This week on HST

HST Programs: March 10 - March 16, 2008

Some selected highlights

GO 11169: Collisons in the Kuiper Belt

Visions of the Kuiper Belt

The Kuiper Belt lies beyond the orbit of Neptune, extending from ~30 AU to ~50 AU from the Sun, and includes at least 70,000 objects with diameters exceeding 100 km. Setting aside Pluto, the first trans-Neptunian objects were discovered in the early 1990s. Most were relatively modest in size, with diameters of a few hundred km and photometric properties that suggested an icy composition, similar to Pluto and its main satellite, Charon. Over the last three years, however, a handful of substantially larger bodies have been discovered, with diameters of more than 1000 km; indeed, one object, Eris (2003 UB13), is slightly larger than Pluto (2320 km) and 25% more massive. We know the mass for Eris because it has a much lower mass companion, Dysnomia, which orbits Eris with a period of 16 days (see this recent press release ). Pluto, itself, has three companions: Charon, which is about 1/7th the mass of Pluto, and the much smaller bodies, Hydra and Nix, discovered from HST observations in early 2005. Observations of other Kuiper Belt Objects (KBOs), mainly using HST, reveal that a significant fraction are binary. This may indicate that the Kuiper Belt is a dangerous place to live, with frequent collisions between KBOs, leading to fragmentation and satellite formation. The present program aims to probe this issue through multi-wavelength observations of a substantial number of KBOs, sampling a broad range of properties.

GO 11144: Building on the Significant NICMOS Investment in GOODS: A Bright, Wide-Area Search for z>=7 Galaxies

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

The Great Observatory Origins Deep Survey (GOODS) is a multi-wavelength survey that covers two 150 sq. arcmin. fields, 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. The prime aim of the GOODS program is to reconstruct the history of galaxy formation, star formation and nuclear galactic activity from the epoch of reionisation to the present. The present HST program builds on past results and aims to push observations to the highest redshifts, searching for galaxies at z > 7. Previously obtained GOODS data have been used to identify z-H dropouts - objects visible on F160W NICMOS images, but not on F098 ACS images. deeper NICMOS images of those candidates will be obtained in the J-band (F110W filter) to confirm whether the fluxes are consistent with 7 < z < 8 star-forming galaxies.

GO 11148: High Contrast Imaging of Dusty White Dwarfs

Artist's impression of a comet spiralling in to the white dwarf variable, G29-38

During the 1980s, one of the techniques used to search for brown dwarfs was to obtain near-infrared photometry of white dwarf stars. Pioneered by Ron Probst (KPNO), the idea rests on the fact that while white dwarfs are hot (5,000 to 15,000K for the typcail targets0, they are also small (Earth-sized), so they have low luminosities; consequently, a low-mass companion should be detected as excess flux at near- and mid-infrared wavelengths. In 1988, Ben Zuckerman and Eric Becklin detected just this kind of excess around G29-38, a relatively hot DA white dwarf that also happens to lie on the WD instability strip. However, follow-up observations showed that the excess peaked at longer wavelengths than would be expected for a white dwarf; rather, G 29-38 is surrounded by a dusty disk. Given the orbital lifetimes, those dust particles must be regularly replenished, presumably from rocky remnants of a solar system. G 29-38 stood as a lone prototype for almost 2 decades, until a handful of other dusty white dwarfs were identified from Spitzer observations within the last couple of years. The present program will use coronagraphic imaging with NICMOS to search for direct evidence of the suspected disks.

GO 11202 The Structure of Early-type Galaxies: 0.1-100 Effective Radii

HST16309+8230, a disk galaxy, distorted due to gravitational lensing by a foreground elliptical

Despite their apparently simple appearance, the processes responsible for the formation and evolution of elliptical galaxies remain somewhat obscure. It is clear that most star formationin these systems must occur at early epochs, since these systems are highly gas poor at even moderate redshifts. Grabitational lensing provies one of the more important tools for investigating these systems, since it can probe the detailed form of the mass distribution, and test for the presence of sub-structure in the underlying dark matter, as predicted by some theoretical models. The present program is combining high-resolution, multi-colour HST imaging with ground-based low-resolution VLT/Keck spectroscopic observations of over 50 strong lensing systems. The resultant datasets can be used to investigate the structure of elliptical galaxies over a wide range of radii, and test the predictions of relevant theoretical models.