This week on HST

HST Programs: March 17 - March 23, 2008

Some selected highlights

GO 11107: Imaging of Local Lyman Break Galaxy Analogs: New Clues to Galaxy Formation in the Early Universe

Mosaic of HST images of M82, the best-known starburst galaxy

Current Big Bang cosmological models predict that the universe should have undergone a global re-ionisation at redshifts between 6 and 20. The first generation of stars is generally tapped as the most likely source of the ionising radiation, perhaps enhanced through merger-stimulated starbursts. Direct observations of those galaxies are not possible at present; the James Webb Space Telescope is expected to open up observations of these systems. In consequence, there is considerable interest in identifying galaxies at lower redshifts that could serve as analogues for the z>6 systems. Over the last few years, the Galaxy Evolution Explorer (GALEX) has proved an important new tool in identifying candidate objects. GALEX has conducted an all-sky survey at ultraviolet wavelengths, and has uncovered sigificant numbers of UV luminous galaxies at low and moderate redshifts. Many of these galaxies are starbursts, undergoing substantial outbursts of star formation. These galaxies have been categorised as "compact UV luminous galaxies" (UVLGs). These appear to be galaxies that are undergoing small-scale mergers, leading to extensive dissipation and vigorous star formation. The present program is using the ACS/SBC prism and WFPC2 to obtain ultraviolet spectra and R-band images of 31 systems, probing the star formation history and its variation with environment.

GO 11120: A Paschen-Alpha Study of Massive Stars and the ISM in the Galactic Center

A multi-wavelength composite image of the Galactic Centre (red - 90 cm radio data; green, mid-IR data; blue, X-ray)

The Galactic Centre lies in the heart of the constellation of Sagittarius, and at a distance of ~8 kiloparsecs from the Sun. Galactic nuclei are the likely end-point for mass accretion, and are generally the site of highly energetic activity; the Galactic Centre is no exception. AO near-infrared imaging has been brought to bear on this issue, resolving a number of stars within the core, close to the compact radio source Sagittarius A*. Monitoring of those sources over the last decade has shown that they are in rapid orbital motion around a very massive central object, now clearly identified as a ~3 million solar-mass black hole. Moving beyond the core, to distances of tens of parsecs from centre, observations have revealed molecular gas and young star forming regions. The aim of the present proposal is to use the NICMOS NIC3 camera to survey a ~32 x 13 arcminute region (~75 pc x 30 pc) in two narrowband filters: F187N, ceentred on the Paschen alpha line; and F190N, providing the continuum flux. These obbservations will provide a detailed map of the star forming activity within the Galactic nucleus.

GO 11165: The Radius of the Super-Neptune HD 149026b

Key events in a planetary transit

Transiting exoplanet systems are a veritable goldmine of information for extrasolar planetary studies: not only is the orbital inclination reliably defined in those systems, but the diameter (and hence the average density) is directly measureable from the eclipse depth, and the atmospheric composition can be probed through line absorption or re-radiated thermal flux. After a relatively slow start, some 36 transiting systems are currently known (see the Extrasolar Planet Encyclopedia ). All of the systems are short-period "hot Jupiters" (or even lower mass "hot neptunes"), and those observations reveal a wide range of properties. HD 149026b is one of the most interesting discoveries. Orbiting a sun-like star (spectral type G0) some 79 parsecs from the Sun, HD 140926b has a period of 2.8766 days, a mass of 0.36 Jupiter masses and a radius 73+/-5% that of Jupiter. Models indicate that it has a substantial core, with some 67 earth masses of heavy elements (i.e. not H or He). The present program will use NICMOS grism observations to derive a more accurate light curve and a more reliable radius measurement.

GO 11233: Multiple Generations of Stars in Massive Galactic Globular Clusters

NGC 2808, a globular cluster with multiple stellar populations

Globular clusters are remnants of the first substantial burst of star formation in the Milky Way. With typical masses of a few x 105 solar masses, distributed among several x 106 stars, the standard picture holds that these are simple systems, where all the stars formed in a single starburst and, as a consequence, have the same age and metallicity. Until recently, the only known exception to this rule was the cluster Omega Centauri, which is significantly more massive than most clusters and has both double main sequence and a range of metallicities among the evolved stars. Omega Cen has been joined by at least one more cluster, NGC 2808, which shows evidence for three distinct branches to the main sequence. The origin of this feature is notknown, but it may be significant that NGC 2808 is also one of the more massive clusters, and might therefore be able to survive several burst of star formation (or, conversely, be the product of a multi proto-globular merger). The present program aims to use WFPC2 to obtain high-precision photometry of other massive globulars, such as NGC 1851, M80 and M13.