This week on HST

HST Programs: November 22, 2010 - November 28, 2010

Selected highlights

GO 11702: Search for Very High-z Galaxies with WFC3 Pure Parallel

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 17 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 fields. This is one of two such programs in the cycle 17 portfolio.

GO 11741: Probing Warm-Hot Intergalactic Gas at 0.5 < z < 1.3 with a Blind Survey for O VI, Ne VIII, Mg X, and Si XII Absorption Systems

Probing the intergalactic medium via QSO absorption lines

One of the key issues facing modern cosmology is the "missing baryon" problem. In brief, a census of all the constituents in the local universe accounts for less than half of the baryonic mass expected based on measurements of the fractional abundanmce of deuterium and observations of the cosmic microwave background. It is generally believed that the missing material lurks in the form of extremely hot gas in the intergalactic medium. The most effective means of probing that medium, and testing this hypothesis, is to search for the appropriate absorption lines in the spectrum of a background source. QSOs are particularly effective cosmic searchlights, since they have strong continuum flux levels at the ultraviolet wavelengths where most of the important absorption lines fall. Following SM4, and the installation of the Cosmic Origins Spectrograph, HST is now well equipped to tackle this type of program, and search fgor a full accounting of the baryonic universe. The present program weill use COS to obtain spectra of nine QSOs at redshifts beyond z=0.89, and will search for warm-hot intergalactic gas in the redshift range 0.5 < z < 1.3.

GO 12181: The Atmospheric Structure of Giant Hot Exoplanets

Probing the atmosphere of a transiting exoplanet

The first exoplanet, 51 Peg b, was discovered through radial velocity measurements in 1995. 51 Pegb was followed by a trickle, and then a flood of other discoveries, as astronomers realised that there were other solar systems radically different from our own, where "hot jupiters" led to short-period, high-amplitude velocity variations. Then, in 1999, came the inevitable discovery that one of those hot jupiters. HD 209458b, was in an orbit aligned with our line of sight to the star, resulting in transits. Since that date, the number of known transiting exoplanet systems has grown to more than 100, most detected through wide-field photometric surveys with the Kepler satellite providing the highest sensitivity dataset. These transiting systems are invaluable, since they not only provide unambiguous measurements of mass and diameter, but they also provide an opportunity to probe the atmospheric structure by differencing spectra taken during and between primary secondary transit. Such observations are best done from space: indeed, the only successful atmospheric observations to date have been with HST and Spitzer. The present program aims to set these measurements on a systematic basis by targeting 13 transiting exoplanets. The WFC3-IR G141 grism will be used to search for characteristic near-infrared spectral features in those systems.

GO 12311: Multiple Stellar Populations in Galactic Globular Clusters

NGC 2808, a globular cluster with multiple stellar populations

Globular clusters are remnants of the first substantial burst of star formation in the Milky Way. With typical masses of a few x 105 solar masses, distributed among several x 106 stars, the standard picture holds that these are simple systems, where all the stars formed in a single starburst and, as a consequence, have the same age and metallicity. Until recently, the only known exception to this rule was the cluster Omega Centauri, which is significantly more massive than most clusters and has both double main sequence and a range of metallicities among the evolved stars. Omega Cen has been joined by several additional clusters, including NGC 2808, which shows evidence for three distinct branches to the main sequence. The origin of this feature is remains uncertain, but it may be significant that NGC 2808 is also one of the more massive clusters, and might therefore be able to survive several burst of star formation (or, conversely, be the product of a multi proto-globular merger). Evidence for multiple populations has also been found in other clusters, including NGC 1851, 47 Tucanae and NGC 6752 - all relatively massive clusters. The present program aims to use high-precision UV (F275W) and far-red (F814W) WFC3 observations of those clusters, together with M4 and M22, to probe the detailed structure along the main sequence.