This week on HST

HST Programs: March 12 - March 18, 2012

Selected highlights

GO 12192: A SNAPSHOT Survey of Interstellar Absorption Lines

A map of the Local Stellar Neighbourhood

Understanding the nature and structure of gas within the interstellar medium is a key step towards understanding how material is recycled and how energetic processes, such as stellar winds and outflows, feed energy into the overall system. UV spectroscopy plays a key role in probing these effects: hot, background objects that produce relatively few intrinsic absorption features serve to map the the velocities and temperatures within the intervening gas along the line of sight. Observations of quasars are used to probe galaxy halos at moderate and high redshift; observations of hot stars provide similar information for gas in the Milky Way. The present program is using high-resolution, echelle observations with STIS to target O and B stars with a few kpc of the Sun, probing the interstellar medium along the line of sight. All of these stars have prior low-resolution observations at far-UV wavelengths with FUSE, providing a reliable guide to the flux levels. The overall goal is to determine the density, temperature and abundance distributions within the ISM along a wide variety of sight-lines throught the Galacic disk and inner halo.

GO 12488: SNAPshot observations of gravitational lens systems discovered via wide-field Herschel imaging

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 that were being detected and investigates 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. HST has carried out a number of programs following up candidate lenses identified from the Sloan Digital Sky Survey (eg GO 10886 , GO 11289 , GO 12210 ). The present program is using WFCE on HST to obtain follow-up near-infrared (F110W) images of up to 200 candidate lenses selected from the Herschel Astrophysical Terahertz Large Area (H-ATLAS) and the Herschel Multi-tiered Extra-galactic (HerMES) surveys. The HST data will verify the nature of those candidates, and provide the angular resolution necessary to model the mass distribution.

GO 12504: Bridging the Brown Dwarf/Jupiter Temperature Gap with a Very Cold Brown Dwarf

The very low-mass binary system, CFBDSIR J1458+10AB

Brown dwarfs are objects that form in the same manner as stars, by gravitational collapse within molecular clouds, but which failt to accrete sufficient mass to raise the central temperature above ~2 million Kelvin and ignite hydrogen fusion.In consequence, these objects, which at solar abundances have masses less than 0.075 MSun or ~75 M<\sub>Jup, lack a sustained source of energy, and cool and fade on relatively short astronomical (albeit, long anthropological) timescales. Following their discovery over a decade ago, considerable observational and theoretical attention has focused on the evolution of their intrinsic properties, particularly the details of the atmospheric changes. At their formation, most brown dwarfs have temperatures of ~3,000 to 3,500K, comparable with early-type M dwarfs, but they rapidly cool, with the rate of cooling increasing with decreasing mass. As temperatures drop below ~2,000K, dust condenses within the atmosphere, molecular bands of titanium oxide and vanadium oxide disappear from the spectrum to be replaced by metal hydrides, and the obejcts are characterised as spectral type L. Below 1,300K, strong methane bands appear in the near-infrared, characteristics of spectral type T. At lower temperatures, other species, notably ammonia, are expected to become prominent, and attempts are currently under way to find examples of these "Y" dwarfs. The search is complicated by the fact that such objects are extremely faint instrinsically, so only the nearest will be detectable. Wide-field surveys have been undertaken at infrared wavelengths with both ground-based telescopes (eg UKIDDS) and satellite observatories (eg WISE). However, an alternative approach is to "look uner the lamp-post": both stars and brown dwarfs are often found as binary or multiple systems, so one can take a sample of low-mass obejcts known to be within the Solar Neighbourhood, and look for even lower luminosity companions. That technique served in the past to identify van Biesbroeck 10, the first ultracool dwarf; GD 165B, the first L dwarf; and Gl 229B, the first T dwarf. The prime target of this proposal, CFBDSIR J1458+10B, was found in a similar manner, through deep infrared imaging targetting known nearby ultracool dwarfs. The primary, a ~T9 dwarf, was discovered in the course of the Canada-France Brown Dwarf Survey survey (hence CFBDS), lies at a distance of ~23 parsecs, has a luminosity of ~10-6 LSun and a temperature around 550K. The secondary was uncovered at a separation of 0.11 arcseconds through Keck laser AO imaging, is 2 magnitudes fainter than the primary at H, has a luminosity of ~2 x 10-7 LSun and is the coolest brown dwarf currently known, with a surface temperature estimated as only 370 K, comparable with boiling water. HST will be used to obtain far-red and near-infrared photometry, with the aim of better characterising the spectral energy distribution at those wavelengths.

GO 12283/12568: WISP - A Survey of Star Formation Across Cosmic Time

A region of massive star formation

Star formation is the key astrophysical process in determining the overall evolution of galactic systems, the generation of heavy elements, and the overall enrichment of interstellar and intergalactic material. Tracing the overall evolution through a wide redshift range is crucial to understanding how gas and stars evolved to form the galaxies that we see around us now. The present program builds on the ability of HST to carry out parallel observations, using more than one instrument. While the Cosmic Origins Spectrograph is focused on obtaining ultraviolet spectra of unparalleled signal-to-noise, this program uses the near-infrared grisms mounted on the Wide-Field Camera 3 infrared channel to obtain low resolution spectra between 1 and 1.6 microns of randomly-selected nearby fields. The goal is to search for emission lines characteristic of star-forming regions. In particular, these observations are capable of detecting Lyman-alpha emission generated by star formation at redshifts z > 5.6. A total of up to 40 "deep" (4-5 orbit) and 20 "shallow" (2-3 orbit) fields will be targeted in the course of this observing campaign.