This week on HST

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

Selected highlights

GO 11579: The Difference Between Neutral- and Ionized-Gas Metal Abundances in Local Star-Forming Galaxies with COS

ACS HST image of I Zwicky 18, the archetypical metal-poor dwarf galaxy

In standard cosmologies, big bang nucleosynthesis is responsible for the production of hydrogen, helium and a trace abundance of lithium. The first stars - and the first galaxies - formed from this near-pristine material, and stellar nucleosynthesis accounts for the production of heavier elements (`metals', in astronomical parlance). As stars evolve and die, they return processed material to the interstellar medium, enriching the metal content and increasing the metallicity of subsequent stellar generations. Thus, within the Milky Way, the stars in the stellar halo (age 11-13 Gyrs) are 10 to 1000 times more metal-poor than the Sun's neighbours in the Galactic Disk. However, a small number of dwarf galaxies are known that seem to have lain fallow, avoiding star formation and self-enrichment, for almost a Hubble time. The classic example is the dwarf galaxy I Zwicky 18, with a metallicity of [M/H]~-1.2. Investigations of the abundance in those systems tend to rest on analyses of the emission line spectra generated by the ionised gas. With the installation of the Cosmic Origins Spectrograph on HST, it is now possible to supplement those data with analyses of the neutral medium. The present program uses the Solar Blind Channel on the ACS to identify suitable regions within the target galaxies; COS is then used to obtain UV spectra of distant sources lying behind those star formation regions. The neutral gas within the foreground galaxy introduces absorption features in the far-UV that can be used to provide an independent estimate of metallicity.

GO 12011: Magnetic Heating of the Outer Atmospheres of Very Low Mass Dwarfs

Artist's impression of a flaring low-mass star.

One of the major surprises delivered by the Einstein Observatory was the high level of chromospheric activity present in cool, late-type dwarfs. Proportionately,. low-mass dwarfs are significantly more active than (and often as luminous as) solar-type stars, and are frequently subject to high energy flares and outbursts. This was surprising since these cool dwarfs are fully convective, and are therefore incapable of supporting the rotational dynamo that is believed to power magnetic activity in sun-like stars. In the succeeding 30 years, subsequent observatories, notably Chandra and HST, with increasingly sensitive X-ray and UV detectors have pushed observations to fainter flux limits and lower luminosity stars. Those observations demonstrate that activity persists through spectral class M and is even present among ultracool L dwarfs, probably powered through shear dynamos maintrained by convective motions. The present program targets two late-type M dwarf: LHS 3003 (M7) and LHS 2065 (M9) and will use COS observations of the far-UV spectra to probe the conditions in the transition region between the chromosphere and corona, and search for coronal structures.

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 12210: SLACS for the Masses: Extending Strong Lensing to Lower Masses and Smaller Radii

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. Until recently, the most common background sources were quasars. Galaxy-galaxy lenses, however, offer a distinct advantage, since the background source is extended, and therefore imposes a stronger constraints on the mass distribution of the lensing galaxy than a point-source QSO. The Sloan Digital Sky Survey is a powerful tool for identifying candidate galaxy-galaxy lenses, and has provided targets for HST imagiung programs in several previous cycles. The presentprogram is using HST-ACS imaging to survey a further 135 strong lens candidates. The HST data will verufy the nature of those candidates, and provide the angular resolution necessary to model the mass distribution.