This week on HST

HST Programs: Septermber 12, 2011 - September 18, 2011

Selected highlights

GO 11616: The Disks, Accretion, and Outflows (DAO) of T Tau stars

Wide-field image, from NOAO, of T Tauri and its immediate environs

The T Tauri stage of evolution occurs early in a star's lifetime, within ~10 Myrs of its birth, when it still retains a dense, dust and gas-rich circumstellar disk. During this phase, there is substantial accretion of material onto the central star. This leads to heating of the inner regions of the accretion disk, and significant emission at ultraviolet and X-ray wavelengths. Previous HST programs (e.g. GO 10840 ) have used the STIS and the ACS/SBC to investigate these processes at FUV wavelengths. The present program will extend those investigations using COS, which provides more than an order of magnitude more sensitivity and resolution. The survey will target 32 T Tauri stars, including 26 "classical" T Tauris and 6 "weak-lined" T Tauris (the latter are surrounded by less disk material, and are generally believed to be at a later stage of evolution than the CTTs). COS will be used to measure the emission profiles of an extensive number of lines, probing opacities, temperatures and densities in the disk and outflow regions.

GO 12060/12062: CANDELS: Galaxy Assembly and the Evolution of Structure over the First Third of Cosmic Time

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

CANDELS is one of three Multi-Cycle Treasury Program, whose observations will be executed over the next three HST Cycles. It builds on past investment of both space- and ground-based observational resources. In particular, it includes coverage of the two fields of the Great Observatory Origins Deep Survey (GOODS), centred on the northern Hubble Deep Field (HDF) in Ursa Major and the Chandra Deep Field-South in Fornax. In addition to deep HST data at optical and near-infrared wavelengths, the fields have been covered at X-ray wavelengths by Chandra (obviously) and XMM-Newton; at mid-infrared wavelengths with Spitzer; and ground-based imaging and spectroscopy using numerous telescopes, including the Kecks, Surbaru and the ESO VLT. This represents an accumulation of almost 1,000 orbits of HST time, and comparable scale allocations on Chandra, Spitzer and ground-based facilities. The CANDELS program is capitalising on this large investment, with new observations with WFC3 and ACS on both GOODS fields, and on three other fields within the COSMOS, EGS and UDS survey areas (see this link for more details). The prime aims of the program are twofold: reconstructing the history of galaxy formation, star formation and nuclear galactic activity at redshifts between z=8 and z=1.5; and searching for high-redshift supernovae to measure their properties at redshifts between z~1 and z~2. The program incorporates a tiered set of observations that complement, in areal coverage and depth, the deep UDF observations, while the timing of individual observations will be set to permit detection of high redshift SNe candidates for subsequent follow-up.

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 12725: A Deep Search for Satellites in the Pluto System: Providing Critical, Safety-of-Flight Support to NASA's New Horizons Mission

Hubble Space Telescope images of the Pluto system, including the newly discovered moon, P4

Pluto, one of the largest members of the Kuiper Belt and, until recently, the outermost planet in the solar system, has been in the news over the last year or two. Besides the extended "planet"/"dwarf planet" debate, Pluto is the primary target of the New Horizons Mission. In 1978, James Christy discovered from analyses of photographic plates that Pluto has a relatively large companion moon, Charon, with a diameter of ~1200 km, or almost half that of Pluto itself. In 2005, Hubble observations led to the discovery of two small moons, christened Nix and Hydra. These two new moons are 5,000 fainter than Pluto itself, implying diameters as small as ~30-50 km if the surface composition is similar to Pluto itself. Within the past year, a series of observations were taken in support of the New Horizons mission, using WFC3 to search for faint rings due to dust particles that might jeopardise the space craft and require a course correction. No rings werer detected; however, a fourth small satellite, "P4", was discovered. This object is even fainter than Nix and Hydra, and may well be as small as 13 km in size. The present observations, again in support of new Horizons, will use WFC3 to push to even fainter magnitudes to both better characterise the P4 orbit and search for even fainter moons.