This week on HST

HST Programs: March 19 - March 25, 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 12479: Low-z analogs of high-redshift Lyman Alpha emitters

Galaxies in the GOODS-North field

High-redshift (z>5) galaxies are generally characterised by strong emission lines, notably at Lyman alpha. This is generally believed to be correlated with substantial star formation activity in these systems. These high redshift galaxies are also generally associated with significant dust absorption, complicating the physical interpretation of the observed parameters. Observations have shown that such systems are first detectable around redshift 1, and these lower redshift systems are subject to much less interpretive uncertainties due to environmental factors. The present program aims to use the WFC3 UVIS G280L grism to survey the full GOODS-North field to search for strong Ly-alpha emitters with redshifts 0.7 , z < 1.8. In order to minimise confusion and limit overlapping spectra, these observations are being taken on 24 pointings, giving a total areal coveage of ~140 square arcminutes. Follow-up observations will be used to expand wavelength coverage for candidate systems, confirming their nature and investigating their intrinsic properties.

GO 12550: Physics and Chemistry of Condensate Clouds across the L/T Transition - A SNAP Spectral Mapping Survey

Brown dwarfs are likely to have complex atmospheric structures that resemble Jupiter

Brown dwarfs are failed stars - objects that form like stars, by gravitational collapse within giant molecular clouds, but which have insufficient mass to raise the central temperature above 107 K, and which therefore are unable to ignite hydrogran fusion and maintain a long-lived central energy source. As such, these objects reach a maximum surface temperature of perhaps 3,000K some tens of millions of years after their formation, and subsequently cool and fade into oblivion. As they cool, they move through spectral types M, L and T, with the oldest brown dwarfs now likely to have temperatures close to 300K and emergent spectra characterised by water and ammonia bands, the putative signatures of the spectral class Y. As these dwarfs cool from L to T (~1500 to ~1200K), the atmospheres undergo significant changes, with heavier elements condensing to form dust. That dust can form clouds, perhaps giving the dwarf's surface a banded appearance, similar to Jupiter. or leading to the generation of localised cloud features. Since the rotation periods for these obejcts are usually a matter of a few hours, the appearance and disappearance of asymmetric features might lead to periodic photometric or spectroscopic variability, and at least photometric variability has been detected for a handful of ultracool dwarfs. The clouds themselves may appear and disappear over relatively short timescales, leading to longer-term photometric variations at particular wavelengths. A Cycle 18 program (GO 12314) focuses on a handful of brown dwarfs near the L/T transition, using the WFC3 grism to obtain multi-orbit, high-accuracy monitoring of their spectral behaviour. That program succeeded in recovering significant variability, with periods from 5-10 hours. The present program builds on those results by extending coverage to a much larger sample of late-L/early-T dwarfs. As a SNAP prorgam, each will be observed for only ~40 minutes. insufficient to determine periodicities, but, given the high precision of HST, sufficient to identify new candidates and gain some insight into the frequenciy of the phenomenon.

GO 12603: Understanding the Gas Cycle in Galaxies: Probing the Circumgalactic Medium

A computer simulation of galactic gas accretion and outflow

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution therefore is tied very closely to how gas is accreted, recycled, circulated through the halo and disk, 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 probe gas in the circumgalactic medium for a large sample of relatively local disk galaxies. The targets are drawn from the GALEX Arecibo SDSS Survey (GASS), with the aim of combining the various observations to map atomic, molecular and ionised gas in these systems. The galaxies lie at redshifts between 0.02 and 0.05, and COS will be used to observe QSOs whose sightlines pass within 250 kpc of the galaxy core. Those sightlines run through the halos of the galaxies, and the QSOs therefore provide a pencilbeam backlight that probes hot circumgalactic gas.