This week on HST

HST Programs: February 11 - February 17, 2008

Some selected highlights

GO 11148: High Contrast Imaging of Dusty White Dwarfs

Artist's impression of a comet spiralling in to the white dwarf variable, G29-38

During the 1980s, one of the techniques used to search for brown dwarfs was to obtain near-infrared photometry of white dwarf stars. Pioneered by Ron Probst (KPNO), the idea rests on the fact that while white dwarfs are hot (5,000 to 15,000K for the typcail targets0, they are also small (Earth-sized), so they have low luminosities; consequently, a low-mass companion should be detected as excess flux at near- and mid-infrared wavelengths. In 1988, Ben Zuckerman and Eric Becklin detected just this kind of excess around G29-38, a relatively hot DA white dwarf that also happens to lie on the WD instability strip. However, follow-up observations showed that the excess peaked at longer wavelengths than would be expected for a white dwarf; rather, G 29-38 is surrounded by a dusty disk. Given the orbital lifetimes, those dust particles must be regularly replenished, presumably from rocky remnants of a solar system. G 29-38 stood as a lone prototype for almost 2 decades, until a handful of other dusty white dwarfs were identified from Spitzer observations within the last couple of years. The present program will use coronagraphic imaging with NICMOS to search for direct evidence of the suspected disks.

GO 11195: Morphologies of the Most Extreme High-Redshift Mid-IR-luminous Galaxies II: The `Bump' Sources

HST images of interacting ultra-luminous IR galaxies

Luminous infrared galaxies (LIRGs) are systems that have total luminosities exceeding 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. Until recently, very few candidate such systems were known at high redshifts; consequently, analyses and investigations of their origins had to rely on observations of low- and moderate-redshift analogues. The team leading this HST proposal have used a combination of mid-infrared (24 micron) and near-infrared observations to identify tens of candidates, and Spitzer follow-up spectroscopy has confirmed that many lie at redshifts 2 < z < 2.5. The sources appear to fall into two broad categories: thopse with bright 24-micron fluxes with power-law spectral energy distributions and SiO absorption at mid-IR wavelengths; and "bump" sources, with an SED that peaks near 1.6 microns (rest wavelength) and PAH absorption. The brighter sources were targeted in a Cycle 15 program; the present program uses NICMOS and WFPC2 to obtain high sensitivity, high angular-resolution observations of 22 of the "bymp" systems.

GO 11225: The Wavelength Dependence of Accretion Disk Structure

The first Einstein cross, the gravitational lensed QSO, G2237+0305

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 to probe the mass distributions on a variety of scales: of galaxies (primarily via multiply-imaged quasars); of galaxy clusters (arcs and arclets); and at the largest scales (weak lensing). However, lensing can also provide insight on the small-scale properties of the object being lensed. In a lensed QSO, the light from the QSO follows different paths to produce the separate images; each of those paths has a different length; consequently, flux variations in the source show up at different times in the separate images. The present program aims to take advantage of this property to probe the structure of the accretion disks surrounding the central black hole in a number of lensed QSOs. The program will combine ultraviolet observations with the ACS/SBC on HST with Chandra X-ray data. Studying the variation as a function of wavelength should probe the accretion disk structure, since light from the inner regions are expected to dominate at shorter wavelengths, while the outer regions dominate at longer wavelengths.

GO 11229: The Search for Evolution of Emission from Dust in Supernovae with HST and Spitzer

Supernova 1987A in the LMC, an HST image acquired by WFPC2 in 1998

Type II supernovae are produced by core collapse in stars exceeding ~7 solar masses. These massive stars support a complex series of nuclear reactions on the main sequence, ranging from hydrogen fusion near the surface to silicon reactions that build up the iron core. This produces an internal onion-ring structure, with a wide range of chemical species that are blasted into the interstellar medium when the core exceeds the Chandrasekhar mass an implodes. These supernovae are widely hypothesised as a potent source of interstellar dust, with the material condesning within the cooling ejecta. However, observations have so far provided only lukewarm support for these theoretical expectations. This proposal combines optical and near-infrared HST observations of recent type II supernovae, using WFPC2 and NICMOS, respectively, with mid-infrared imaging with IRAC and MIPS on Spitzer. The aim is to use the multiwavelength imaging to probe the evolution of the dust content in the surrounding ejecta. NICMOS observations of SN2006BC in NGC 2397.