This week on HST

HST Programs: December 6, 2010 - December 12, 2010

Selected highlights

GO 11585: Tracing the distribution of gas and galaxies using three closely-spaced background QSOs

A computer simulation of gas in the intergalactic medium

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution therefore depends on how gas is accreted, recycled, circulated through the halo and 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 subsequent recycling, the gas is expected to be reside predominantly at high temperatures. The most effective means of detecting hot gas is through ultraviolet spectroscopy, where gas associated with relatively nearby galaxies and galaxy clusters systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. The sampling of galactic halos and the intergalactic medium is usually very sparse, since one cannot place quasars "to order". This program, which is focused on studying the gas in the IGM, takes advantage of the serendipitous presence of three bright quasars (redshifts z~0.7 to 1.2) within~1 arcminute on the sky. The Cosmic Origins Spectrograph will be used to obtain far-UV spectra, covering absorption from both the warm (~10,000 K) and the warm-hot (~100,000 K) intergalactic medium. These observations will be combined with hydrodynamical simulations to characterise the densities, metallicies and kinematics of IGM clouds along these lines of sight.

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 12320: The Ages of Globular Clusters and the Population II Distance Scale

Hubble Heritage image of the globular cluster, M15

Globular clusters are the oldest structures within the Milky Way that are directly accessible to observation. They are relatively simple systems, with relatively simple colour-magnitude diagrams (albeit with some complexities adduced from recent HST observations, see GO 11233 ). Matching those CMDs against theoretical models allows us to set constraints on the age of the oldest stars in the Galaxy, and hence on the age of the Milky Way and the epoch of galaxy formation. However, the accuracy of those age determinations rest crucially on the accuracy of the cluster distance determinations. The clusters themselves lie at distances of several kpc at best, and tens of kpc at worst; thus, direct trigonometric parallax measurements must await microacrsecond astrometric missions. The classical method of deriving distances is main sequence fitting - using nearby stars, with similar chemical abundances and accurate parallax measurements, to map out the main sequence in absolute units, and then scaling the cluster data to fit. The problem with this method is that metal-poor subdwarfs are rare, so even Hipparcos was only able to obtain accurate distances to a handful of stars. The present program aims to improve the distance measurements by using the Fine Guidance Sensors on HST to determine sub-millarcsecond trigonometric parallaxes to 9 subdwarfs, almost doubling the sample available for MS fitting.

GO 12328: 3D-HST: A Spectroscopic Galaxy Evolution Treasury

A grism spectrum of a high-redshift source

Throughout its twenty-plus year career, HST has strongly influenced our understanding of numerous research areas within astronomy and astrophysics. Among those, galaxy evolution stands out as a discipline that has been essentially revolutionised by Hubble observations. The original Hubble Deep Field, the product of 10 days observation in December 1995 of a single pointing of Wide Field Planetary Camera 2, demonstrated conclusively that galaxy formation was a far from passive process. The images revealed numerous blue disturbed and irregular systems, characteristic of star formation in galaxy collisions and mergers. Building on this initial progam, the Hubble Deep Field South (HDFS) provided matching data for a second southern field, allowing a first assessment of likely effects due to field to field cosmic variance; the Hubble Ultra-Deep Field (UDF) probed to even fainter magitude with the Advanced Camera for Surveys (ACS); recent deep near-infrared imaging with WFC3 has pushed the redshift limit to beyond z~8; and the Multi-Cycle Treasury CANDELS program is providing a tiered set of observations that complement, in areal coverage and depth, the deep UDF observation. All of these programs rely on deep, high-sensitivity, high resolution imaging. However, WFC3 is also eqipped with grism/prism optical elements, and therefore offers the potential for low-resolution spectroscopy at optical and near-infrared wavelengths. The present program capitalises on that ability, using the near-infrared WFC3/G141 and far-red ACS/G800L grisms to survey subsets of the GOODS-South, AEGIS, UDS and COSMOS fields covered by the CANDELS survey. These data will provide spectroscopic means of probing star formation at redshifts in the range 1 < z < 3.5.