This week on HST

HST Programs: April 2 - April 8, 2007

Some selected highlights

GO 10786: Rotational state and composition of Pluto's outer satellites

Hubble Space Telescope image of Pluto, Charon and the two new moons, Nix & Hydra

Pluto, one of the largest members of the Kuiper Belt and, until recently (still, for some), the outermost planet in the solar system, has been in the news over the last year or two. Besides the great "planet"/"dwarf planet" debate, Pluto is the primary target of the New Horizons Mission, and Hubble observations in 2005 led to the discovery of two small moons. Together with Charon, itself only discovered in 1978, these additions make Pluto a 4-body system. Christened Nix and Hydra, the two new moons are 5,000 fainter than Pluto itself, implying diameters as small as ~30-50 km if the surface composition is similar to Pluto itself. The present program aims to better characterise these bodies through multicolour observatioins with WFPC2. The observations will also look for systematic photometric variations that might probe the rotation period. with the aim of testing whether these moons are in synchronous rotation with Pluto itself.

GO 10859 : Precise Measurements of Sgr A* Flare Activity

Chandra X-ray images of the Galactic Centre region; the bottom right panel is centred on Sgr A*

Sagittarius A* is bright non-thermal radio source, originally discovered in the 1970s, associated with the supermassive black hole lying at the Galactic centre. X-ray radiation from Sgr A* was first tentatively detected by ROSAT in the 1990s, with confirming observations by Chandra and XMM. Recent high-resolution infrared observations by Genzel et al (at MPE, Garching) and Ghez et al (at UCLA) have succeeded in measuring the orbital motions of stars near Sgr A*, allowing an estimate of its mass of ~3.6 million solar masses. Besides continuous quiescent emission, Sgr A* exhibits flares at both X-ray and optical wavelengths, probably due to material being accreted by the black hole. This proposal aims to use NICMOS to obtain high spatial resolution nera-infrared observations simultaneously with X-ray observations taken by XMM, and ground-based sub-millimetre observations, made with the Caltech Submillimetre Observatory on Mauna Kea. The broad wavelength coverage provided by these observation will test theoretical models of accretion-driven high-energy flares.

GO 10917: Afterglows and Environments of Short-Hard Gamma-Ray Bursts

Artist's impression of a gamma-ray burst

Gamma ray bursts are described colloquially as the biggest bangs since the Big Bang. Originally detected by US spy satellites in the 1960s, these short-lived bursts of high energy radiation resisted characterisation for over 30 years. It is only within the last decade that the Galactic vs. Extragalactic debate on their origins has been setled in favour of the latter. Generically, gamma ray bursts are believed to originate in the death throes of an extremely massive star, as it collapses to form either a black hole or a highly magnetised neutron star. Most occur at moderate to high redshifts. With the identification of optical counterparts to increasing number of bursts, different patterns of behaviour are emerging, indicating progenitors with a range of intrinsic properties. In particular, two broad classifications have been devised: short-hard bursts (SHBs), with durations shorter than 2 seconds; and long-soft bursts (LSBs). The LSBs appear to originate in the collapse of very massive stars, while the SHBs are generally attributed to coalescing binary systems (probably pairs of netron stars or black holes). Gamma ray bursts are, by their nature, unpredictable; thus, follow-up observations are made in Target of Opportunity mode. This proposal focuses specifically on SHBs, and aims to use NICMOS and WFPC2 to track the afterglow and examine the local environment.

GO 11083: The Structure, Formation and Evolution of Galactic Cores and Nuclei

A wide-field view of the Virgo cluster

The Virgo cluster, lying at a distance of ~20 MPc, is the nearest large galaxy cluster. The cluster embraces more than 2,000 galaxies, with masses ranging from ~3 x1012 MSun for the central giant elliptical, M87, to ~109 MSun dwarf systems, predominantly ellipticals but with a smattering of irregulars. In Cycle 11, the same proposal used the Advanced Camera for Surveys to observe more than 100 elliptical and S0 galaxies in Virgo, obtaining deep images in the SDSS g (green) and z (far-red) passbands. These observations show a systematic variation with luminosity in the surfacve brightness profile. Giant ellipticals have a relatively flat brightness profile in the central regions, while dwarfs tends to have compact nuclei. The aim of the current proposal is to use ultraviolet (WFPC2/F225W) and near-infrared (NIC1/F160W) images of the central regions to probe star formation and the star formation history on scales of 0.1-1.0 arcsecond (10-100 parsecs).