Daily #4017

HUBBLE SPACE TELESCOPE - Continuing to collect World Class Science

DAILY REPORT #4017

PERIOD COVERED: UT December 28, 2005 (DOY 362)

OBSERVATIONS SCHEDULED

ACS/HRC 10512

Search for Binaries Among Faint Jupiter Trojan Asteroids

We propose an ambitious SNAPSHOT program to survey faint Jupiter
Trojan asteroids for binary companions. We target 150 objects, with
the expectation of acquiring data on about 50%. These objects span
Vmag = 17.5-19.5, a range inaccessible with ground-based adaptive
optics. We now have a significant sample from our survey of brighter
Trojans to suggest that the binary fraction is similar to that which
we find among brighter main-belt asteroids, roughly 2%. However, our
observations suggest a higher binary fraction for smaller main-belt
asteroids, probably the result of a different formation mechanism
{evident also from the physical characteristics of the binaries}.
Because the collision environment among the Trojans is similar to that
of the Main Belt, while the composition is likely to be very
different, sampling the binary fraction among the fainter Trojans
should help us understand the collisional and binary formation
mechanisms at work in various populations, including the Kuiper Belt,
and help us evaluate theories for the origin of the Trojans.
Calibration of and constraints on models of binary production and
collisional evolution can only be done using these large-scale,
real-life physical systems that we are beginning now to find and
utilize.

ACS/HRC/WFC 10729

ACS CCDs daily monitor

This program consists of a set of basic tests to monitor, the read
noise, the development of hot pixels and test for any source of noise
in ACS CCD detectors. The files, biases and dark will be used to
create reference files for science calibration. This programme will be
for the entire lifetime of ACS. Changes from cycle 13:- The default
gain for WFC is 2 e-/DN. As before bias frames will be collected for
both gain 1 and gain 2. Dark frames are acquired using the default
gain {2}. This program cover the period Oct, 2 2005- May, 29-2006. The
second half of the program has a different proposal number: 10758.

ACS/WFC 10574

Witnessing Galaxy Transformation in Galaxy Groups at z 1

The recent discover of five galaxy groups in the Lynx supercluster
region offers us the exciting opportunity to observe for the first
time groups in the process of collapsing into a merging pair of
clusters at z 1. Our current picture of structure formation suggests
that substantial evolution of galaxy properties can occur in groups
and filaments well before they enter the environs of massive clusters.
However, neither current theoretical models nor observations give us a
complete understanding of the relative importance of the different
physical processes that control the structural and spectral
transformations that occur prior to, during, and after infall into a
dense environment. We propose direct observation of these newly
discovered dynamically young structures in the Lynx region, in order
to provide a critical benchmark in testing not only whether galaxy
evolution occurs mostly prior to entry into the densest regions but
will also constrain the relative importance of initial conditions in
determining the fate of galaxy systems. Our analysis of these proposed
ACS measurements will be complemented with an unique dataset we have
already in the optical, infrared, mid-infrared, and X-ray.

WFPC2 10608

Probing the star formation law in the extreme outer limits of M83, a
prototypical XUV-disk galaxy

The Galaxy Evolution Explorer {GALEX} has discovered a new sub-class
of spiral galaxy, which we refer to as extended UV-disk {XUV-disk}
galaxies. They are distinguished by conspicuous UV-bright star
clusters located at galactocentric radii extending to many times the
optical {R25} extent, and appear to represent a population of spiral
galaxies still actively building, or augmenting, their stellar disk.
However, XUV-disks pose a mystery in the form of a relative lack of
HII regions {traced by H-alpha emission} associated with outer disk,
UV-bright stellar clusters. M83 is an XUV-disk prototype and the focus
of this proposal. It has an H-alpha surface brightness profile
characterized by a steep decline at the radius beyond which the
gaseous disk is thought to become dynamically stable {against collapse
and ensuing star formation}, but GALEX UV profiles show no "edge" at
this location. Our HST study of M83 aims to resolve this puzzling
discrepancy, confirmed in several XUV-disks, by searching for
Lyman-continuum producing O stars that are either absent or present
without nebulosity. HST provides the only means of resolving
individual massive stars in the FUV band at M83's distance. Without
HST, we lose the critical ability to photometrically classify O and B
stars. Our multiwavelength observations will also constrain the
history of star formation in the outer disk over Gyr timescales by
characterizing the evolved stellar population, both using resolved
giants and color analysis of the diffuse background.

WFPC2/ACS/WFC 10590

Star-Formation History of an Unmerged Fragment: the Leo A Dwarf Galaxy

The Leo A dwarf irregular is the only known Local Group galaxy that on
the weight of current evidence has been suggested to have experienced
its first star formation within the past 2-3 billion years. As a
galaxy that could have been almost purely gaseous during the epoch of
giant galaxy assembly, Leo A is the best nearby candidate to be a
redshift zero analogue to the major building blocks of the Milky Way.
We propose to obtain deep optical images of Leo A with the ACS/WFC to
achieve three main goals: 1} To establish the fractions of
star-formation, by mass, that occurred prior and subsequent to the
main epoch of hierarchical merging {redshift z ~ 2-4, Age ~ 10-12.5
Gigayears}; 2} to measure the time variation in Leo A's star-formation
rate over the past 10 Gyr, based on statistical analyses of its {V-I,
I} color-magnitude diagram; and 3} to measure the radial distributions
of young and old stellar populations and quantify the degree to which
the optically prominent, young population is embedded in an extended,
low-surface brightness sheet or halo of ancient stars. Because of the
distance modulus {24.5 mag} and high degree of stellar crowding at the
level of the oldest main-sequence turnoffs, the observations necessary
to achieve these goals are unobtainable except with HST. The ONLY way
to reliably derive the star-formation history of Leo A over its entire
lifetime is with photometry to magnitudes of {B, I} = {28.6, 27.9},
the level of the oldest main-sequence turnoff in Leo A. These data
would confirm and extend the limited inferences obtained from WFPC2
photometry over 2 magnitudes less deep, and provide the first
opportunity to measure the complete star-formation history of a
potential "living fossil" analogue to the building blocks of the Milky
Way. We propose to use WFPC2 in parallel to measure radial variations
in the stellar populations between the galaxy's core and outskirts.
Because the expected 2-gyro jitter ellipse is comparable to the pixel
scale of ACS/WFC, we rely on point-spread function fitting photometry,
and we require no special scheduling constraints, our proposed program
would be virtually unaffected by entry into 2-gyro mode.

FLIGHT OPERATIONS SUMMARY:

Significant Spacecraft Anomalies: (The following are preliminary
reports of potential non-nominal performance that will be
investigated.)

HSTARS: (None)

COMPLETED OPS REQUEST: (None)

COMPLETED OPS NOTES: (None)

SCHEDULED SUCCESSFUL
FGS GSacq 08 08
FGS REacq 07 07
OBAD with Maneuver 30 30

SIGNIFICANT EVENTS: (None)