This week on HST

HST Programs: July 23 - July 29, 2012

Selected highlights

GO 12459: 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 MACS0416-2403 at z=0.42.

GO 12471: The Bottom of the Iceberg: Faint z~2 Galaxies and the Enrichment of the IGM

WFC3 grism imaging of a section of the GOODS-North field

Observations with the Hubble telescope have impacted many science areas, but probably none more so than investigations of galaxy formation and evolution in the high redshift universe. Deep field images, starting with the oroginal HDF and progressing through the HDF-South, GOODS-North and South, the Ultra-Deep Field and the infrared UDF, have been coupled with observations at other wavelengths from space and ground to probe galaxy structure at redshifts extending to z=9 and beyond. Most analyses of galaxy evolution as a function of redshift favour the general conclusion that the global star formation rate peaked at redshifts in the range z=2 to 3. Extensive star formation in proto-galaxies at those redshifts leads to extensive gaseous outflows and strong interactions. The present program aims to build on past research by using the spectroscopic capabilities offered by the WFC3-IR grism to survey fields centred on 15 high-redshift QSOs whose spectra have been used to probe the line-of-sight structure of the intergalactic medium. The grism observations will permit dientification of fainter galaxies that may be associated with the strong absorbers detected in the QSO spectra.

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 12512: Debris Disk Chemistry from Spatially Resolved Spectroscopy

HST image of the debris disk around two nearby A stars, including HR 4796, one of the stars targeted by this program

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 structure of those disks is affected by massive bodies (i.e. planets and asteroids), which, through dynamical interactions and resonances, can produce rings and asymmetries. Analysis of the rangle of morphological structure in these systems provides insight into the distribution of properties of planetary systems. HST currently provides the most effective (in some cases, the only) means of achieving the high-contrast required for the detection of scattered light from these disks in the presence of the bright parent stars, and a number of HST post-SM4 programs are being devoted to obtaining high-resolution images for newly discovered systems (eg GO 12228, 12291, 12573, 12576). The present program takes a different tack, aiming to obtain spectroscopic imaging of disks around three bright nearby stars. The aim is to use the Space Telescope Imaging Spectrograph in coronagraphic mode, where an occulting bar is placed over the central star. The observations will be made with the G430L and G750L low-resoltuion gratings, covering the near-UV, optical and far-red regions of the spectrum. The resulting spectra will provide insight on the composition of the dust grains in these relatively young systems.