This week on HST

HST Programs: October 24, 2011 - October 30, 2011

Selected highlights

GO 12103: 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 MACS 0717+3745.

GO 12248: How Dwarf Galaxies Got That Way: Mapping Multiphase Gaseous Halos and Galactic Winds Below L*

A computer simulation of galactic gas accretion and outflow

Galaxy formation, and star formation within a galaxy, requires the presence of gas. The detailed evolution of individual galactic systems therefore depends 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 low- to moderate redshift QSOs (0.1 < z < 1) that lie at small angular separations from sub-L* mass galaxies (0.02 < M/L* < 0.3) at redshifts between z=0.01 and 0.06. 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 12301: Precision Age-dating of Star Clusters in Stephan's Quintet

HST WFC3 images of four of the five galaxies in Stephan's quintet: the foreground galaxy is to the upper left

Stephan's Quintet is a striking grouping of five moderately sized galaxies in the constellation of Pegasus. It is named after its discoverer, Edouard Stephan of Marseille Observatory, who catalogued the system in 1877, and is the archetypical example of a "compact group". In fact, only four of the five galaxies are actually physically associated, with radial velocities close to ~6600 km/sec, implying distances of ~90 Mpc. Those four galaxies have similar morphological characteristics, with reddish-hued bulge components.NGC 7317, at the bottom left of the image, shows little evidence for ongoing star formation. However, the pair of galaxies at the upper right, NC 7318A and 7318B, are clearly more active which, with the presence of tidal features, is a likely indicator of gravitational interactions. The fifth galaxy in the group, and the fourth in the present image, NGC 7320, is the largest in apparent size, has a radial velocity of only 790 km/sec, indicating that it lies much closer, at a distance of ~10 Mpc. That galaxy shows an abundance of young clusters and HII regions. These galaxies were targeted for observation by Wide Field Camera 3 as part of the Early Release Observations following Servicing Mission 4, resulting in the image shown above.Those images use observations with the F438W (B), F606W (V/R), F665N (H-alpha) and F814W (I) filters on the WFC3 UVIS channel. The present program aims to capitalise on those data, adding observations with the F336W (U) and F547M (V) filters; the latter filter explicitly excludes emission due to H-alpha.

GO 12474: The frequency and chemical composition of rocky planetary debris around young white dwarfs

Artist's impression of a comet spiralling in to the white dwarf variable, G29-38

During the 1980s, one of the techniques used to search for brown dwarfs was to obtain near-infrared photometry of white dwarf stars. Pioneered by Ron Probst (KPNO), the idea rests on the fact that while white dwarfs are hot (5,000 to 15,000K for the typcail targets0, they are also small (Earth-sized), so they have low luminosities; consequently, a low-mass companion should be detected as excess flux at near- and mid-infrared wavelengths. In 1988, Ben Zuckerman and Eric Becklin detected just this kind of excess around G29-38, a relatively hot DA white dwarf that also happens to lie on the WD instability strip. However, follow-up observations showed that the excess peaked at longer wavelengths than would be expected for a white dwarf; rather, G 29-38 is surrounded by a dusty disk. Given the orbital lifetimes, those dust particles must be regularly replenished, presumably from rocky remnants of a solar system. G 29-38 stood as a lone prototype for almost 2 decades, until a handful of other dusty white dwarfs were identified from Spitzer observations within the last couple of years.In subsequent years, a significant number of DA white dwarfs have been found to exhibit narrow metallic absorption lines in their spectra. Those lines are generally attributed to "pollution" of the white dwarf atmospheres. Given that the diffusion time for metals within the atmospheres is short (tens to hundreds of years), the only reasonable means of maintaining such lines in ~20% of the DA population is to envisage continuous accretion from a surrounding debris disk. The present program aims to address this question by using COS to obtain UV observations of young white dwarfs, probing correlations with progenitor mass and examining the detailed composition of the accreted materials.