This week on HST

HST Programs: June 29, 2010 - July 3, 2010

Selected highlights

GO 11586: Exceptional Galactic Halo Globular Clusters and the Second Paramete

The distant halo cluster, NGC 7006

Globular clusters are members of the Galactic halo population, which formed during the first extensive period of star formation in the Milky Way. As such, the properties of the 106 to 107 stellar constituents can provide crucial insight into the earliest stages of galaxy formation. As extremely old systems, globulars have an extensive population of evolved stars, including red giants, asymptotic giants and horizontal branch stars. The morphology of the horizontal branch varies considerably across the cluster population. To first order, the variation has some metallicity dependence, with metal-rich clusters having a short, red horizontal branch. Metal-poor clusters, however, show significant diversity in behaviour, which ahs long been identified as the "second parameter problem". The nature of that second parameter remains unclear, although age is one of the favourites. The present program aims to probe that question through deep imaging of six distant clusters in the Galactic halo. These observations, using the ACS in the F606W and F814W filters, will provide the first deep colour-magnitude diagrams for these systems, and allow detailed comparison of their properties with the better-studied systems in the inner halo.

GO 11598: How Galaxies Acquire their Gas: A Map of Multiphase Accretion and Feedback in Gaseous Galaxy Halos

A computer simulation of galactic gas accretion and outflow

Galaxy formation, and the overall history of star formation within a galaxy, clearly depends on the presence of gas, and therefore on how gas is accreted, recycled, circulated through the halo and, perhaps, ejected back into the intergalactic medium. Tracing that evolutionary history is difficult, since gas passes through many different phases, some of which are easier to detect than others. During accretion and, probably, subsequent recycling, the gas is expected to be reside predominantly at high temperatures. The most effective means of detecting such gas is through ultraviolet spectroscopy, where gas within nearby systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. The present program is using the Cosmic Origins Spectrograph to observe z>1 QSOs that lie at small angular separations from SDSS galaxies at redshifts between z=0.15 and 0.35. The sightlines run through the halos of the galaxies, and the QSOs therefore provide a pencilbeam backlight that probes hot gas in the foreground systems.

GO 11659: Probing the Interior of SN1006

Chandra X-ray image of the SN1006 supernova remnant

In May 1006, Chinese, Arabic and European astronomers and stargazers independently identified a "guest star" in the region of the sky that would become known as the constellation Lupus. The star exceeded Venus in brightness at its peak, even threatening to rival the Moon, and remained visible until at least the end of that year. Indeed, there are suggestions that there a secondary brightening that preserved the star at naked eye magnitude until early 1008. The associated remnant was not identified until 1965, when Milne and Gardner resolved the radio source PKS 1459-41 into a circular shell, approximately 30 arcminutes in diameter. That remnant has since been studied extensively, notably by the Chandra satellite. HST is clearly poorly suited to mapping the remnant. However, there are a number of bright UV sources that lie behind the remnant, and those can be use to probe the remnant. This proposal will use COS to obtain spectra of these three sources, and trace the chemical and physical properties of the gas based on the absorption line profiles.

GO 11700: Bright Galaxies at z>7.5 with a WFC3 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 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.