This week on HST

HST Programs: January 16 - January 22, 2012

Selected highlights

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 12477: Weak lensing masses of the highest redshift galaxy clusters from the South Pole Telescope SZ survey

The ACS optical/far-red image of the galaxy cluster, Abell 2218, including an extensive number of 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. Galaxy clusters can be identified at moderate redshifts by searching for signatures of the Sunyaev-Zeldovich effect: high energy electrons in the hot intercluster medium interact with radiation from the cosmic microwave background to distort the microwave spectrum. The South Pole Telescope is a 10-metre microwave/millimetre telescope located at Amundsen-Scott South Pole Station on the Antarctiva high plateau, close to the geographic South Pole. That telescope has been used to search for galaxy clusters. As intense mass concentrations, these systems are 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 use the Wide Field Camera on the Advanced Camera for Surveys to obtain observations of seven clusters with masses between 3 x 1014 and 1015 MSun, and with redshifts in the range 0.93 < z < 1.2. The HST data will be used to measure weak lensing, and combined with ground-based and space-based observations (Chandra, XMM, Spitzer) to refine the mass estimates for these clusters.

GO 12549: The Formation History of the Ultra-Faint Dwarf Galaxies

The Bootes I dwarf galaxy (SDSS data)

The Milky Way possesses at least 15 satellite galaxies. The most prominent are the Large and Small Magellanic Clouds, irregular galaxies which have been known since at least the tenth century. The remaining systems are gas-poor dwarf spheroidal systems, with luminosities less than 108 LSun and masses less than 108 MSun. All of these dwarfs were discovered from wide field imaging surveys. The two brightest systems, Scuptor and Fornax, were found on plates taken in the 1930s by Harvard's Boyden Observatory, in Bloemfontein, South Africa. The brighter systems in the northern hemisphere, including Ursa Minor and Draco, were identified from photographic plates taken as part of the first Palomar Sky Survey in the 1950s, and the brighter southern systems, including Carina and Sextans (L ~ 105 LSun, were uncovered in 80s and 90s on plates taken with UK Schmidt telescope, Siding Spring, Australia. These systems have such low densities that they are barely discernible to the eye; in fact, Sextans was discovered based from analysis of automated plate scans. The advent of CCD surveys and improved analysis techniques have allowed astronomers to push to even fainter, more diffuse systems. In the past fwe years, SDSS has yielded a new class of systems, dubbed ultra-faint dwarfs, with lminosities less than 104 LSun and mass-to-light ratios exceeding 100. The faintest of these systems, Bootes I, has a total luminosity of only ~300 LSun, or only ten times more luminous than Vega. The theoretical expectatio is that these systems represent dark-matter dominated satellites predicted by Lambda-CDM models. The present program will use the Advanced Camera for Surveys (ACS) to obtain deep F606W (V/R) and F814W (I) images of six systems, Bootes I, Canes Venatici, Hercules, Leo IV, Ursa Major I and Coma Berenices. The aim is to use detailed data on the colour-magnitude diagram to probe the chemical composition, age and star formation histories of these systems.

GO 12607: Using Hubble to Measure Volatile Abundances and the D/H Ratio in a Bright ToO Comet

Comet Garradd C/2009 P1, taken by Chris Cook on September 2, 2011 from Cape Cod (for further details, and more spectacular images, see www.cookphoto.com )

Comets are the least substantial and most spectacular inhabitants of the Solar System. Near the Sun, great comets can spawn tails that are millions in miles in length, stretching over tens of degrees across the sky. The source of this splendour is a small, icy nucleus, typically no more than 10-20 kilometres in size. Comets are believed to have played an important role in the early Solar System, delivering water to Earth shortly after its formation. However, the extent of that role is as yet unclear, since observations have shown the proportion of deuterated water (HDO) is significantly higher in at least some comets (eg Comet Hale-Bopp, the Great Comet of 1997) than on Earth. However, other comets (such as Comet 103P/Hartley 2) appear to have D/H ratios much close to the ~1/6,200 ratio found in Earth's oceans. The present program aims to shed further light on this issue by adding observations of an additional bright comet. The target chosen is Comet Garradd C/2009, discovered by Gordon Garradd from Siding Spring using the Uppsala 0.5-metre Schmidt telescope. At that time, it was approximately 8 AU from the Sun and fainter than 17th magnitude. Perihelion passage occured in late December at ~1.25 AU, by which time the comet was 6th or 7th magnitude. The brightness, plus the fact that the comet is following a hyperbolic orbit, strongly suggesting that this is its first passage, make it an excellent target for the current program. The Space Telescope Imaging Spectrograph (STIS) will be used to obtain near-UV spectra of the nucelus.