This week on HST

HST Programs: February 9 - February 15, 2009

Selected highlights

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. These two programs build on a previous ACS survey of 88 systems from the IRAS Revised Bright Galaxy Sample (GO 10592) in the F439W and F814W passbands. Program GO 11196 is using the ACS/SBC and WFPC2 to obtain far- (F140LP) and near- (F218W) UV imaging of 27 galaxies; a separate4 program, GO 11235, used NICMOS to obtain near-infrared H-band images of the galaxy cores. 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 correlations between near-infrared emission and the mid-IR structures identified from Spitzer IRAC imaging; the presence of coherent structure, bars or bridges, that might funnel gas into the nuclei; 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 several interacting system are scheduled for this week.

GO 11706: The parallax of the transiting planet XO-3

Artist's impression of a planetary transit against an active solar-type star

Transiting extrasolar planets offer the opportunity to gain valuable insight into the interior structure and atmospheres of gas giants beyond the Solar System. Besides providing direct measures of mass (with no complications for v sin(i)) and radius (from accurate time-series photometry), spectroscopic observations obtained during either transit or planetary eclipse can probe the atmospheric structure and chemical composition. The present proposal targets the transiting system designated XO-3, which was discovered in 2007 by an international team of professional and amateur astronomers using a fleet of telescopes with very modest apertures - the primary survey telescope, the XO telescope, is a pair of 200-mm telephoto lenses - but a very wide field of view. These small telescopes are used to survey large areas of the celestial sphere, searching for photometric variations characteristic of planetary transits (i.e. periodic dips in brightness of 1-2%); transit candidates are then verified using higher accuracy photometric observations with larger telescopes, and finally radial velocity measurements to confirm the companion mass. XO-3b, the third system discovered in the course of this program, is a ~11.8 MJ object in a 3.2-day period around a 9th magnitude F5 dwarf. Photometric and spectrosopic parallaxes place the star at a distance of around 250 parsecs, with an uncertainty of 15-20%. The present program will use the HST Fine Guidance Sensors to measure a trigonometric parallax accurate to 0.2 milliarcseconds, corresponding to uncertainties of ~5% in distance.

GO 11970: HST Observations of Titan's Escaping Atmosphere in Transit and in Emission

Saturn's satellite, Titan, as seen from Cassini

Titan and Saturn undergo a series of mutual phenomena every ~20 years, when Titan's orbit carries it across the body of the planet as viewed from earth. These phenomena have been taking place over the past couple of years, as Titan's ring plane aligns with the terrestrial viewpoint, but they will come to an end in late 2009. During these transits, Titan, and Titan's atmosphere, will be silhouetted against the Saturnian disk, allowing the potential detection of structure within the satellite's atmosphere. HST has the opportunity to observe Titanian transits on three occasions this year - January 23rd, February 8th and February 24th. The present observations focus on the second transit The ACS/SBC will be used to obtain images in the F115LP, F125LP and F140LP filters, before, during and after the transit takes place. Outside the transit, the observations will be used to search for atmospheric emission, primarily by Lyman alpha.

GO 11986: Completing HST's Local Volume Legacy

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 (Program GO 10915, The ACS Nearby Galaxy Survey) 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. That 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, as program GO 11307: Completing the ACS Galaxy Survey with WFPC2.