This week on HST

HST Programs: March 18 - March 24, 2013

Selected highlights

GO 12893: Study of Small and Cool Kepler Planet Candidates with High Resolution Imaging

The Kepler satellite

Kepler is a NASA Discovery-class mission, designed to search for extrasolar planets by using high-precision photometric observations to detect transits. Launched on 7 March 2009, Kepler continuously monitors ~100,000 (mainly) solar-type stare within a ~100 square degree region in Cygnus. The mission has been an astounding success. Ground-based observations have successfully detected a couple of dozen transiting planets (e.g. HD 209458); almost all are "hot jupiters", gas giants on short-period orbits which produce a photometric dip of ~10-2 with a periopd of a few days, with a smattering of neptune-sized "super-Earths". Kepler, in contrast, has so far identified 1,790 exoplanet host stars and a total of 2,321 transiting exoplanets. More significantly, the exquisite precision of Kepler's photometric observations enables it to detect the 0.01% transit signature of earth analogues in these systems. A subset of stellar binaries provide one of the main sources of confusion in searching for planetary transits, since "grazing" transits can mimic the planetary signature. This is particularly an issue with Kepler, since the optical system is designed to provide a broad psf, spreading the stellar flux over a large area on the detector to allow high photometric accuracy. As a result, faint eclipsing stellar binaries will contribute to the source counts. Moreover, since the target field is (intentionally) within the Milky Way, there is a significant potential for unresolved stars within the (relatively broad) Kepler psf to increase the total signal, and hence dilute the depth of transits, giving the appearance of a smaller diameter exoplanet. This program is using the high spatial resolution imaging provided by HST to study a subset of the Kepler Earth-like candidates to assess the potential of this effect.

GO 12930: WISE Discovered Ly-alpha Blobs at High-z: The missing link?

Lab-1, the largest Lyman-alpha blob currently known, at z=3.1

Lyman-alpha blobs are large concentrations of gas that have been detected through their strong emission of Lyman alpha radiation. Most of those found have been foudnt rhough imaging at optical wavelengths, and therefore tend to lie at reshifts exceeding z=2. Some of these form coherent structures, including 3-D ~70-Mpc-scale filamentary structures. The present program aims to capitalise on recent discoveries made by the Wide-field infrare Survey Explorer. WISE has identified a number of Ly-alpha emitting blobs that have strikingly different energy distributions at near- and mid-infrared wavelengths, strongly suggestive of the presence of substantial quantities of dust. These objects have radically different energy distributions that the optically-identified LABs at z>2. The present program builds on a Cycle 19 program (GO 12481) and will use H-band observations with the WFC3-IR camera to obtain high-resolution images of these unusual systems in ther est-frame optical wavelengths, mapping both the overall energy distribution and the detailed morphology in Lyman-alpha and the rest-frame UV continuum.

GO 12942: Testing the Merger Hypothesis for Black Hole/Galaxy Co-Evolution at z~2

Composite optical/radio image of CenA, the elliptical merger that harbours the nearest AGN

Quasars are highly energetic sources that can achieve luminosities substantially exceeding 1012 LSun. These objects are generally believed to be powered by accretion onto a central supermassive black hole, with M > 107 MSun. Many QSOs reside within galaxies that are morphologically similar to elliptical galaxies, which are predominantly gas poor at th present epoch. This raises the issue of how one fuels the central, active black hole. One possibility is through mergers, with the QSO host assimilating gas-rich neighbours. The persent program aims to test this hypothesis through observations of luminous, dusty QSOs at redshifts z>2. The targets were selected by cross-referencing the FIRST radio catalogue with 2MASS near-IR survey data, combined with follow-up spectroscopic confirmation of the nature of the sources. The goal of this program is to use near-infrared imaging with the WFC3-IR camera to search for morphological evidence of merging systems that coudl power the central black hole.

GO 13178: Spitzer Trigonometric Parallaxes of the Solar Neighborhood's Coldest Brown Dwarfs

The stellar menagerie: Sun to Jupiter, via brown dwarfs

Brown dwarfs are objects that form in the same manner as stars, by gravitational collapse within molecular clouds, but which do not accrete sufficient mass to raise the central temperature above ~2 million Kelvin and ignite hydrogen fusion. As a result, these objects, which have masses less than 0.075 MSun or ~75 M<\sub>Jup, lack a sustained source of energy, and they fade and cool 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 objects are characterised as spectral type L. Below 1,300K, strong methane bands appear in the near-infrared, characteristics of spectral type T. At present, the coolest T dwarfs known have temperatures of ~650 to 700K. At lower temperatures, other species, notably ammonia, are expected to become prominent, and a number of efforts have been undertaken recently 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. Identifying such ultra-ultracool dwarfs was a goal of the WISE satellite mission, which recently completed its all-sky survey. WISE has succeeded in identifying close to a dozen very red, and therefore very cool, stellar sources. Initial follow-up observations with both Spitzer and HST suggest that at least may have effective temperatures less than 400K, although subsequent analyses suggests that those temperatures may be udnerestimated. These are the first examples of the new spectral class of Y dwarfs. The present program follows up on the initial discoveries by combining astrometry from Spitzer imaging, deriving proper motions and parallaxes, with low-resolution spectroscopy with the G102 and G141 grisms on WFC3-IR. The overall goal is to establish reliable distances, luminosities and temperatures, defining these sources as benchmarks in the lowest reaches of the brown dwarf regime.