This week on HST

HST Programs: September 3 - September 9, 2007

Gyro 2 failed on Friday August 31. HST switched to 2-gyro mode operations 18 months ago, in anticipation of this eventuality. Gyro 6 is now running, and normal operations have resumed using Gyros 1 & 6.
NICMOS was not available for scheduling this week; observations will resume next week.

Program Number Principal Investigator Program Title Links
10787 Jane Charlton, The Pennsylvania State University Modes of Star Formation and Nuclear Activity in an Early Universe Laboratory Abstract
10818 Judith Cohen, California Institute of Technology Very Young Globular Clusters in M31 ? Abstract
10915 Julianne Dalcanton, University of Washington ACS Nearby Galaxy Survey Abstract
11084 Dan Zucker, Institute of Astronomy, Cambridge Probing the Least Luminous Galaxies in the Local Universe Abstract
11100 Marusa Bradac, Stanford University Two new `bullets' for MOND: revealing the properties of dark matter in massive merging clusters Abstract
11115 Christopher T. Russell, University of California - Los Angeles Photometric Imaging of Asteroid 2 Pallas Abstract
11122 Bruce Balick, University of Washington Expanding PNe: Distances and Hydro Models Abstract
11178 William M. Grundy, Lowell Observatory Probing Solar System History with Orbits, Masses, and Colors of Transneptunian Binaries Abstract
11201 Nitya Kallivayalil, Harvard University Systemic and Internal motions of the Magellanic Clouds: Third Epoch Images Abstract
11206 Kai G. Noeske, University of California - Santa Cruz At the cradle of the Milky Way: Formation of the most massive field disk galaxies at z>1 Abstract
11211 George Fritz Benedict, University of Texas at Austin An Astrometric Calibration of Population II Distance Indicators Abstract
11212 Douglas R. Gies, Georgia State University Research Foundation Filling the Period Gap for Massive Binaries Abstract
11213 Gerard T. van Belle, California Institute of Technology Distances to Eclipsing M Dwarf Binaries Abstract
11229 Margaret Meixner, Space Telescope Science Institute SEEDS: The Search for Evolution of Emission from Dust in Supernovae with HST and Spitzer Abstract
11289 Jean-Paul Kneib, Laboratoire d'Astronomie Spatiale SL2S: The Strong Lensing Legacy Survey Abstract
11292 Mark R. Showalter, SETI Institute The Ring Plane Crossings of Uranus in 2007 Abstract
11312 Graham Smith, University of Birmingham The Local Cluster Substructure Survey (LoCuSS): Deep Strong Lensing Observations with WFPC2 Abstract

Some selected highlights

GO 11100: Two new `bullets' for MOND: revealing the properties of dark matter in massive merging clusters

1E0657-56, the original Bullet cluster: Chandra observations of the X-ray emission are shown in red; the inferred dark matter distribution is shown in blue. The Bullet cluster, 1E0657-56, was publicised in a press release in August 2006, during the Prague IAU meeting. Originally identified through the strong X-ray emission, the "cluster" actually comprises two galaxy clusters that are undergoing a collision. The X-ray emission, generated by hot gas, is offset from the galaxian cluster members - as expected, given that the intercluster baryonic material in the two clusters will interact dissipationally during the collision. Crucially, however, the dark matter distributions, as determined from weak lensing, remain centred on the galaxies. This runs contrary to the predictions of modified gravity theories, such as MOND. The present program aims to use WFPC2 to obtain deep images of two further cluster mergers, MACSJ0025.4-1222 and MACSJ2243.3-0935, and will use weak lensing analysis to determine the dark matter distributions in those two systems.

GO 11115: Photometric Imaging of Asteroid 2 Pallas

False-colour image of asteroid 951 Gaspra, taken by the Galileo spacecraft. Pallas if fiftyfold larger in diameter. Pallas was the second known inhabitant of the asteroid belt, discovered by Heinrich Olbers on March 28, 1802, almost 16 months after Piazzi's discovery of Ceres. Indeed, Olbers discovered Pallas in the course of obtaining follow-up observations of Ceres. With a diameter of ~530 kilometres, Pallas is the third largest asteroid in the main belt. Ground-based observations of Pallas suggest that the asteroid is approximately triaxial in shape, with the shortest axis as much as 50-70 kilometres smaller than the longest axis. Pallas is currently at opposition, and near its closest approach to Earth. HST observations offer the potential of a more accurate determination of those quantities, as well as measurement of the surface properties and the albedo. The Planetary Camera on WFPC2 will be used to image Pallas in 5 passbands, from F336W (U-band) to F814W (I-band).

GO 11201: Systemic and Internal motions of the Magellanic Clouds: Third Epoch Images

The Large Magellanic Cloud (upper left) with the Small Magellanic Cloud (right) and the (foreground) Galactic globular cluster47 Tucanae The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are the most massive satellites of the Milky Way galaxy. The orbital motions of these systems can be used to probe the mass distribution of Milky Way, and backtracking the orbits can shed light on how the three systems have interacted, In particular, the well known Magellanic Stream, stretching between the two Clouds, is thought to be a product either of interactions between the Clouds, or of ram-stripping of gas from the LMC on its last passage through the Plane of the Milky Way. The present program builds on observations obtained at two epochs with the now-defunct (but perhaps soon to be revived) ACS High Resolution Camera (ACS/HRC). The previous programs targeted known QSOs lying behind the Clouds; the QSOs serve as fixed reference points for absoltue astrometry of the numerous foreground LMC/SMC stars. First epoch observations were made in late 2002 (GO 9462), with the follow-up imaging in late 2004 (GO 10130). The tangential motions of the Clouds amount to only a few milliarcseconds, but the high spatial resolution and high stability of HST imaging makes such measurements possible, even with only a 2-year baseline. Surprisingly, the initial results suggest that the 3-D motions of both clouds are much higher than expected, suggesting either that the LMC/SMC/MW is either dynamically very young, or unbound. The present program will use WFPC2 to obtain third-epoch data in the same fields, providjng a crucial test of the initial results

Past weeks:
page by Neill Reid, updated 10/8/2007