This week on HST

HST Programs: December 14 - December 20, 2009

Selected highlights

GO 11149: Characterizing the Stellar Populations in Lyman-Alpha Emitters and Lyman Break Galaxies at 5.7<7 in the Subaru Deep Field

Optical imaging of the Subaru deep field

The Subaru Deep Field (SDF) lies at 13h24m38.9s+27d29'25", near the North Galactic Pole and more than 30 arcminutes from the nearest bright star or galaxy. The field has been imaged to faint magnitudes (V>27.5, JH>26.0) at both optical and near-infrared wavelengths using, respectively, Suprime-Cam and CIRCO mounted on the Subaru 8.2-metre telescope, located on the summit of Mauna Kea, Hawaii. The goal is to use these data to probe the high redshift universe, searching for "drop-outs" with colours consistent with redshifts z > 6 - sources where the Lyman limit is redshifted longward of 7000 Angstroms, leading to no flux in the optical r-band and extremely red (r-i) colours. High redshift systems are expected to be relatively rare, so these ground-based observations offer an advantage over HST in terms of areal coverage (Suprime-Cam covers 30'x37'); however, the higher angular resolution and increased sensitivity of HST offer significant advantages in characterising candidate galaxies. The present proposal is using NICMOS on HST to obtain follow-up F110W/F160W (J/H) images of 20 spectroscopically confirmed 5.7 < z < 7 Ly-alpha emitters or lyman-break galaxies in the SDF. In addition, the IRAC camera on Spitzer is being used to obtain mid-IR data for the same galaxies. The observations will be used to fully characterise the spectral energy distributions and morphologies of these sources.

GO 11588: Galaxy-Scale Strong Lenses from the CFHTLS survey

ACS images of galaxy-galaxy Einstein ring lenses

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 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. The CFHT Legacy survey was a concerted effort by Canada and France to use almost half of their dark/grey time on the Canada-France-Hawaii telescope to obtain deep images with the MegaCam 1x1 degree imager. In ttoal, about 450 nights were devoted to this project between mid-2003 and early 2009, covering almost 600 square degrees to varying depths. The resultant survey provides a powerful means for identifying candidate galaxy-galaxy lenses. This program is using WFC3 SNAP imaging to verify the nature of those candidates, and provide the angular resolution necessary to model the mass distribution.

GO 11644: A dynamical-compositional survey of the Kuiper belt: a new window into the formation of the outer solar system

The view from Sedna: an artist's impression

The Kuiper Belt lies beyond the orbit of Neptune, extending from ~30 AU to ~50 AU from the Sun, and includes at least 70,000 objects with diameters exceeding 100 km. Setting aside Pluto, the first trans-Neptunian objects were discovered in the early 1990s. Most are relatively modest in size, with diameters of a few hundred km and photometric properties that suggested an icy composition, similar to Pluto and its main satellite, Charon. Over the last three years, a handful of substantially larger bodies have been discovered, with diameters of more than 1000 km; one of the objects, 2003 UB313, is comparable in size to Pluto (2320 km.). At the same time, ground-based surveys, such as the Deep Ecliptic Survey, the Canada-France Ecliptic plane Survey and the Palomar Quest Survey, scanned the ecliptic for fainter, lower-mass objects, with the aim of using their properties to assess the likely chemical composition and dynamical history of the early Solar System. The present program will use Wide Field Camera 3 to push up to 2 magnitudes fainter than these ground-based studies, providing reliable estimates of compositions for a representative sample of KBOs.

GO 12044: Observations of the Coldest Brown Dwarf

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 obejcts 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 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. Indeed, finding such ultra-ultracool dwarfs is a goal of the recently launched WISE satellite. In the meantime, Spitzer used the IRAC instrument to conduct a deep survey of 10 square degrees at mid-infrared wavelengths, identifying several Y-dwarf candidates. These HST observations use WFC3 to target the brightest such candidate.