This week on HST

HST Programs: May 21 - May 27, 2012

Selected highlights

GO 12443: Galaxy Assembly and the Evolution of Structure over the First Third of Cosmic Time - III

Emission line galaxies from the UDS field

CANDELS is one of three Multi-Cycle Treasury Program, whose observations will be executed over the next three HST Cycles. It builds on past investment of both space- and ground-based observational resources. In particular, it includes coverage of the two fields of the Great Observatory Origins Deep Survey (GOODS), centred on the northern Hubble Deep Field (HDF) in Ursa Major and the Chandra Deep Field-South in Fornax. In addition to deep HST data at optical and near-infrared wavelengths, the fields have been covered at X-ray wavelengths by Chandra (obviously) and XMM-Newton; at mid-infrared wavelengths with Spitzer; and ground-based imaging and spectroscopy using numerous telescopes, including the Kecks, Surbaru and the ESO VLT. This represents an accumulation of almost 1,000 orbits of HST time, and comparable scale allocations on Chandra, Spitzer and ground-based facilities. The CANDELS program is capitalising on this large investment, with new observations with WFC3 and ACS on both GOODS fields, and on three other fields within the COSMOS, EGS and UDS survey areas (see this link for more details). The prime aims of the program are twofold: reconstructing the history of galaxy formation, star formation and nuclear galactic activity at redshifts between z=8 and z=1.5; and searching for high-redshift supernovae to measure their properties at redshifts between z~1 and z~2. The program incorporates a tiered set of observations that complement, in areal coverage and depth, the deep UDF observations, while the timing of a subset of the observations will be set to permit detection of high redshift SNe candidates, for subsequent separate follow-up.The present observations focus on wider-field component, obtaining ACS and WFC3 observations in the COSMOS, EGS and UDS fields.

GO 12473: An Optical Transmission Spectral Survey of hot-Jupiter Exoplanetary Atmospheres

An artist's impression of the hot Jupiter circling a solar-type star

The first planet orbiting another star was discovered in 1995 around the relatively rbight G dwarf, 51 Pegasi. 51 Pegb confounded expectations as a jovian-mass gas giant in a 3-day orbit, placing it closer to the parent star than Mercury in our Solar System. Over the past nearly 20 years, numerous other systems have been identified, with the most recent flood of identifications coming from the Kepler satellite, which is pushing the detection limit to objects as small as earth in orbits with semi-major axes exceeding 1 AU. Many of these new detections (and all of the Kepler detections) are transiting systems. Transiting systems offer a potential gold-mine for extrasolar planetary studies, since not only is the orbital inclination well defined, but the diameter (and hence the average density) can be measured directly from the eclipse depth, while the atmospheric composition can be probed through line absorption or re-radiated thermal flux. The results from these measurments can be used to test, and improve, theoretical models of extrasolar planets. These observations are best done from space (indeed, the only unequivocally successful atmospheric observations to date have been with HST and Spitzer). The present program targets nine systems, all discovered through ground-based surveys (the WASP and HAT surveys), and all comprising jovian-mass planets in short-period orbits. The program will use STIS to obtain optical spectra, covering the full wavelength range with the G430L and G750L gratings, while WFC3 will be used to target near-IR spectra with the G141 grism.

GO 12586: Detecting Isolated Black Holes through Astrometric Microlensing

A rather spectacular version of black hole lensing.

Gravitational lensing is a consequence of general relativity. Its effects were originally quantified by Einstein himself in the mid-1920s. In the 1930s, Fritz Zwicky suggested that galaxies could serve as lenses, but lower mass objects can also also lens background sources. Bohdan Paczynski pointed out in the mid-1980s that this offered a means of detecting dark, compact objects that might contribute to the dark-matter halo. Paczcynski's suggestion prompted the inception of several large-scale lensing surveys, notably MACHO, OGLE, EROS and DUO. Those wide-field imaging surveys have target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. The duration of each event depends on several factors, including the tangential motion of the lens and its mass. Long-term events are generally associated with a massive lens. Duration alone is not sufficient to identify a lens as a black hole - a source with very low tangential motion relative to the Sun can produce the same effect. However, microlensing not only leads to flux amplification, but also to small astrometric motions, caused by the appearance and disappearance of features in the lensed light. Those motions serve as a mass discriminant - higher mass lenses produce larger amplitude motions. The expected astrometric signal from a black hole lens is > 1.4 millarcseconds, just measureable with HST. This program aims to capitalise on this fact by searching for lensing by black holes in the Galactic field. The observations target long-duration lensing events in the Galactic Bulge.

GO 12606: Verifying the White Dwarf Mass-Radius relation with Sirius B and other resolved Sirius-like systems

All single stars, and most binary stars, with masses less than ~7 solar masses are expected to end their lives as white dwarfs - extremely compact objects made of degenerate material, compressing ~0.3 to 1.4 solar masses of material into a sphere little larger in radius than the Earth. Theoretical evolutionary models predict a broad correlation between the mass of the main-sequence star and the mass of the remnant, although there is significant scatter in the observed initial-final mass relation. The models also predict that white dwarfs should follow mass-radius relations that depend on the composition, temperature and internal structure. The present program aims to test the predictions of those models by determining accurate masses and radii for a sample of white dwarfs in resolved binary systems. STIS spectra will be used to measure accurate Balmer line profiles for these hot degenerates, and those pofiles can be analysed to yield effective temperatuers and surface gravities. Moreover, the H-beta line profile has a sharp core that allows accurate measurement of the apparent radial velocity of the system. This measured velocity has two main components: the star's peculiar velocity relative to the Sun; and the gravitational redshift induced by the high field on a degenerate white dwarf. Sicne these stars are members of wide binary systems, observations of the main sequence companion can be used to determine the former quantity and hence allow emasurement of the latter, and set constraints on the white dwarf mass.