This week on HST

HST Programs: May 12 - May 18, 2008

Some selected highlights

GO 11142: Revealing the Physical Nature of Infrared Luminous Galaxies at 0.3

NICMOS image of the nearby luminous IR galaxy, Arp 299

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 is targeting systems with redshifts in the range 0.3 < z < 2.7, combining imaging at near-infrared (NICMOS on HST) and mid-infrared (MIPS on Spitzer) wavelengths. All of the systems already have Spitzer mid-infrared spectra, allowing not only an accurate characterisation of the over all flux distribution, and a \ determination of the total luminosity, but also providing insight into the galaxian dust content and chemical evolution.

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 11518: Mutual eclipses of a Kuiper belt-satellite system

Composite HST image of the Kuiper Belt binary, WW31

The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun (see David Jewitt's Kuiper Belt page for details). Over 500 KBOs are currently known out of a population of perhaps 70,000 objects with diameters exceeding 100 km. Approximately 2% of the known KBOs are binary. This is a surprisingly high fraction, given the difficulties involved in forming such systems and the relative ease with which they can be disrupted. It remains unclear whether these systems formed from single KBOs (through collisions or 3-body interactions) as the Kuiper Belt and the Solar System have evolved, or whether they represent the final tail of an initial (much larger) population of primordial binaries. The KBO binaries include Pluto (one of the largest known KBOs, regardless of whether one considers it a planet or not) and 2003 EL61, the fifth largest KBO. Recent observations of the latter dwarf planet suggest that orbit of the small inner satellite is almost precidely aligned with our line of sight. Consequently, we may be in a position to observe mutual eclipses, occultations and transits. Accurate observations of those events (timing and photometry) can provide invaluable information on the size, shape, density and mass of the bodies in this system. The HST observations are being used to monitor the relative positions of 2003 EL61 its satellite over the inner part of the orbit. The new observations will enable the derivation of refined orbital parameters and generate specific predictions for the mutual events.

GO 11551: When degenerate stars collide: Understanding A New Explosion Phenomena

Artist's impression of a GRB in action

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected over the last 30+ years, and analyses indicate that they can be divided into two classes with durations longer or shorter than 2 seconds. The short bursts appear to release more high energy radiation, so the two subsets are known as long/soft and short/hard bursts. The long/soft bursts appear to originate in the collapse of very massive stars, while the short/hard bursts are believed to be coalescing binary systems (probably pairs of neutron stars or black holes). The first optical counterpart to a gamma ray burst was identified in 1998, allowing confirmation of their extragalactic nature. Since then, more than 50 bursts have been detected at X-ray wavelengths, with approximately half detected at either optical or radio wavelengths; almost all are long/soft bursts. The present observations focus on a recent short/hard burst, GRB 080503, where follow-up ground-based observations (with the Keck telescopes) revealed an apparent optical counterpart that appeared ~1 day after the GRB detection. This detection has been construed as possible evidence for "mini-supernovae", generated in the high density ejecta produced by the binary merger. The (rapidly scheduled) follow-up HST WFPC2 observations aim to measure the energy distribution of this optical transient, and determine whether the observed properties are consistent with mini-supernova hypothesis.