This week on HST

HST Programs: June 25 - July 1, 2012

Selected highlights

GO 12457: Through a Lens, Darkly - New Constraints on the Fundamental Components of the Cosmos

The cluster MACS J1206.2-0.47, imaged by HST as part of the CLASH program

The overwhelming majority of galaxies in the universe are found in clusters. As such, these systems offer an important means of tracing the development of large-scale structure through the history of the universe. Moreover, as intense concentrations of mass, galaxy clusters provide highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. The present program aims to capitalise fully on HST's imaging capabilities, utilising the refurbished Advanced Camera for Surveys and the newly-installed Wide-Field Camera 3 to obtain 17-colour imaging of 25 rich clusters. The data will be use to map the mass profiles of the clusters and probe the characteristics of the high-redshift lensed galaxies. Since ACS and WFC3 can be operated in parallel, the program will also use parallel imaging in offset fields to search for high-redshift supernovae. The present observations target the cluster RXJ2129+0005 at z=0.24.

GO 12473: An Optical Transmission Spectral Survey of hot-Jupiter Exoplanetary Atmospheres

An artist's impression of the hot Jupiter circling a solar-type star

The first planet orbiting another star was discovered in 1995 around the relatively rbight G dwarf, 51 Pegasi. 51 Pegb confounded expectations as a jovian-mass gas giant in a 3-day orbit, placing it closer to the parent star than Mercury in our Solar System. Over the past nearly 20 years, numerous other systems have been identified, with the most recent flood of identifications coming from the Kepler satellite, which is pushing the detection limit to objects as small as earth in orbits with semi-major axes exceeding 1 AU. Many of these new detections (and all of the Kepler detections) are transiting systems. Transiting systems offer a potential gold-mine for extrasolar planetary studies, since not only is the orbital inclination well defined, but the diameter (and hence the average density) can be measured directly from the eclipse depth, while the atmospheric composition can be probed through line absorption or re-radiated thermal flux. The results from these measurments can be used to test, and improve, theoretical models of extrasolar planets. These observations are best done from space (indeed, the only unequivocally successful atmospheric observations to date have been with HST and Spitzer). The present program targets nine systems, all discovered through ground-based surveys (the WASP and HAT surveys), and all comprising jovian-mass planets in short-period orbits. The program will use STIS to obtain optical spectra, covering the full wavelength range with the G430L and G750L gratings, while WFC3 will be used to target near-IR spectra with the G141 grism.

GO 12492: The Nature of the Binary Companions to the Blue Straggers in the Old Open Cluster NGC 188

The old open cluster, NGC 188

Blue stragglers are found in old open clusters and globular cluster systems. Most appear to lie on the main sequence, but in a location `up and to the right' of the main-sequence turnoff; that is, these stars have higher mass than the turnoff stars, and, if coeval, ought to have evolved through the giant branches to become white dwarfs. The resolution of this paradox appears to lie in stellar mergers; blue stragglers represent the remnants of violent dynamical processes that can only occur within high density star clusters. Recent models identify three possible formation routes for these stars: mass transfer in binary systems; stellar collisions through dynamical encounters; and mergers of close pairs within triple systems. Recent observations of the old galactic open cluster, NGC 188, have shown that many of the blue stragglers in this system are binary systems today, with wide companions on long period orbits. A key test for the different formation lies in determining the nature of those wider companios: if mass transfer is the emchanism, the companions should all be whikte dwarfs; if the blue stragglers formed as mergers within triple systems, the wide companions are still likely to be unevolved main-sequence stars. The present prorgam aims to test these mdoels by using the ACS/SBC to obtain high-fidelity UV photometry, searching for the UV upturn that would represent a hot, white dwarf companion.

GO 12572: The Brightest of Reionizing Galaxies Pure Parallel Survey

The ACS optical/far-red image of the Hubble Ultra Deep Field

Galaxy evolution in the early Universe is a discipline of astronomy that has been transformed by observations with the Hubble Space Telescope. The original Hubble Deep Field, the product of 10 days observation in December 1995 of a single pointing of Wide Field Planetary Camera 2, demonstrated conclusively that galaxy formation was a far from passive process. The images revealed numerous blue disturbed and irregular systems, characteristic of star formation in galaxy collisions and mergers. Building on this initial progam, the Hubble Deep Field South (HDFS) provided matching data for a second southern field, allowing a first assessment of likely effects due to field to field cosmic variance, and the Hubble Ultra-Deep Field (UDF) probed to even fainter magitude with the Advanced Camera for Surveys (ACS). The highest redshift objects found in the UDF have redshifts approaching z~7. Pushing to larger distances, and greater ages, demands observatons at near-infrared wavelengths, as the characteristics signatures of star formation are driven further redward in the spectrum. Wide Field Camera 3, installed in Servicing Mission 4, is well suited to these observations, and a number of programs are in place in Cycle 19 that address these issues. Indeed, WFC3 is employed in pure parallel mode by several programs. These take advantage of other science programs, usually with COS, that involve 2-5 orbit pointings on sources at high galactic latitude. The WFC3 pointing is unplanned, since it depends on the orientation adopted for the prime observations, but 2-5 orbits of IR imaging can reach galaxies at redshifts exceeding z=7 (potentially even z~8) in high latitude field.