of the Intergalactic Globular Cluster Population in Abell 
1185

We request deep V and I observations with ACS to examine the 
properties of a newly discovered population of intergalactic globular 
clusters in the core of the rich galaxy cluster Abell 1185. Our 
previous WFPC2 observations of this field {GO-8164} revealed an excess 
of five times the number of objects at the expected magnitudes of 
globular clusters compared to the Hubble Deep Fields. The colors and 
luminosity function of these intergalactic globular clusters will
place strong constraints on their origin, which in turn will yield new 
insights to the evolution of galaxy populations in dense environments

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 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.

(continued)
---