This week on HST

HST Programs: June 11 - June 17, 2012

Selected highlights

GO 12549: The Formation History of the Ultra-Faint Dwarf Galaxies

The Bootes I dwarf galaxy (SDSS data)

The Milky Way possesses at least 15 satellite galaxies. The most prominent are the Large and Small Magellanic Clouds, irregular galaxies which have been known since at least the tenth century. The remaining systems are gas-poor dwarf spheroidal systems, with luminosities less than 108 LSun and masses less than 108 MSun. All of these dwarfs were discovered from wide field imaging surveys. The two brightest systems, Scuptor and Fornax, were found on plates taken in the 1930s by Harvard's Boyden Observatory, in Bloemfontein, South Africa. The brighter systems in the northern hemisphere, including Ursa Minor and Draco, were identified from photographic plates taken as part of the first Palomar Sky Survey in the 1950s, and the brighter southern systems, including Carina and Sextans (L ~ 105 LSun, were uncovered in 80s and 90s on plates taken with UK Schmidt telescope, Siding Spring, Australia. These systems have such low densities that they are barely discernible to the eye; in fact, Sextans was discovered based from analysis of automated plate scans. The advent of CCD surveys and improved analysis techniques have allowed astronomers to push to even fainter, more diffuse systems. In the past fwe years, SDSS has yielded a new class of systems, dubbed ultra-faint dwarfs, with lminosities less than 104 LSun and mass-to-light ratios exceeding 100. The faintest of these systems, Bootes I, has a total luminosity of only ~300 LSun, or only ten times more luminous than Vega. The theoretical expectatio is that these systems represent dark-matter dominated satellites predicted by Lambda-CDM models. The present program will use the Advanced Camera for Surveys (ACS) to obtain deep F606W (V/R) and F814W (I) images of six systems, Bootes I, Canes Venatici, Hercules, Leo IV, Ursa Major I and Coma Berenices. The aim is to use detailed data on the colour-magnitude diagram to probe the chemical composition, age and star formation histories of these systems.

GO 12550: Physics and Chemistry of Condensate Clouds across the L/T Transition - A SNAP Spectral Mapping Survey

Brown dwarfs are likely to have complex atmospheric structures that resemble Jupiter

Brown dwarfs are failed stars - objects that form like stars, by gravitational collapse within giant molecular clouds, but which have insufficient mass to raise the central temperature above 107 K, and which therefore are unable to ignite hydrogran fusion and maintain a long-lived central energy source. As such, these objects reach a maximum surface temperature of perhaps 3,000K some tens of millions of years after their formation, and subsequently cool and fade into oblivion. As they cool, they move through spectral types M, L and T, with the oldest brown dwarfs now likely to have temperatures close to 300K and emergent spectra characterised by water and ammonia bands, the putative signatures of the spectral class Y. As these dwarfs cool from L to T (~1500 to ~1200K), the atmospheres undergo significant changes, with heavier elements condensing to form dust. That dust can form clouds, perhaps giving the dwarf's surface a banded appearance, similar to Jupiter. or leading to the generation of localised cloud features. Since the rotation periods for these obejcts are usually a matter of a few hours, the appearance and disappearance of asymmetric features might lead to periodic photometric or spectroscopic variability, and at least photometric variability has been detected for a handful of ultracool dwarfs. The clouds themselves may appear and disappear over relatively short timescales, leading to longer-term photometric variations at particular wavelengths. A Cycle 18 program (GO 12314) focuses on a handful of brown dwarfs near the L/T transition, using the WFC3 grism to obtain multi-orbit, high-accuracy monitoring of their spectral behaviour. That program succeeded in recovering significant variability, with periods from 5-10 hours. The present program builds on those results by extending coverage to a much larger sample of late-L/early-T dwarfs. As a SNAP prorgam, each will be observed for only ~40 minutes. insufficient to determine periodicities, but, given the high precision of HST, sufficient to identify new candidates and gain some insight into the frequenciy of the phenomenon.

GO 12603: Understanding the Gas Cycle in Galaxies: Probing the Circumgalactic Medium

A computer simulation of galactic gas accretion and outflow

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution therefore is tied very closely to how gas is accreted, recycled, circulated through the halo and disk, and, perhaps, ejected back into the intergalactic medium. Tracing that evolutionary history is difficult, since gas passes through many different phases, some of which are easier to detect than others. During accretion and, probably, subsequent recycling, the gas is expected to be reside predominantly at high temperatures. The most effective means of detecting such gas is through ultraviolet spectroscopy, where gas within nearby systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. The present program is using the Cosmic Origins Spectrograph to probe gas in the circumgalactic medium for a large sample of relatively local disk galaxies. The targets are drawn from the GALEX Arecibo SDSS Survey (GASS), with the aim of combining the various observations to map atomic, molecular and ionised gas in these systems. The galaxies lie at redshifts between 0.02 and 0.05, and COS will be used to observe QSOs whose sightlines pass within 250 kpc of the galaxy core. Those sightlines run through the halos of the galaxies, and the QSOs therefore provide a pencilbeam backlight that probes hot circumgalactic gas.

GO 12668: Proper Motion Survey of Classical and SDSS Local Group Dwarf Galaxies

The low-mass dwarf galaxy, Leo II

The Milky Way, M31 and M33 are the three largest galaxies in the Local Group. The system, however, includes more than 25 other members, the majority being dwarf spheroidal galaxies that are satellites of either M31 or the Milky Way. Those galaxies have old, evolved stellar populations, and even the most prominent have masses that are less than a few x 107 MSun, or 10-4 that of the Milky Way. All of these galaxies are moving in the potential set by the overall Local Group system, but dominated by M31 and the Milky Way. Determining full space motions for the dwarfs therefore provide a means of constraining that potential. Even thought the galaxies, and their brightest stellar constituents, are faint, measuring radial velocity is a relatively straightforward procedure. Deriving tangential motions is not, since the typical proper motions of these systems are a few mas/year at best. The present proposal aims to capitalise on the exceptional resolution and high stability of HST to address this issue. WFC3 will target 7 dwarf galaxies, imaging fields that are centred on a background QSO. That QSO serves as a reference point for measurement of the transverse motion of stars in the foreground dwarf galaxy.