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06 May 2003

HUBBLE SPACE TELESCOPE

DAILY REPORT # 3356

PERIOD COVERED: DOY 125

Part 2 of 4

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. 

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/WFC/HRC 9477

TRGB Distance to the Maser Galaxy NGC4258

Masers surrounding the nucleus of the galaxy NGC 4258 have been used
to derive a geometric distance to this galaxy from proper motions and
accelerations of individual sources assumed to be in Keplerian
orbits. This provides us with a unique opportunity to test the zero
point of the Cepheid Period- Luminosity relation in specific, and the
extragalactic distance scale in general. A Cepheid distance has also
been determined to NGC 4258 by us following the exact methodology
adopted by the HST Key Project on the Extragalactic Distance Scale.
The two answers differ, with the maser distance being smaller by 12
We are proposing to use a third and totally independent means to
assess the sense and significance of the difference that may need to
be applied to the extragalactic distance scale. By using the tip of
the red giant branch {TRGB} method we will obtain a Population II
distance measurement to the maser galaxy NGC 4258 good to a precision
of better than 10 Resolving this issue is important because if the
maser distance is adopted and used to recalibrate the HST Cepheid Key
Project distances, then the Hubble constant increases from H_o = 70
to 80 km/sec/Mpc. Both distance scales must be scrutinized and tested
very closely before any recalibration takes place.

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/WFPC2 9481

Pure Parallel Near-UV Observations with WFPC2 within High-Latitude
ACS Survey Fields

In anticipation of the allocation of ACS high-latitude imaging
survey{s}, we request a modification of the default pure parallel
program for those WFPC2 parallels that fall within the ACS survey
field. Rather than duplicate the red bands which will be done much
better with ACS, we propose to observe in the near-ultraviolet F300W
filter. These data will enable study of the rest-frame ultraviolet
morphology of galaxies at 0 * Origin: SpaceBase(tm) Pt 1 -14.4- Van BC Canada 604-473-9358 (1:153/719.1)

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