range of astrophysical problems and will, therefore, be made public immediately. Our science goals are to: {1} spatially resolve the dust clouds and filaments which strongly affect mid-UV and H-alpha derived SF rates, {2} test how the large-scale correlation between H-alpha and mid-UV flux breaks down on pc scales, and {3} model the propagation of star formation by comparing the SF over time scales of ~100 Myr {via mid-UV} and ~5 Myr {via H-alpha}. This will {4} significantly improve our insight into, and calibration of SF in UV-bright galaxies at high z, and into the cosmic SF history.

FGS 9883

Parallaxes of Extreme Halo Subgiants: Calibrating Globular Cluster
Distances and the Ages of the Oldest Stars

The ages of the oldest stars are a key constraint on the evolution of
our Galaxy, the history of star formation, and cosmological models.
These ages are usually determined from globular clusters. However, it
is alternatively possible to determine ages of extreme Population II
subgiants in the solar neighborhood based on trigonometric parallaxes,
without any recourse to clusters. This approach completely avoids the
vexing issues of cluster distances, reddenings, and chemical
compositions. There are 3 known nearby, extremely metal-deficient Pop
II subgiants with Hipparcos parallax errors of 6-11% which are
available for such age determinations. At present, based on the latest
isochrones, the derived ages of these stars {HD 84937, HD 132475, and
HD 140283} are all close to 14 Gyr, uncomfortably close to or higher
than current estimates of the age of the universe. However, the errors
in the Hipparcos parallaxes imply uncertainties of at least 2 Gyr in
the ages of the 3 stars. We propose to measure parallaxes of these
three Pop II subgiants using HST’s Fine Guidance Sensor 1R. We expect
to reduce the Hipparcos parallax error bars by factors of 5-6,
providing the most stringent test yet of current theoretical stellar
models of Pop II stars and pushing the age uncertainties to below 0.5
Gyr. These data will also provide a major new constraint on the
distance scale of globular clusters, with wide implications for
stellar evolution and the calibration of Pop II standard candles.

NIC2 9875

The Fundamental Plane of Massive Gas-Rich Mergers

We propose deep NICMOS H-band imaging of a carefully selected sample
of 33 luminous, late-stage galactic mergers. This program is part of a
comprehensive investigation of the most luminous mergers in the nearby
universe, the ultraluminous infrared galaxies {ULIGs}. The
high-resolution HST images will complement an extensive set of
ground-based data that include long-slit NIR spectra from a recently
approved Large VLT Programme. This unique dataset will allow us to
derive with unprecedented precision structural -and- kinematic
parameters for a large unbiased sample of objects spanning the entire
ULIG luminosity function. These data will refine the fundamental plane
of massive gas-rich mergers and enable us to answer the following
questions: {1} Do ultraluminous mergers form elliptical galaxies, and
in particular, giant ellipticals? {2} Do ULIGs evolve into optically
bright QSOs? The results from this detailed study of massive mergers
in the local universe will be relevant to understanding galaxy
formation and evolution at earlier epochs, and in particular, the
dusty sub-mm population that accounts for more than half of the star
formation at z > 1.

NIC/NIC3 9865

The NICMOS Parallel Observing Program

We propose to continue managing the NICMOS pure parallel program.
Based on our experience, we are well prepared to make optimal use of
the parallel opportunities. The improved sensitivity and efficiency of
our observations will substantially increase the number of
line-emitting galaxies detected. As our previous work has
demonstrated, the most frequently detected line is Halpha at

ACS/WFC 9860

ESSENCE: Measuring the Dark Energy Equation of State

The accelerating universe appears to be dominated by a dark energy
with a significant negative pressure. The ratio of the pressure to
density of this mysterious energy {its equation of state} is an
observable which can differentiate between the proliferating candidate
theories. We propose to estimate the dark energy equation of state by
observing Type Ia supernovae at redshifts near z=0.7 with HST in
concert with the on-going ESSENCE NOAO Survey program that is
discovering and studying supernovae between 0.3


Whirling Dervish Dynamos: Magnetic Activity in CV Secondaries

The mass-losing secondary stars of cataclysmic variables {CVs} are the
most rapidly rotating cool dwarfs observable. Other rapid rotators
show a maximal, “saturated” level of magnetic activity {e.g., X-ray
emission}, but there are hints from contact binaries and young
clusters that activity may be suppressed at the highest rotation
rates. CV secondaries are thus important probes of magnetic dynamos at
rotational extremes. Implications for CV evolution {e.g., the “period
gap”, accretion variability} may also be profound. Unfortunately,
study of CV secondaries is hampered by pesky accretion-related
phenomena and reflection effects. As a result, little systematic work
has been done. To explore activity in these stars, we therefore
propose to study far-UV spectra of AM Her-type systems {which have no
accretion disks} in deep photometric minima in which accretion is shut
off. Magnetic-related emission from the secondary will be separated
{in velocity} from residual accretion emission by observations near
quadratures. Lower chromospheric irradiation due to the white dwarf
primary will be removed by modeling, yielding the true level of
magnetic activity on the secondary. We will compare the results to
other dMe stars and draw implications for magnetic dynamos and
activity at rotational extremes, and for CV evolution and behavior.

NIC1 9843

NICMOS Observations of Cool Brown Dwarf Doubles

We propose to use NICMOS to observe two brown dwarf systems discovered
using HST/WFPC2 imaging. Each of the two late-L dwarf primaries has a
secondary that is much fainter. Based on the limited optical
photometry available, the secondaries lie at the L/T Dwarf transition,
and may lie anywhere in the range from L9 {cooler than any known L
dwarf} or early-T. NICMOS photometry will be used to characterize the
spectral energy distribution and search for methane absorption.

SpaceRef staff editor.