This week on HST

HST Programs: February 6 - February 12, 2012

Selected highlights

GO 12461: Supernova follow-up for MCT Programs

High redshift supernovae from HST observations in previous cycles

CANDELS and CLASH are two of three Multi-Cycle Treasury Programs whose observations will be executed over HST Cycles 18, 19 and 20. Both programs have components that focus on the identification and subsequent follow-up of candidate high redshift supernovae. CLASH focuses on galaxy clusters (eg ), with 17-colour observations using ACS and WFC3. But while the cluster is centred on one camera, the second camera is being used to take deep images in fields offset by several arcminutes, and the multiple exposures obtained in those regions can be used to search for high redshift supernovae. CANDELS is a tiered wide-field imaging program that 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, those 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. CANDELS 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 existence of previous datasets permits searching for high-redshift supernovae, with the prime aim of measuring their properties at redshifts between z~1 and z~2. The present program is focused on a high-redshift candidate identifeid within the COSMOS field.

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 12532: The Scale Sizes of Globular Clusters: Tidal Limits, Evolution, and the Outer Halo

The giant elliptical, M87, and its extensive population of associated globular clusters.

Globular clusters are key remnants of the first major episode of star formation in the Milky Way Galaxy. Other galaxies, both spirals and ellipticals, have their own populations of globulars. The clusters associated with M31 are well resolved (both HST and, to a lesser extent, ground-based observations permit the derivation of colour-magnitude diagrams), but in most cases the clusters are only identifiable based on their slightly extended profiles, or through statistical analysis of starcounts in the immediate vicinity of target galaxies. Despite years of study, significant undertanties still remain regarding the physical properties of golbulars, and how those properties vary with either extrinsic or intrinsic parameters. The present program aims to probe the physical size of globular clusters, which have long been thought to have been governed largly by tidal stripping within the gravitational field of the parent galaxy. However, the recent discovory of "anomalous extended clusters" in M31 and other systems has cast some doubt on that proposition. Hundreds of globulars have been identified in M87, the core galaxy in the Virgo cluster. This program aims to map the entire halo of this system, using WFC3 and ACS to obtain deep, high angular resolution images capable of measuring the average size, and the distributionin size, of clusters as a function of radial separation from the parent galaxy.

GO 12573: STIS Coronagraphy of New Debris Disks from the WISE All-Sky Survey

HST image of the face-on debris disk in the G2 dwarf, HD 107146

Planet formation occurs in circumstellar disks around young stars. Most of the gaseous content of those disks dissipates in less than 10 million years, leaving dusty debris disks that are detectable through reflect light at near-infrared and, to a lesser extent, optical wavelengths. The structure of those disks is affected by massive bodies (i.e. planets and asteroids), which, through dynamical interactions and resonances, can produce rings and asymmetries. Analysis of the rangle of morphological structure in these systems provides insight into the distribution of properties of planetary systems. HST currently provides almost the only means of achieving the high-contrast required for the detection of scattered light from these disks in the presence of the bright parent stars. While many such systems have been observed, only a relatively small number of disks have been imaged successfully at visual or near-infrared wavelengths. The present program aims to expand the sample by targetting six solar-type stars that are known to have circumstellar disks based on mid-infrared observations with WISE, the Wide-field Infrared Survey Explorer. This sample constitutes the brightest new discoveries, and therefore the most likely candidates for detection by Hubble. The present proposal will use the occulting bar on the Space Telescope Imaging Spectrograph to carry out coronagraphic imaging at visual wavelengths.