This week on HST

HST Programs: July 16 - July 22, 2007

Some selected highlights

GO 10849: Imaging Scattered Light from Debris Disks Discovered by the Spitzer Space Telescope around 21 Sun-like Stars

HST ACS image of the face-on debris disk around the nearby G dwarf, HD 107146

While much debate has raged in recent months over exactly how to define a planet, there is very little debate in the astronomical community about where planets form: they form in circumstellar disks. During the earliest stages of their existence, the disks are dusty, gas-rich and high opacity; for example, see NICMOS images of T Tauri stars and IRAS sources and current HST proposals 10540, 10810 and 10864. After only ~10 million years, however, the gas dissipates, leaving a young planetary system with a rich content of dust, rocks, planetoids and planets. This period corresponds to the high bombardment phase in earth's history, when the Moon was formed. To the outside observer, the dusty disk has low surface brightness, and is much less prominent than the gaseous disk. HST can image these disks via scattered light at near-infrared and, in a few cases, optical wavelengths - probably the most spectacular example is Beta Pic (see the recent HST ACS images ) - but the most effective means of detection is to search for excess thermal radiation at mid-IR wavelengths with Spitzer. The present proposal is a follow-on to a Spitzer Legacy program that surveyed nearby G dwarfs. The aim is to use the NICMOS coronagraph to resolve the structure of the disks suspected to be present in these 21 nearby solar-type stars.

GO 11082: NICMOS Imaging of GOODS: Probing the Evolution of the Earliest Massive Galaxies, Galaxies Beyond Reionization, and the High Redshift Observational Universe

ACS images of a section of the GOODS fields

The Great Observatories Origins Deep Survey, is a large-scale program that is designed to probe galaxy formation and evolution at redshifts from z~1 to z~6. GOODS covers two ~150 sq. arcminute fields, one centred on the Hubble Deep Field and the second on the Chandra Deep Field South, and combines deep oppical/far-red imaging (F435W, F606W, F775W and F850LP filters) using ACS on HST with deep IRAC (3.6 to 8 micron) and MIPS (25 micron) imaging with Spitzer. Chandra data are also available for the bulkd of the field. The present program is supplementing those data with near-infrared H-band (F160W) imaging with NICMOS, coupled (eventually) with parallel ultraviolet observations using the ACS/SBC.

GO 11196: An Ultraviolet Survey of Luminous Infrared Galaxies in the Local Universe

A NICMOS image of the interacting LIRG, NGC 6090

Luminous infrared galaxies (LIRGs) have total luminosities that exceed 1011.4 LSun, with most of the energy emitted at wavelengths longward of 10 microns. Many (perhaps most) of these galaxies are interacting or merging disk galaxies, with the excess infrared luminosity generated by warm dust associated with the extensive star formation regions. Many systems also exhibit an active nucleus, and may be in the process of evolving towards an S0 or elliptical merger remnant. The present program builds on a previous ACS survey of 88 systems from the IRAS Revised Bright Galaxy Sample (GO 10592) in the F439W and F814W passbands. The present program is using the ACS/SBC and WFPC2 to obtain far- (F140LP) and near- (F218W) UV imaging of 27 galaxies. Combined with the previously obtained B- and I-band data, these observations will probe the distribution of star formation activity and the presence of bars and bridges, funneling gas towards active regions the age distribution of star clusters the relationship between star formation and AGN activity the overall structural properties of the LIRGS as a function of luminosity and environment The observations will also provide a detailed UV images for a reference sample of nearby galaxies. Observations of the interacting system, NGC 17, are scheduled for this week.

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.