This week on HST

HST Programs: May 21 - May 27, 2007

Some selected highlights

GO 10786: Rotational state and composition of Pluto's outer satellites

Hubble Space Telescope image of Pluto, Charon and the two new moons, Nix & Hydra

Pluto, one of the largest members of the Kuiper Belt and, until recently (still, for some), the outermost planet in the solar system, has been in the news over the last year or two. Besides the great "planet"/"dwarf planet" debate, Pluto is the primary target of the New Horizons Mission, and Hubble observations in 2005 led to the discovery of two small moons. Together with Charon, itself only discovered in 1978, these additions make Pluto a 4-body system. Christened Nix and Hydra, the two new moons are 5,000 fainter than Pluto itself, implying diameters as small as ~30-50 km if the surface composition is similar to Pluto itself. The present program aims to better characterise these bodies through multicolour observatioins with WFPC2. The observations will also look for systematic photometric variations that might probe the rotation period. with the aim of testing whether these moons are in synchronous rotation with Pluto itself.

GO 10880: The host galaxies of QSO2s: AGN feeding and evolution at high luminosities

Artist's impression of the black hole and surrounding torus in an AGN

The peculiar nature of active galaxies was first discerned almost 100 years, when Edward A. Fath obtained a photographic spectrum on the Lick 36-inch of the nucleus of the the spiral galaxy, M77. Unlike most systems, which had star-like absorption line spectra, M77 (now more popularly known as NGC 1068) exhibited several strong emission lines that were also known to be present in gaseous nebulae. The emission was also concentrated near the galactic nucleus. We now know that these lines are due to hydrogen and ionised oxygen and nitrogen, generated by hot gas within a toroidal disk around a central supermassive black hole. There are still a number of significant questions concerning these active galactic nuclei (AGNs): when do the black holes form? how are they fed? how do they evolve with time? and how do they affect the surrounding galaxy? The present program aims to tackle some aspects of these questions by using WFPC2 to image QSOs at redshifts between 0.3 and 0.4, investigating the central morphology at high angular resolution.

GO 10886: The Sloan Lens ACS Survey: Towards 100 New Strong Lenses

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. Until recently, the most common background sources were quasars. Galaxy-galaxy lenses, however, offer a distinct advantage, since the background source is extended, and therefore imposes a stronger constraints on the mass distribution of the lensing galaxy than a point-source QSO. The Sloan sky survey provides a powerful tool for identifying candidate galaxy-galaxy lenses; this program is using HST-ACS imaging to verify the nature of those candidates, and provide the angular resolution necessary to model the mass distribution.

GO 11080: Exploring the Scaling Laws of Star Formation

A composite BVJ image of NGC 972, one of the galaxies targeted in this program

Understanding the star formation process is crucial to our developing viable theories of galaxy formation. The present program aims to investigate this process by obtaining NICMOS Paschen-alpha images of the central regions of an all-sky sample 84 nearby spiral and S0 galaxies. The near-infrared images will be combined with CO and HI interferometric radio maps, and, where such data are available, mid-infrared imaging by Spitzer, probing the relative distributions of gas, dust and star formation in these galaxies. In addition to studying how the star formation rate (SFR) varies as a function of gas density, the NICMOS data will permit high spatial-resolution investigations of where (and whether) Schmidt-type star-formation "laws" (power-law relations between the SFR and gas density) are valid in external galaxies.