This week on HST

HST Programs: April 20 - April 26, 2009

Selected highlights

GO 11704: The Ages of Globular Clusters and the Population II Distance Scale

Hubble Heritage image of the globular cluster, M15

Globular clusters are the oldest structures within the Milky Way that are directly accessible to observation. They are relatively simple systems, with relatively simple colour-magnitude diagrams (albeit with some complexities adduced from recent HST observations, see GO 11233 ). Matching those CMDs against theoretical models allows us to set constraints on the age of the oldest stars in the Galaxy, and hence on the age of the Milky Way and the epoch of galaxy formation. However, the accuracy of those age determinations rest crucially on the accuracy of the cluster distance determinations. The clusters themselves lie at distances of several kpc at best, and tens of kpc at worst; thus, direct trigonometric parallax measurements must await microacrsecond astrometric missions. The classical method of deriving distances is main sequence fitting - using nearby stars, with similar chemical abundances and accurate parallax measurements, to map out the main sequence in absolute units, and then scaling the cluster data to fit. The problem with this method is that metal-poor subdwarfs are rare, so even Hipparcos was only able to obtain accurate distances to a handful of stars. The present program aims to improve the distance measurements by using the Fine Guidance Sensors on HST to determine sub-millarcsecond trigonometric parallaxes to 9 subdwarfs, almost doubling the sample available for MS fitting.

GO 11974: High-resolution imaging for 9 very bright, spectroscopically confirmed, group-scale lenses

Strong galaxian lens from the SLACS survey

Gravitational lensing provides a powerful method of tracing the mass distribution in both galaxy clusters and individual galaxies, while at the same time amplifying the light from background galaxies to allow detailed investigation of their properties. This proposal will obtain WFPC2 observations of a 9 strong lenses, identified from the ground-based Sloan Digital Sky Survey. By themselves, the SDSS data have neither sufficient resolution nor sensitivity to map the detailed structure of the lens, and hence the underlying galaxian potential. The aim is to combine the WFPC2 data with detailed models to measure the mass distribution and light profiles of the lensing galaxies, and constrain the morphological properties of the background lensed galaxy.

GO 11979: WFPC2 Imaging of Fomalhaut b: Determining its orbit and testing for H-alpha emission

HST-ACS image of the planetary-mass companion of Fomalhaut

Fomalhaut, or alpha Piscis Austrini, is one of the Sun's closest neighbours, an A-type star with a mass approximately twice that of the Sun and an age between 100 and 300 million years, lying at a distance of only ~7.7 parsecs. Observations with the IRAS satellite in the early 1980s revealed the presence of significant excess radiation at mid-infrared wavelengths, indicating the presence of substantial dust within a disk that is being irradiated by the luminous central star. Since then, observations of Fomalhaut and nearby stars of that ilk have led to a much more detailed characterisation of the debris disk phase. In particular, Spitzer has mapped warm dust in these systems, while HST imaging has provided exquisite resolution in reflected light. It is now recogised that debris disks are the evolutionary stage where planet formation has likely run to completion, the gas has fully dissipated but the disk remains well populated with dusty material spanning a wide range of sizes. Indeed, it is likely that this phase coincides with the heavy bombardment epoch within the Solar System. ACS imaging of Fomalhaut reveals extensive structure in the disk, notably a sharply-defined, eccentric inner edge to the disk, which led to the prediction of a ~Saturn-mass planet at that location. Subsequent ACS/HRC observations led to the identification of that planet, the first direct imaging of a "conventional" exoplanet. The exoplanet is variable at optical wavelengths, suggesting that the observed flux is not only contributed by reflected light from the planetary "surface". The present WFPC2 observations are designed to test whether H-alpha emission might be a significant contributor.

GO 11987: The Recent Star Formation History of SINGS Galaxies

Spitzer IRAC image of the one-armed spiral, NGC 4725; the 5.8 and 8 micron data (red) highlight warm dust clouds

SINGS is the Spitzer Infrared Nearby Galaxy Survey, a comprehensive imaging and spectroscopic study of 75 nearby galaxies (D < 30 Mpc). The program combines Spitzer mid-infrared observations (With IRAC, MIPS and the IRS) with optical data from ground-based telescope and HST, radio observations and UV imaging with GALEX. The overall goalm of the program is to map the distribution of gas and dust in these nearby systems, and hence probe the nature and distribution of physical processes within the ISM. Many of the SINGS targets lack high-resolution ultraviolet imaging, and the present communtiy program was allocated Director's Discretionary Time of HST for WFPC2 F336W (U-band) and (in a few cases) F656N (H-alpha) imaging of a subset of the sample. The data will be of particular use in resolving and providing reliable age estimates for individual star clusters.