This week on HST

HST Programs: May 2, 2011 - May 8, 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 12177: 3D-HST: A Spectroscopic Galaxy Evolution Treasury

Part of the GOODS/Chandra Deep Field South field, as imaged by HST

One of the exciting new capabilities offered by the post-SM4 Hubble Telescope is multi-object, low-resolution, near-infrared spectroscopy, using the two grisms available on the IR channel of Wide-Field Camera 3. These observations provide an important avenue for complementing wide-field imaging surveys. In particular, the present program aims to build on the extensive database currently being accumulated as part of the CANDELS Multi-Cycle Treasury program. CANDELS, itself, rests on past HST Treasury programs, and will provide multi-tiered imaging of five fields. 3D-HST will supplement portions of four fields (GOODS-south, AEGIS, the UDS and COSMOS fields) with WFC3/G141 and ACS/G800L grism data. The spectroscopic data will provide important additional information on the galaxy redshift distribution, and on the star formation characteristics in the redshift range 1 < z < 3.5. The data should also be useful in identifying quasars at high redshifts, potentially extending beyond z~6.

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 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.