This week on HST

HST Programs: January 17, 2011 - January 23, 2011

Selected highlights

GO 11728: The Impact of Starbursts on the Gaseous Halos of Galaxies

A computer simulation of galactic gas accretion and outflow

The detailed history of galaxy formation and assembly depends on the availability of gas for star formation, and on how processes engendered by that star formation affects the distribution and intrinsic properties of that gas. In particular, feedback in the form of winds and ionising radiation from either young stellar associations or massive black holes may be capable of interfering with, and even interrupting, the overall assembly process. The present program aims to investigate this issue by probing the nature of halo gas in the vicinity of galaxies that have undergone recent (<100 Myr old) starbursts. The halo gas is expected to 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 moderate-redshift QSOs that lie at small angular separations from the starburst galaxies. 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 12056: A Panchromatic Hubble Andromeda Treasury

M31: the Andromeda spiral galaxy

M31, the Andromeda galaxy, is the nearest large spiral system to the Milky Way (d ~ 700 kpc), and, with the Milky Way, dominates the Local Group. The two galaxies are relatively similar, with M31 likely the larger system; thus, Andromeda provides the best opportunity for a comparative assessment of the structural properties of the Milky Way. Moreover, while M31 is (obviously) more distant, our external vantage point can provide crucial global information that complements the detailed data that we can acquire on individual members of the stellar populations of the Milky Way. With the advent on the ACS and, within the last 2 years, WFC3 on HST, it has become possible to resolve main sequence late-F and G dwarfs, permitting observations that extend to sub-solar masses in M31's halo and disk. Initially, most attention focused on the extended halo of M31 (eg the Cycle 15 program GO 10816 ), with deep imaging within a limited number of fields revealing the complex metallicity structure within that population. With the initiation of the present Multi-Cycle Treasury program, attention switches to the M31 disk. "PHAT" will conduct a multi-waveband survey of approximately one third of disk and bulge, focusing on the north-east quadrant. Observations will extend over the next three cycles, and will provide a thorough census of upper main-sequence stars and star forming regions, matching the stellar distribution against the dust and gas distribution.

Temporal variation in the intensities of the lensed components of the Einstein Cross

Gravitational lensing is a consequence 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 also increases the apparent brightness of the background sources. This effect can be used to our advantage, in enabling detailed observations of high-redshift sources that be too faint to observe under normal circumstances, but it can also lead to statistical biases in parameters such as luminosity functions. These effects are likely to be of most importance for higher redshift sources, where the longer pathlength leads to a higher probability of the light encountering a foreground lens. The present program aims to address this issue for by using WFC3 to obtain high resolution images F105W ("Y"-band) of a subset of 54 QSOs with redshifts in the range 5.7 < z < 6.4.

GO 12290: Do Rocky Extrasolar Minor Planets Have a Composition Similar to Bulk Earth?

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

White dwarf stars are the final evolutionary state of stars like the Sun. Comprised of degenerate material that compacts more than half the mass of the Sun into a radius comparable with that of the earth, white dwarfs have no internal energy source, and gradually cool and fade with time. At higher temperatures (> 10,000K), their spectra are dominated by broad lines of either hydrogen (DA white dwarfs) or helium (DBs). A handful of spectroscopic oddities, with prominent metal lines (Ca, Mg) were discovered in the 1970s and 80s. Those elements are predicted to diffuse relatively rapidly within the white dwarf atmosphere, and those objects were typically explained as resulting from interstellar accretion. However, high resolution optical spectroscopy obtained in the late 1990s revealed that a significant fraction of DA white dwarfs have narrow metallic absorption lines. Crucially, one of the stars found to have such absorption features is G29-38, the white dwarf originally tagged as harbouring a brown dwarf companion based on its showing a strong mid-infrared excess. Given the relatively high frequency of occurrence and the short residence time in the atmosphere, interstellar accretion is not an option, and those lines must be maintained by continuous accretion from a circumstellar debris disk (which contributes the IR excess in G 29-38). That debris disk, in turn, must be regularly replenished, presumably from collisions between rocky remnants of a solar system. Thus, probing the abundance distribution of the accreted material should illuminate the composition of the residual asteroidal material. The present program aims to tackle this issue by using the Cosmic Origins Spectrograph to obtain UV spectra of a sample of 6 white dwarfs known to exhibit metallic absorption lines.