This week on HST

HST Programs: June 30 - July 6, 2008

Some selected highlights

GO 11168: The IMF in the Hidden Galactic Starburst W49A

Ground-based images (from ESO) of the star forming regions, W49A

W49 was the 49th source catalogued by Gert Westerhout in his thesis survey, using the Dwingeloo 25-metre telescope, for 22-cm continuum sources lying within a section of the Galactic Plane. Subsequent observations resolved W49 into two sources, separated by 12.5 arcseconds. W49B proves to be a supernova remnant, while W49A is an active star forming region lying at a distance of ~11 kiloparsecs from the Sun in the constellation of Aquila. W49A is clearly very young, with observations revealing several massive OB clusters lying within a giant molecular cloud that is still at an early stage of disruption. These clusters are highly reddened, however, due to both dust within the parent molecular cloud and foreground material along the line of sight, which runs squarely along the Galactic Plane. Consequently, infrared observations offer the only means of probing lower mass stars and the IMF within these obscured clusters. The present program will use the NIC2 camera on NICMOS to image five fields across the W49A complex.

GO 11188: First Resolved Imaging of Escaping Lyman Continuum

Lyman-break galaxy in the Hubble Deep Field: the four panels show the same region at (clockwise from upper left) 3000, 4500, 6060 and 8140 Angstroms; the centred object is a Lyman-break galaxy at z~3.

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 Lyman-alpha emission escapes from galaxies, and how starbursts contribute to reionisation at intermediate redshifts. The present program aims to address this issue through multiwavelength observations of several Lyman Break Galaxies (LBGs) at redshifts z~3. These galaxies have been identified because they "drop out" (disappear) at wavelengths shortward of the (redshifted) Lyman limit (912 Angstroms in the galaxy's rest frame). This program will combine deep F336W WFPC2 observations, covering the (redshifted) ionising radiation, with F606W data, sampling the continuum at (rest frame) 1500 Angstroms.

GO 11210: The Architecture of Exoplanetary Systems

Artist's impression of a young planetary system

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. This program focuses 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.

GO 11307: Completing ACS Nearby Galaxy Survey with WFPC2

SIRTF image of NGC 2976 (from the SINGS program)

Colour-magnitude diagrams derived from photometric surveys have proven invaluable in developing our understanding of the main properties of the galactic stellar populations. Large ground-based telescopes allowed extension of this type of analysis to the principal satellites of the Milky Way and, to a limited extent, the Andromeda spiral. With the advent of HST, particularly following SM3B and the installation of the Advance Camera for Surveys, those fundamental CMD studies can be extended to higher-density star fields, fainter magnitudes and intrinsically lower luminosity stars. Until recently, studies have concentrated on nearer Local Group galaxies; the ambitious aim of the original incarnation of this program was to conduct a systematic survey of all major star-forming galaxies within ~3.5Mpc of the Milky Way, together with a number of galaxies in the M81 group at a distance of ~4 Mpc. The program included a total of 45 galaxies, ranging from massive spiral systems to dwarf galaxies. Initially, observations were made using the wide-field camera on ACS, sampling selected fields in the wide-V (F606W) and I (F814W) passbands. Following the ACS failure in January 2007, the program was re-designed, focusing on WFPC2 observations of the larger galaxies within ~3.5 Mpc.