This week on HST

HST Programs: May 7 - May 13, 2012

Selected highlights

GO 12255: Probing Ultracool Atmospheres and Substellar Interiors with Dynamical Masses

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

Brown dwarfs are objects that form like stars, but lack sufficient mass to drive the central temperature above a few million degrees, and therefore never succeed in igniting core hydrogen fusion. Discovered in the early- to mid-1990s (although predicted theoretically since the early 1960s), these objects initially have surface temperatures of ~3,500K, but cool rapidly and move through spcetral types M, L and T. Following their discovery, considerable theoretical attention has focused on the evolution of their intrinsic properties, particularly the details of the atmospheric changes in the evolution from type L to type T and beyond. The overall timescales associated with this process remains unclear. Mass is a crucial factor in mapping those changes, but mass is also the most difficult quantity to measure in a reliable fashion. The present program aims to tackle this issue through observations of low-mass binary systems. Previous observing programs have been used to measure astrometric orbits, and hence derive dynamical masses. The present program aims to use STIS to obtain optical spectra of each component in three binary systems. The data will cover the Li 6708 A absorption feature, whose presence is a key age indicator.

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 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 12616: Near-IR Imaging of the Most Distant Spectroscopically-Confirmed Galaxies in the Subaru Deep Field

Optical imagiung 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 the WFC3-IR camera on HST to obtain follow-up F110W/F160W (J/H) images of more than 40 spectroscopically confirmed 5.7 < z < 7 Ly-alpha emitters or lyman-break galaxies in the SDF. These data can be combined with mid-infrared imaging using the IRAC camera on Spitzer to fully characterise the spectral energy distributions and morphologies of these sources.