This week on HST

HST Programs: March 31 - April 6, 2008

Some selected highlights

GO 10916: Study of SN Ejecta in the Core-Collapse Supernova Remnant G292.0+1.8: Cas A's Older Cousin

Chandra X-ray image of G292.0+1.9, a ~3000-year old supernoa remnant

Supernovae are generally believed to originate through two mechanisms: accretion onto a white dwarf in a close binary system, driving the white dwarf above the Chandrasekhar limit; and the core implosion of very massive (> 7 solar masses) stars. Both processes result in explosive nucleosythesis that enriches the interstellar medium, with the ejecta forming a rapidly expanding shell. Supernovae are rare: Tycho's star (1604) was the last Galactic supernova identified by contemporary astronomers, although the radio remnant Cas A (identiied as 3C 461 in the 1959 Third Cambridge Catalogue of Radio Stars) may have been recorded, if not recognised as unusual, by Flamsteed in 1680. G292.0+1.8 is ten times older, with an age of ~3,000 years, and the remnant appears to possess an outer shell that is rich in metals, with filamentary structure traced by emisison from highly excited oxygen and suphur. The present proposal will use WFPC2 to image the remnant with the F475W filter, covering the 5007 Angstrom [O III] line. The six-image mosaic will be matched against Spitzer and Chandra data to map the detailed structure of this relatively young remnant.

GO 11083: The Structure, Formation and Evolution of Galactic Cores and Nuclei

A wide-field view of the Virgo cluster

The Virgo cluster, lying at a distance of ~20 MPc, is the nearest large galaxy cluster. The cluster embraces more than 2,000 galaxies, with masses ranging from ~3 x1012 MSun for the central giant elliptical, M87, to ~109 MSun dwarf systems, predominantly ellipticals but with a smattering of irregulars. In Cycle 11, the same proposal used the Advanced Camera for Surveys to observe more than 100 elliptical and S0 galaxies in Virgo, obtaining deep images in the SDSS g (green) and z (far-red) passbands. These observations show a systematic variation with luminosity in the surfacve brightness profile. Giant ellipticals have a relatively flat brightness profile in the central regions, while dwarfs tends to have compact nuclei. The aim of the current proposal is to use ultraviolet (WFPC2/F225W) and near-infrared (NIC1/F160W) images of the central regions to probe star formation and the star formation history on scales of 0.1-1.0 arcsecond (10-100 parsecs).

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. The present program builds on a previous ACS survey of 88 systems from the IRAS Revised Bright Galaxy Sample (GO 10592) in the F439W and F814W passbands. The present program is using the ACS/SBC and WFPC2 to obtain far- (F140LP) and near- (F218W) UV imaging of 27 galaxies. 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 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 the interacting system, NGC 17, are scheduled for this week.

GO 11298: Calibrating Cosmological Chronometers: White Dwarf Masses

HST image of the white dwarf companion to Sirius - which isn't a target of the present program

White dwarfs are the evolutionary end point for most stars with masses less than ~7 MSun. These compact degenerate objects lack any internal heat source, and therefore gradually cool from their initial temperatures of ~100,000-200,000K. As they cool, the luminosity decreases from Mbol ~ 2-3 (for the immediate post-PN object) to Mbol ~ 17 (for 10-12 Gyr-old Galactic halo white dwarfs). The rate of cooling can be predicted using sophisticated models of white dwarf interiors. These models show that the rates are mass dependent, but the overwhelming majority of field white dwarfs are expected to have masses in the range 0.6-0.7 MSun, reflecting the steep slope to the IMF above ~1 MSun (high mass stars are rare, so high mass remnants, like Sirius B, are also rare). Confirming that hypothesis demands reliable mass measurements for individual white dwarfs. Fortunately, a number of white dwarfs are known in binary systems, and a subset of those systems are close enough to each other and to the Sun that their orbits can be mapped (and hence their dynamical masses determined) using the Fine Guidance Sensors on HST. The present program targets 4 white dwarf/white dwarf binary systems.