This week on HST

HST Programs: January 4 - January 10, 2016

Selected highlights

GO 13653: Elementary Abundances of Planetary Systems

Probing the atmosphere of a transiting exoplanet

The first exoplanet, 51 Peg b, was discovered in 1995 through high-precision radial velocity measurements. 51 Pegb was followed by a trickle, and then a flood of other discoveries, as astronomers realised that there were other solar systems radically different from our own, where "hot jupiters" led to short-period, high-amplitude velocity variations. Such systems were not widely predicted since the formation regime for massive planets was generally held to be beyond the "ice line" in solar systems, regions where the protoplanetsry disk is sufficiently cool that volatiles can be accumulated to form the icy cores of gas giants. Thus, the existence of such objects at small stellar separations implies either substantial planetary migration, or a different formation mechanism. Understanding the detailed composition of these exoplanets, particularly the relative fraction of volatile elements like carbon and oxygen, can offer insight into formation regime. The present program aims to tackle this question through co-ordinated ground- and space-based observations of the transiting exoplanet XO-2b, a jovian mass planet in a ~2.6 day orbit (semi-major axis 0.039 AU) around a K0 dwarf. The WFC3-IR G141 grism will be used to search for the characteristic near-infrared spectral features due to water in the exoplanet atmospheres.

GO 13765: A Cepheid-Based Distance to the Benchmark AGN NGC 4151

Ground-based image of NGC 4151

NGC 4151 is an intermediate-type spiral lying at a distance of approximately 19 Mpc from the Milky Way. Originally catalogued by Sir John Heschel, spectrscopic observations in the early 20th century revealed the presence of strong emission lines associated with the nucleus. With NGC 1068, NGC 4151 became a prototype for the class of galaxies catalogued by Carl Seyfehrt. We now know that' these features are associated with the accretion of hot gas onto a central supermassive black hole. As one of the nearest such galaxies, NGC 4151 has been subject to intense observations over the last 50 years, including analyses of the mass of the central black hole through modeling gas and stellar dynamics, and through reverberation mapping. These two measurments should be compatible, but the former relies on having a good distaince estimate. At present, NGC 4151's distance is based on purely statistical correlation, such as the Tully-Fisher relation, and therefore known to an accuracy of only 20% at best. The present program aims to redress this situation by searching for long-period Cepheid variables. These variables have their highest amplitued variation at blue wavelengths, so the WFC3 UVIS camera is being sued to search for candidates. Follow-up observations are being obtained with the WFC3-IR camera, since the Cepheid period-luminosity relation is tighter at IR wavelengths, and observations less sensitive to foreground absorption by dust.

GO 13767: Bright Galaxies at Hubble's Detection Frontier: The redshift z~9-10 BoRG pure-parallel survey

The ACS optical/far-red image of the Hubble Ultra Deep Field

Galaxy evolution in the early Universe is a discipline of astronomy that has been transformed by observations with the Hubble Space Telescope. The original Hubble Deep Field, the product of 10 days observation in December 1995 of a single pointing of Wide Field Planetary Camera 2, demonstrated conclusively that galaxy formation was a far from passive process. The images revealed numerous blue disturbed and irregular systems, characteristic of star formation in galaxy collisions and mergers. Building on this initial progam, the Hubble Deep Field South (HDFS) provided matching data for a second southern field, allowing a first assessment of likely effects due to field to field cosmic variance, and the Hubble Ultra-Deep Field (UDF) probed to even fainter magitude with the Advanced Camera for Surveys (ACS). The highest redshift objects found in the UDF have redshifts approaching z~7. Pushing to larger distances, and greater ages, demands observatons at near-infrared wavelengths, as the characteristics signatures of star formation are driven further redward in the spectrum. Wide Field Camera 3, installed in Servicing Mission 4, is well suited to these observations, and a number of programs are in place in Cycle 17 that address these issues. Indeed, WFC3 is employed in pure parallel mode by several programs. These take advantage of other science programs, usually with COS, that involve 2-5 orbit pointings on sources at high galactic latitude. The WFC3 pointing is unplanned, since it depends on the orientation adopted for the prime observations, but 2-5 orbits of IR imaging can reach galaxies at redshifts exceeding z=7 (potentially even z~8) in high latitude fields. The present program builds on similar programs in Cycles 17 and 19, and aims to detect the brightest galaxies at z~9, within 600 Myrs of the Big Bang.

GO 14038: HST Frontier Fields - Observations of Abell 370

HST observations of the Frontier Fields cluster, Abell 370

The overwhelming majority of galaxies in the universe are found in clusters. As such, these systems offer an important means of tracing the development of large-scale structure through the history of the universe. Moreover, as intense concentrations of mass, galaxy clusters provide highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. The present program builds on the highly successful CLASH program,which used 17-colour ACS/WFC3 images to map 25 galaxy clusters, tracing the mas profile and the dark matter distribution. in addition, the observations identified several lensed galaxies at redshifts that enter the JWST domaine, with the most distant object lying at a redshift z~11, within a few hundred million years of the Big Bang. The Frontier Fields program is a large-scale Director's Discretionary program that capitalises on the latter characteristic by targeting 4-6 strong-lensing galaxy clusters for very deep optical and near-infrared imaging. WFC3 and ACS will be used to observe the clusters, with simultaneous imaging obtained in parallel of a nearby "blank" field. Since the observations need to made at a specific orientation, they are being taken in two sets, ~6 months apart, alternating between detectors. Abell 370 at z=0.375 is the sixth and final target. The present observations will provide the first epoch data, with ACS covering the cluster and WFC3 centred on the parallel field.