This week on HST

HST Programs: November 19 - November 25, 2007

Some selected highlights

GO 10798: Dark Halos and Substructure from Arcs & Einstein Rings

Hubble imanges of Einstein ring gravitational lenses

Gravitational lensing provides a powerful method of tracing the mass distribution of individual galaxies and galaxy clusters; at the same time, the amplified the light from background galaxies provides a means of probing the early stages of galaxy formation. The Sloan Digital Sky Survey (SDSS) has proven a powerful tool for identifying new lens candidates, although high angular resolution HST observations are essential for confirming those candidates as genuine gravitational lensing systems (see Program GO 10886). The present program focuses on that sample, targeting confirmed lenses for deep, multicolour imaging that aims to measure the overall mass distribution of the lens galaxies, and quantify the existence of small-scale sub-structure. The current cosmological paradigm, cold dark matter, predicts that sub-clustering within galactic halos should scale with increasing redshift; observations of lenses at different redshifts will serve to test this prediction.

GO 11109: Characterization of the UV absorption feature in asteroid (1) Ceres

HST images of Ceres

The asteroid Ceres has undergone more changes in classification than most astronomical objects. Discovered by Giuseppe Piazzi on New Year's day 1801, Ceres was first identified as a planet lying between Mars and Jupiter (consistent with the Titius-Bode sequence). It soon became clear that Ceres was much smaller than any known planet (a diameter of only 950 km), and William Herschel suggested that it should be called an "asteroid" (star-like body), since it was unresolved in telescopes of the time. Within 6 years. Pallas, Juno and Vesta had been added to the roster, and the descriptor "minor planet" was applied. Most recently, Ceres has changed status again, becoming, with Pluto, a "dwarf planet", the only such body within the asteroid belt. Nomenclature apart, observations of Ceres can provide important insight into the chemical composition of the bodies within the main asteroid belt. Ceres and Vesta will be targeted by the NASA Discovery Mission, Dawn, launched on 27 September this year. The present proposal aims to use the objective prism on the ACS/Solar Blind Channel to obtain uv spectra covering the wavelength range 1200 to 2000 Angstroms. The prime goal is to obtain an improved characterisation of a broad absorption feature residing at these wavelengths, and obtain a better understanding of its origin.

GO 11175: UV Imaging to Determine the Location of Residual Star Formation in Galaxies Recently Arrived on the Red Sequence

Galaxy mergers and the red sequence

The overwhelming majority of galaxies are found in clusters. Observations show that almost all well-defined cluster systems at low and moderate redshift have a significant population of elliptical galaxies which have red colours, indicative of old stellar populations and minimal current star formation. The elliptical galaxies outline a distinct sequence in the colour-magnitude (or colour-mass) diagram, the so-called red sequence. Over the last few years, there has been considerable interest in understanding the origins of this sequence: how did the ellipticals form (most theories envisage mergers of gas-rich systems at moderate redshifts)? when did star formation cease in these galaxies (most galaxies in clusters at redshifts 1.5 < z < 3 seem to have active star formation)? are there environmentally-dependent effects? This proposal aims to address some of these questions through WFPC2 BVI and ACS/SBC observations of a number of low-redshift (0.04 < z , 0.10) galaxies that appear to have only recently arrived on the red sequence. The WFPC2 data will provide detailed morphologies, while the SBC ultraviolet imaging will be ued to search for traces of residual star formation.

GO 11225: The Wavelength Dependence of Accretion Disk Structure

The first Einstein cross, the gravitational lensed QSO, G2237+0305

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 to probe the mass distributions on a variety of scales: of galaxies (primarily via multiply-imaged quasars); of galaxy clusters (arcs and arclets); and at the largest scales (weak lensing). However, lensing can also provide insight on the small-scale properties of the object being lensed. In a lensed QSO, the light from the QSO follows different paths to produce the separate images; each of those paths has a different length; consequently, flux variations in the source show up at different times in the separate images. The present program aims to take advantage of this property to probe the structure of the accretion disks surrounding the central black hole in a number of lensed QSOs. The program will combine ultraviolet observations with the ACS/SBC on HST with Chandra X-ray data. Studying the variation as a function of wavelength should probe the accretion disk structure, since light from the inner regions are expected to dominate at shorter wavelengths, while the outer regions dominate at longer wavelengths.