This week on HST

HST Programs: August 10 - August 16, 2009

SMOV still under way, but science observations being made.

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 (on HST) and mid-infrared (MIPS on Spitzer) wavelengths. The initial observatiosn for this Cycle 16 program were taken with NICMOS, and the program has been transferred to WFC3 for Cycle 17. 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 11669: The Origins of Short Gamma-Ray Bursts

An artist's impression of a gamma-ray burst

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 coalescing binary systems (probably pairs of netron stars or black holes). The first optical counterpart to a gamma ray burst was identified in 1998, allowing confirmation of their extragalactic nature, and, since then, more than 60 bursts have been detected at X-ray wavelengths, and half that number detected at either optical or radio wavelengths; all of these detections are long/soft bursts. The present program will use a two-pronged approach to probe the nature of soft GRB progenitors by gaining a better understanding of the nature of the surrounding stellar population. One approach is statistical, with high angular resolution WFC3 imaging used to examine the colours of stellar populations in a sample of galaxies that have hosted past GRBs - are they characteristic of young, star-forming regions, or of older, more mature environments? The drawback with this study lies in the modest accuracies associated with the GRB positions in those galaxies. Complementing that analysis, the program will trigger ToO observations of two yet-to-be discovered GRBs, one in a star-forming galaxy and the other in an elliptical. The HST imaging of those sources should lead to more precise positoins, and hence a clearer idea of the local stellar environment.

GO 11695: Searching for the Bottom of the Initial Mass Function

Multi-colour image of the Chamaeleon I region

Chamaeleon I is a star-forming region that lies within a molecular cloud complex at a distance of ~150 parsecs from the Sun. The young cluster has been the subject of extensive ground-based observations, which have succeeded in identifying over 200 members, spread over an area of ~0.5 square degrees, with masses ranging from 2-3 solar masses to below the hydrogen burning limit. In addition, a subset of the cluster was targeted for deep HST observations with ACS and NICMOS in Cycle 13, while WFPC2 was used recently to search for evidence for disks and jets among the lower-mass memebrs. The cluster is much less massive than the Orion Nebula Cluster (ONC), and appears to break into two sub-units. Matched against theoretical isochrones, the colour-magnitude data suggest that the cluster has an age comparable to the ONC, with estimates of 3-4 Myrs for the southern sub-unit and 5-6 Myrs for the northern. The deep Cycle 13 observations were used to probe the form of the stellar mass function at the lowest masses, and found no evidence for a cut-off at low masses. The present observations build on the Cycle 13 program by providing second-epoch data that will allow separation of cluster and field to apparent magnitudes that correspond to masses as low as 3 MJupiter for cluster members.