This week on HST

HST Programs: February 21, 2011 - February 27, 2011

Selected highlights

GO 12065: Through a Lens, Darkly - New Constraints on the Fundamental Components of the Cosmos

The ACS optical/far-red image of the galaxy cluster, Abell 2218; note the lensed arcs

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 14-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.

GO 12228: Probing for Exoplanets Hiding in Dusty Debris Disks: Inner <10 AU Disk Imaging, Characterization, and Exploration

HST-ACS image of the disk surrounding the nearby M dwarf, AU Mic

Planet formation occurs in circumstellar disks around young stars. Most of the gaseous content of those disks dissipates in less than 10 million years, leaving dusty debris disks that are detectable through reflect light at near-infrared and, to a lesser extent, optical wavelengths. The disk structure is affected by massive bodies (i.e. planets and asteroids), which, through dynamical interactions and resonances, can produce rings and asymmetries. Over the past decade, HST and Spitzer have provided complementary information on this subject, with Spitzer measuring thermal radiation from circumstellar dust and HST providing high-resolution mapping of debris disks in reflected light. Most recently, HST ACS coronagraphic imaging have revealed the presence of a planetary object within the disk of the nearby A star, . Planetary companions to the young (60 Myr-old) F star, HR 8799, have also been imaged by both ground-based telescopes and HST. The ACS coronagraph was associated with the High Resolution Camera, which is no longer functioning; nor is NICMOS. However, coronagraphy is still possible using the occulting bar on the Space Telescope Imaging Spectrograph (STIS). The present program aims to expand the catalogue of imaged exoplanets to other nearby young stars that are known to harbour debris disks. The present set of observations targets the young, nearby M dwarf, AU Mic (or Gliese 803).

GO 12233: Strong Gravitational Lensing by Quasars

ACS images of galaxy-galaxy Einstein ring lenses from the Sloan survey

Gravitational lensing is a consequence the theory of general relativity. Its importance as an astrophysical tool first became apparent with the realisation (in 1979) that the quasar pair Q0957+561 actually comprised two lensed images of the same background quasar. In the succeeding years, lensing has been used primarily to probe the mass distribution of galaxy clusters, using theoretical models to analyse the arcs and arclets that are produced by strong lensing of background galaxies, and the large-scale mass distribution, through analysis of weak lensing effects on galaxy morphologies. Gravitational lensing can also be used to investigate the mass distribution of individual galaxies, and, until recently, most attention focused on background quasars lensed by foreground galaxies. The present program, however, is built on a role reversal. Here, the WFC3 UVIS camrea is used to obtain blue (F475W) and far-red (F814W) images of low redshift (z < 0.7) quasars that are acting as strong lenses of background emission line galaxies. The targets are selected from the Sloan Digital Sky Survey, and the high-resolution HST data will be used to constrain the radial mass profiles of the quasar host galaxies.

GO 12307: A public SNAPSHOT survey of gamma-ray burst host galaxies

Artist's impression of a gamma-ray burst

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected over the last 30+ years, and analyses indicate that they can be divided into two classes with durations longer or shorter than 2 seconds. The short bursts appear to release more high energy radiation, so the two subsets are known as long/soft and short/hard bursts.The short/hard bursts appear to arise from coalescing binary systems (probably pairs of neutron stars or black holes), but the long/soft bursts appear to originate in the collapse of very massive stars. The latter sources are therefore almost certainly associated with star formation, so they act as signposts to active star-forming regions in the high redshift universe. Many of these bursts are sufficiently bright that ground-based spectroscopic observations allow reliable measurement of the redshift. In many cases, ground-based observations at later epochs are insufficient to detect the underlying host galaxy, and characterise its properties. This SNAPSHOT program uses the WFC3-IR camera to image the host galaxies of Swift-selected GRBs at redshifts less than 3. The data will permit statical analyses of the luminosities and morphologies of those systems.