This week on HST

HST Programs: January 31, 2011 - February 6, 2011

Selected highlights

GO 11563: Galaxies at z~7-10 in the Reionization Epoch: Luminosity Functions to <0.2L* from Deep IR Imaging of the HUDF and HUDF05 Fields

The ACS optical/far-red image of the Hubble Ultra Deep Field

Galaxy evolution in the early Universe is a discipline of astronomy that has been transformed by observations with the Hubble Space Telescope. The original Hubble Deep Field, the product of 10 days observation in December 1995 of a single pointing of Wide Field Planetary Camera 2, demonstrated conclusively that galaxy formation was a far from passive process. The images revealed numerous blue disturbed and irregular systems, characteristic of star formation in galaxy collisions and mergers. Building on this initial progam, the Hubble Deep Field South (HDFS) provided matching data for a second southern field, allowing a first assessment of likely effects due to field to field cosmic variance, and the Hubble Ultra-Deep Field (UDF) probed to even fainter magitude with the Advanced Camera for Surveys (ACS). The highest redshift objects found in the UDF have redshifts approaching z~7. Pushing to larger distances, and greater ages, demands observatons at near-infrared wavelengths, as the characteristics signatures of star formation are driven further redward in the spectrum. The present program aims to extend observations beyond z~8 to redshifts close to z>9 by using the WFC3-IR camera to obtain deep F850LP (Y), F105W (J) and F160W (H) images centred on the UDF and two flanking fields. Parallel observations with ACS are being used to extend the visible and red imaging data to even fainter magitudes. This program recently resulted in the discovery of the first strong candidate galaxy at redshift z~10, corresponding to a lookback time of ~13.2 billion years.

GO 12233: Strong Gravitational Lensing by Quasars

ACS images of galaxy-galaxy Einstein ring lenses from the Sloan survey

Gravitational lensing is a consequence the theory of general relativity. Its importance as an astrophysical tool first became apparent with the realisation (in 1979) that the quasar pair Q0957+561 actually comprised two lensed images of the same background quasar. In the succeeding years, lensing has been used primarily to probe the mass distribution of galaxy clusters, using theoretical models to analyse the arcs and arclets that are produced by strong lensing of background galaxies, and the large-scale mass distribution, through analysis of weak lensing effects on galaxy morphologies. Gravitational lensing can also be used to investigate the mass distribution of individual galaxies, and, until recently, most attention focused on background quasars lensed by foreground galaxies. The present program, however, is built on a role reversal. Here, the WFC3 UVIS camrea is used to obtain blue (F475W) and far-red (F814W) images of low redshift (z < 0.7) quasars that are acting as strong lenses of background emission line galaxies. The targets are selected from the Sloan Digital Sky Survey, and the high-resolution HST data will be used to constrain the radial mass profiles of the quasar host galaxies.

GO 12275: Measuring gas flow rates in the Milky Way

A map of the high velocity cloud systems surrounding the Milky Way (B. Wakker, U. Wisconsin).

The stellar components of the Milky Way Galaxy are well known: the disk, the central bulge and the old, metal-poor stellar halo. However, the Milky Way is also surrounded by a halo of hot, gas that is itself embedded within a much more tenuous corona of even hotter, ionised gas. Within that structure lie high velocity clouds. Originally discovered in the 1930s as absorption features in stellar spectra, these clouds have velocities that differ significantly from the rotational velocity along that line of sight, and they are generally believed to be undergoing infall into the Galaxy. The origin and nature of these systems remains uncertain, with some favouring a Galactic origin, driven by star formation and feedback between disk and halo, and others supporting their origin within the warm-hot intergalactic medium. HVCs are not self luminous, so indirect methods need to be applied to examine their characteristics. The most effective is to identify stars that lie behind individual systems and, as with their discovery in the 1930s, search the stellar spectra for signature absorption lines produced by material within the cloud. Many, indeed most, of the key absorption features lie at ultraviolet wavelengths, a spectral region that has been opened up with the installation of the Cosmic Origins Spectrograph on HST. Detailed information is currently available for only five HVCs. The present program is using active galactic nuclei (AGNs) as background light sources to probe most of the remaining known HVCs within the Milky Way.

GO 12321: 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 (an extension of the Cycle 17 program, GO 11706) will use the HST Fine Guidance Sensors to measure a trigonometric parallax accurate to 0.2 milliarcseconds, corresponding to uncertainties of ~5% in distance.