This week on HST

HST Programs: October 18, 2010 - October 24, 2010

Selected highlights

GO 11666: Chilly Pairs: A Search for the Latest-type Brown Dwarf Binaries and the Prototype Y Dwarf

NICMOS images of the ultracool L/T binary, 2MASS J22521073-1730134; the northern component, notably fainter at F160W, is the T dwarf.

Ultracool dwarfs are defined as having spectral types later than M7, and therefore include the recently discovered L and T dwarfs. They encompass the lowest mass stars (masses < ~0.1 MSub) and sub-stellar mass brown dwarfs, with surface temperatures ranging from ~2500K (~M7) to <700K (late-type T dwarfs). Following their discovery over a decade ago, considerable theoretical attention has focused on the evolution of the intrinsic properties, particularly the details of the atmospheric changes in the evolution from type L to type T. This point marks the emergence of methane as a dominant absorber at near-infrared wavelengths. Current models suggest the transition occurs at ~1400-1200K, and that the spectral changes are at least correlated with, and perhaps driven by, the distribution and properties of dust layers ("clouds") within the atmosphere. The overall timescales associated with the process remain unclear. The present proposal aims to tackle this issue through identifying, and characterising, ultracool binary systems with extremely cool components. Since these systems are almost certainly coeval, the relative spectral energy distributions of the two components can be used to set constraints on evolutionary models. More than 80 ultracool binary systems are currently known; almost all have relatively small linear separations (<15 AU), and components with mass ratios close to one. The present program targets 27 ultracool dwarfs with spectral types in the range T5 to T9, and will use WFC3 IR observations to search for previously unrecognised close, faint companions.

GO 12163: Structure and Stellar Content of the Nearest Nuclear Clusters in Late-Type Spiral Galaxies

the nearby spiral, NGC 300, one of the galaxies targetted in this program

Extensive observations over the past decade, primarily with HST, have shown that massive, young star clusters are present in the nuclear regions of almost all late-type spiral galaxies. It remains unclear, however, whether those clusters harbour (or generate) the black holes that are commonly found in the central regions of both ellipticals and spiral galaxies with significant bulge components, such as our Milky Way. The present program aims to address this issue through systematic observations of 10 relatively nearby bulgeless spirals. The aim is to combine multi-wavelength, high resolution imaging, using WFC3 on HST, with ground-based spectroscopy of the central regions of these systems. The resultant detailed colour-magnitude diagrams and spectral line strengths will be used in conjunction with stellar population models to constrain the star formation history and stellar content of the central regions, while the kinematics will probe the dynamics of the central regions, and set limits on the likely presence of central black holes in these systems.

GO 12184: A SNAP Survey for Gravitational Lenses Among z~6 Quasars

Temporal variation in the intensities of the lensed components of the Einstein Cross

Gravitational lensing is a consequence 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 also increases the apparent brightness of the background sources. This effect can be used to our advantage, in enabling detailed observations of high-redshift sources that be too faint to observe under normal circumstances, but it can also lead to statistical biases in parameters such as luminosity functions. These effects are likely to be of most importance for higher redshift sources, where the longer pathlength leads to a higher probability of the light encountering a foreground lens. The present program aims to address this issue for by using WFC3 to obtain high resolution images F105W ("Y"-band) of a subset of 54 QSOs with redshifts in the range 5.7 < z < 6.4.

GO 12234: Differentiation in the Kuiper belt: a search for silicates on icy bodies

Some of the larger Solar System trans-Neptunian objects

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. HST itself is being used by a number of programs to push observations to even fainter magnitude limits, and obtain accurate multi-colour photometryy of these objects. Interestingly, the KBOs observed in the course of these HST programs have shown evidence for a range of spectral properties, possibly indicative of difference composition. The present program aims to further probe this issue through narrow-band 1-1.3 micron imaging of 13 KBOs.