This week on HST

HST Programs: January 21 - January 27, 2008

Some selected highlights

GO 10890: Morphologies of the Most Extreme High-Redshift Mid-IR-Luminous Galaxies

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 present program uses NICMOS and WFPC2 to obtain high sensitivity, high angular-resolution observations of 31 candidate systems.

GO 11084: Probing the Least Luminous Galaxies in the Local Universe

Ground-based image of Andromeda V, one of the brighter(!) dwarf companions

Large spiral galaxies tend to be accompanied by a retinue of lesser companions. In the case of the Milky Way, the two most prominent satellites are the Large and Small Magellanic Clouds, irregular galaxies with considerable gas content and extensive ongoing star formation. But there are at least 12 other systems, the majority of which are dwarf spheroidals with masses less than ~108 Solar masses, or less than one-thousandth that of the Milky Way, and no on-going star formation. The nearby Andromeda galaxy has its own swarm of acolytes, with the most recent additions (Andromeda XI, XII and XIII) discovered in 2006. The latter galaxies populate the extremely low luminosity tail of the galaxy luminosity function, and the aim of this proposal is to obtain a better understanding of the stellar population (or even populations) in these low-mass systems. Besides the three Andromeda dwarfs, the program will target the Milky Way satellites Canes Venatici I, Canes Venatici II, Hercules, and Leo IV, together with Leo T, an isolated Local Group member. The WFPC2 camera will be used to obtain deep imaging in F606W (V-band ) and F814W (I-band).

GO 11163: Accreting Pulsating White Dwarfs in Cataclysmic Variables

Artist's impression of a cataclysmic variable system

Cataclysmic variables are close binary systems that are comprised of a low-mass main sequence star and a compact (white dwarf or neutron star) companion. The main sequence star fills its Roche lobe, leading material accreting onto the evolved companion via a circumstellar disk. As the material is accreted, it heats up and radiates at ultraviolet and X-ray wavelengths, and inhomogeneities in the accretion rate can lead to flares and outbursts. There are numerous different types of cataclysmic variables, including dwarf novae, recurrent novae and polars (where the strong magnetic field present in the white dwarf leads to accretion along field lines onto the magnetic poles). The present program focuses on observations of CV systems where the white dwarf is a pulsating variable - the non-radial pulsations are high-order gravity modes, driven by opacity variations in the stellar interior. The appropriate conditions only apply for a finite range of surface temperature - the instability strip. Observations of a handful of pulsating white dwarfs in CVs suggest that they have higher temperatures than isolated pulsators; the present proposal aims to use ACS/SBC UV spectroscopy of a number of new discoveries to verify whether such is the case.

GO 11202 The Structure of Early-type Galaxies: 0.1-100 Effective Radii

HST16309+8230, a disk galaxy, distorted due to gravitational lensing by a foreground elliptical

Despite their apparently simple appearance, the processes responsible for the formation and evolution of elliptical galaxies remain somewhat obscure. It is clear that most star formationin these systems must occur at early epochs, since these systems are highly gas poor at even moderate redshifts. Grabitational lensing provies one of the more important tools for investigating these systems, since it can probe the detailed form of the mass distribution, and test for the presence of sub-structure in the underlying dark matter, as predicted by some theoretical models. The present program is combining high-resolution, multi-colour HST imaging with ground-based low-resolution VLT/Keck spectroscopic observations of over 50 strong lensing systems. The resultant datasets can be used to investigate the structure of elliptical galaxies over a wide range of radii, and test the predictions of relevant theoretical models.