This week on HST

HST Programs: October 17, 2011 - October 23, 2011

Selected highlights

GO 12060/12062: CANDELS: Galaxy Assembly and the Evolution of Structure over the First Third of Cosmic Time

Part of the GOODS/Chandra Deep Field South field, as imaged by HST

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 individual observations will be set to permit detection of high redshift SNe candidates for subsequent follow-up. The present set of observations is part of the GOODS (South) deep field survey.

GO 12188: Tests of Extreme Physics in Very Cool White Dwarfs

HST ACS images of white dwarfs in the globular cluster, M4

White dwarfs are the evolutionary end point for most stars with masses less than ~7 MSun. These compact degenerate objects lack any internal heat source, and therefore gradually cool from their initial temperatures of ~100,000-200,000K. As they cool, the luminosity decreases from Mbol ~ 2-3 (for the immediate post-PN object) to Mbol ~ 17 (for 10-12 Gyr-old Galactic halo white dwarfs). The rate of cooling is mass dependent (high mass dwarfs cool faster), and can be predicted using sophisticated models of white dwarf interiors. As white dwarfs cool, their spectral characteristics change. The majority of degenerates have hydrogen-rich envelopes with some metallic content (e.g. Ca, Mg), and the spectra are dominated by strong Balmer absorption features at temperatures above ~6,000K (DA white dwarfs). At hot temperatures (>30,000K), a subset of white dwarfs have pure helium atmospheres, and consequently show only helium lines (DB white dwarfs). At cooler temperatures, some metal lines persist, and molecular carbon features are occasionally present, but with ever decreasing temperature, the spectra become featureless and white dwarfs enter the all-embracing DC (featureless spectrum )class. The present program will use near-UV STIS spectroscopy to map the the overall spectral energy distribution at these short wavelengths. The results will be matched against state-of-the-art models, setting constraintsn on the detailed atmosphere composition of these cool objects.

GO 12264: The Relationship between Gas and Galaxies for 0

A computer simulation of galactic gas accretion and outflow

Galaxy formation, and the overall history of star formation within a galaxy, demands the presence of abundant gaseous material. The detailed evolution of individual systems depends on how gas is accreted, recycled, circulated through the halo and, perhaps, ejected back into the intergalactic medium. Tracing that evolutionary history is difficult, since gas passes through many different phases, some of which are easier to detect than others. During accretion and, probably, subsequent recycling, the gas is expected to be reside predominantly at high temperatures. The most effective means of detecting such gas is through ultraviolet spectroscopy, where gas within nearby systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. The present program is using the Cosmic Origins Spectrograph to observe four QSOs with redshifts in the range 1 < z < 1.2. All four lie within regions of the sky that have been well surveyed for galaxies, with extensive data already available describing the redshift distribution of large-scale structures along the line of sight. The QSOs provide the background sources, and the goal is to use COS observations with the G130M and G160M gratings to probe hot gas along the line of sight, and match that distribution against the galaxian data.

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.

---

Past weeks:

• Cycle 14 observations (from March 13 2006 to June 30 2006)
• Cycle 15 observations (from July 1 2006)
• Cycle 16 observations (from July 1 2007)
• Cycle 17 observations (from July 13 2009)
• Cycle 18 observations (from August 30 2010)
• Cycle 19 observations (from October 3 2011)

---

page by Neill Reid, updated 10/10/2011