This week on HST

HST Programs: April 24 - April 30, 2006

Some selected highlights

GO 10496: Decelerating and Dustfree: Efficient Dark Energy Studies with Supernovae and Clusters

HST images of intermediate redshift supernovae

The last few years of the twentieth century saw a revolution in cosmology, with the measurement of the acceleration term in expansion at high redshifts and the identification of dark energy as a major cosmological component. Type Ia supernovae are the prime yardstick for measuring the rate of expansion at moderate and high redshifts, and Hubble offers almost the only way of obtaining reliable post-maximum photometry of these objects to determine the full shape of the light curve. Most previous HST supernovae programs have concentrated on field galaxies, such as those illustrated here, but applying appropriate corrections for in situ reddening by dust remains an issue in these systems. The present program aims to minimise the uncertainties by searching for supernovae in massive, high-redshift clusters, with the expectation that the majority of detections lie within dust-poor elliptical galaxies. ACS survey observations of eight clusters are scheduled for the coming week, together with follow-up NICMOS observations of a supernova detected in previous ACS images.

GO 10514: Kuiper Belt Binaries: Probes of Early Solar System Evolution

A composite of HST images of the Kuiper Belt binary, WW31

The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun (see David Jewitt's Kuiper Belt page for details). Over 500 KBOs are currently known out of a population of perhaps 70,000 objects with diameters exceeding 100 km. Approximately 2% of the known KBOs are binary (including Pluto, one of the largest known KBOs, regardless of whether one considers it a planet or not). This is a surprisingly high fraction, given the difficulties involved in forming such systems and the relative ease with which they can be disrupted. It remains unclear whether these systems formed from single KBOs (through collisions or 3-body interactions) as the Kuiper Belt and the Solar System have evolved, or whether they represent the final tail of an initial (much larger) population of primordial binaries. This proposal aims to use ACS/HRC images of known KBOs toidentify new binary systems.

GO 10573: Globular Clusters in the Direction of the Inner Galaxy

Colour-magnitude diagram for the bulge globular cluster, NGC 5927 (ground-based data)

Globular clusters have served as important probes of Galactic structure since the early years of the twentieth century, when Harlow Shapley used their spatial distribution as evidence for his `Large Galaxy' model. Their ages, chemical abundances and kinematics provide insight into the formation and early evolution of the Milky Way. A substantial number of globulars lie in the inner Galaxy, many of which are probably associated with the Bulge, rather than the halo (as originally proposed by Robert Zinn in 1980). The line of sight towards these systems passes through the heart of the Galactic disk, and most are subject to significant reddening by foreground dust. This can prove a severe hindrance to reliable interpretation on the colour magnitude diagrams. This proposal will obtain ACS/WFC observations in the F435W, F555W and F814W filters, and will use the resultant B, V and I data to map the differential extinction across 27 Bulge globular clusters. All of these clusters are inc highly crowded fields, so the ACS observations are necessary to derive accurate CMDs and determine reliable intrinsic properties.

GO 10626: A Snapshot Survey of Brightest Cluster Galaxies and Strong Lensing to z = 0.9

Lensed arcs in an intermediate redshift galaxy cluster

Gravitational lensing provides a powerful method of tracing the mass distribution in galaxy clusters, while amplifying the light from background galaxies. This proposal will obtain ACS observations of a subset of 150 galaxy clusters, searching for the tell-tale arcs that reveal gravitational lensing. The aim is to use those data to measure the mass distribution in the targeted clusters, and constrain the evolution of both the baryonic and dark mass in cluster cores at redshifts less than z=0.9. These images will also yield a well-defined sample of arcs formed by strong lensing; the frequency and detailed distribution (size, multiplicity, redshifts) of these strong lens systems sets strong constraints on the total mass content (and its structure) in the centers of the targeted clusters.