This week on HST

HST Programs: January 26, 2009 - February 1, 2009

Selected highlights

GO 11236: Did Rare, Large Escape-Fraction Galaxies Reionize the Universe?

Lyman alpha image of the radio galaxy, 4C41.17

In Big Bang cosmology, the early history of the unverise is characterised by three distinct phases: the initial expansion, during which time Big Bang nucleosynthesis occurs, and the universe cools from its initial exceedingly high temperatures; recombination, which occurs at a redshift z~1,100 (or an age of ~400,000 years), when the Universe was cool enough to allow neutral hydrogen to become dominant, leading to high opacity and the cosmic microwave background; and reionisation, when energy sources reionised hydrogen, reducing the opacity of the intergalactic medium and restoring transparency. Reionisation is generally believed to have occurred at a redshift between z~6 and z~20, with the ionising sources either (or both) the first generation of stars (Population III starbursts) and/or proto-quasars. The IGM remains ionised thereafter. A key issue in developing an understanding of this process is assessing how readily starburst-generated Lyman-alpha emission escapes from galaxies, and how starbursts contribute to reionisation at intermediate redshifts. This proposal aims to quantify this issue by targeting a large sample of starburst galaxies at redshifts z~0.7. the galaxies all lie within the region covered by the COSMOS survey, and will be observed at ultraviolet wavelengths using the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC).

GO 11942: Increasing the Accuracy of HST Astrometry with FGS1r

Artist's impression of a young planetary system

This FGS program is a follow-up to GO 11210, "The Architecture of Exoplanetary Systems". Immanuel Kant is generally credited with first proposing that the planets in the Solar System coalesced from a flat, rotating disk formed by the Solar Nebula. Direct confirmation of that process only came in the early 1990s, when millimetre-wave interferometers were able to detect molecular gas in Keplerian rotation around a handful of nearby young stars. Since then, there have been numerous other observations, including Hubble's images of proplyds (protoplanetary disks) in the Orion Cluster, and Hubble and Spitzer observations of edge-on disks in other young stars. One of the clear selling points of the Solar Nebula disk model is that it appears to offer a natural path to forming planets with coplanar orbits, matching (most of) our observations of the Solar System. On the other hand, as our knowledge of exoplanetary systems has accumulated over the last decade, it has become clear that dynamical interactions may play a very important role in the evolution of these systems. In particular, disk/planet interactions are generally regarded as responsible for the inward migration of gas giants to form hot Jupiters in <3 day period orbits. Planet-planet interactions could lead to significant changes in orbital inclination. Radial velocity planet searches are uncovering more and more multi-planet systems. Program GO 11210 focused the high precision of HST's astrometric detectors, the Fine Guidance Sensors, on four of those systems. The aim is to complement the existing radial velocity measurements with sub-milliarcsecond precision astrometry, allowing determination of the true orbital paths - specifically, the relative inclination - of the low-mass objects in these systems. The present observations build on this program by adding longer-baseline measurements, providing a better basis for subtracting the systemic proper motion and identifying anomalous motions.

GO 11969: Satellite Search for Dawn Mission Targets, Vesta and Ceres

HST images of the dwarf planet, Ceres

Ceres and Vesta are the two largest members of the main belt asteroids. Indeed, Ceres was the first member discovered, by Giuseppe Piazzi in 1801, while Vesta was the fourth discovery, in 1807 by Heinrich Wilhelm Olbers. Ceres is roughly spherical, with a diameter of 950 km, while Vesta is an oblate spheroid, ~580 x 560 x 485 km. Both have been studied extensively, using both ground-based telescopes and space observatories, including Spitzer and HST, and will be targeted for study by NASA's DAWN mission, launched on September 27 2007. Dawn will encounter Vesta in August 2011 and depart for ceres in May 2012, arriving in February 2015, with the mission ending in July 2015. Both asteroids (or dwarf planets) are known to have heavily cratered surfaces, particularly Vesta, suggesting a history of violent collisions. Indeed, fragments of Vesta are generally believed to account for a significant fraction of HED meteorites (Howardites, Eucrites and Diogenites). The present program aims to use deep WFPC2 imaging with HST to search for very low mass companions to these two dwarf planets, whose presence could be of significance to planning the details of the DAWN encounters.