This week on HST

HST Programs: December 12, 2011 - December 18, 2011

Selected highlights

GO 12209: A Strong Lensing Measurement of the Evolution of Mass Structure in Giant Elliptical Galaxies

ACS images of galaxy-galaxy Einstein ring lenses from the Sloan survey

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 primarily to probe the mass distribution of galaxy clusters, using theoretical models to analyse the arcs and arclets that are produced by strong lensing of background galaxies, and the large-scale mass distribution, through analysis of weak lensing effects on galaxy morphologies. Gravitational lensing can also be used to investigate the mass distribution of individual galaxies. Until recently, the most common background sources were quasars. Galaxy-galaxy lenses, however, offer a distinct advantage, since the background source is extended, and therefore imposes a stronger constraints on the mass distribution of the lensing galaxy than a point-source QSO. The present survey aims to target a particular type of galaxy - luminous giant ellipticals. Spectroscopic data from the recently initiatiated Baryon Oscillation Spectroscopic Survey (BOSS). sampling luminous ellipticals at redshifts between z~0.4 and 0.7, has been used to identify candidate strong lens systems. The present program is using HST I-band (F814W) ACS-WFC imaging to verify the nature of those candidates, and, for confirmed sources, provide the angular resolution necessary to model the mass distribution of the lensing system.

GO 12304: Metallicity distribution functions of 4 Local Group dwarf galaxies

The Leo I dwarf galaxy

The Milky Way has at least fifteen 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 all gas-poor dwarf spheroidal systems, with luminosities less than 109 LSun and masses less than 108 MSun. Most of these systems are relatively metal poor, with metallicities significantly less than one-tenth solar. The present program targets four systems: leo I, Leo II, Phoenix and IC 1613. All four were discovered in the twentieth century: IC 1613 by Max Wolf in 1906; Leo I and leo II by Robert Harrington and Albert Wilson, using the Palomar Schmidt; and the Phoenix dwarf in 1976, when it was initially classed as a globular cluster by Hans-Emil Schuster and Richard West on photographic plates taken by the ESO Schmidt. The present program aims to probe the metallicity distribution of evolved stars within these systems by using WFC3 camera to obtain deep images in a range of filters at both optical and near-infrared wavelengths. The observations will cover giant branch stars brighter than the red clump, and will set strong constraints on the frequency of extermely metal-poor stars in these systems.

GO 12597: Hubble Imaging of a Newly Discovered Main Belt Comet

Asteroid 596-Scheila, a candidate main-belt coment, imaged by Peter Lake in December, 2010

The term 'comet" is generally associated with low-mass, volatile-rich solar system objects that spend most of their life at very lage distances from the Sun, plunging only rarely into the inner regions where they acquire extended tails due to outgassing. Sometimes those obejcts are captured into short-period, eccentric orbits, leading to rapid depletion of the volatile content in rapidly-successive perihelion passages. However, recent years have seen growing evidence of another class of cometes exist: comets with near-circular orbits that place them between Mars and Jupiter, within the realm of the Main Belt of asteroids. One of the first candidate main belt comets, as these objects have been dubbed, is the asteroid Scheila. Discovered by the Heidelberg astronomer August Kopff in 1906, and named after an English student with whom he was acquainted, this is one of the larger known asteroids, with a diameter estimated as ~110 km. Early December 2010, Steve Larson (of Arizona's Lunar and Planetary Laboratory) noted that Scheila had sprouted a coma halo in observations taken by the Catalina Sky Survey. An examination of archival images revealed no evidence for activity throughout October and November, but a possible onset on December 3rd. The asteroid 1979 OW7/1996 N2 exhibited similar behaviour in 1996 and again in 2002; the initial outburst was ascribed to a collision, but the second event suggested that the activity is intrinsic to the asteroid. Most recently, the asteroid 2006 VW139 was imaged during an outburst by the Pan-STARRS telescope. The present HST tgarget-of-opportunity program has been triggered in response to that discovery, and will use high-resolution visual imaging with Wide-Feld Camera 3 to probe the detailed nature of the outburst.

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.