This week on HST

HST Programs: November 15, 2010 - November 21, 2010

Selected highlights

GO 11585: Tracing the distribution of gas and galaxies using three closely-spaced background QSOs

A computer simulation of gas in the intergalactic medium

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution therefore depends on how gas is accreted, recycled, circulated through the halo and 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 subsequent recycling, the gas is expected to be reside predominantly at high temperatures. The most effective means of detecting hot gas is through ultraviolet spectroscopy, where gas associated with relatively nearby galaxies and galaxy clusters systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. The sampling of galactic halos and the intergalactic medium is usually very sparse, since one cannot place quasars "to order". This program, which is focused on studying the gas in the IGM, takes advantage of the serendipitous presence of three bright quasars (redshifts z~0.7 to 1.2) within~1 arcminute on the sky. The Cosmic Origins Spectrograph will be used to obtain far-UV spectra, covering absorption from both the warm (~10,000 K) and the warm-hot (~100,000 K) intergalactic medium. These observations will be combined with hydrodynamical simulations to characterise the densities, metallicies and kinematics of IGM clouds along these lines of sight.

GO 11695: Searching for the Bottom of the Initial Mass Function

Multi-colour image of the Chamaeleon I region

Chamaeleon I is a star-forming region that lies within a molecular cloud complex at a distance of ~150 parsecs from the Sun. The young cluster has been the subject of extensive ground-based observations, which have succeeded in identifying over 200 members, spread over an area of ~0.5 square degrees, with masses ranging from 2-3 solar masses to below the hydrogen burning limit. In addition, a subset of the cluster was targeted for deep HST observations with ACS and NICMOS in Cycle 13, while WFPC2 was used recently to search for evidence for disks and jets among the lower-mass memebrs. The cluster is much less massive than the Orion Nebula Cluster (ONC), and appears to break into two sub-units. Matched against theoretical isochrones, the colour-magnitude data suggest that the cluster has an age comparable to the ONC, with estimates of 3-4 Myrs for the southern sub-unit and 5-6 Myrs for the northern. The deep Cycle 13 observations were used to probe the form of the stellar mass function at the lowest masses, and found no evidence for a cut-off at low masses. The present observations build on the Cycle 13 program by providing second-epoch data that will allow separation of cluster and field to apparent magnitudes that correspond to masses as low as 3 MJupiter for cluster members.

GO 11696: Infrared Survey of Star Formation Across Cosmic Time

A region of massive star formation

Star formation is the key astrophysical process in determining the overall evolution of galactic systems, the generation of heavy elements, and the overall enrichment of interstellar and intergalactic material. Tracing the overall evolution through a wide redshift range is crucial to understanding how gas and stars evolved to form the galaxies that we see around us now. The present program builds on the ability of HST to carry out parallel observations, using more than one instrument. While the Cosmic Origins Spectrograph is focused on obtaining ultraviolet spectra of unparalleled signal-to-noise, this program uses the near-infrared grisms mounted on the Wide-Field Camera 3 infrared channel to obtain low resolution spectra between 1 and 1.6 microns of randomly-selected nearby fields. The goal is to search for emission lines characteristic of star-forming regions. In particular, these observations are capable of detecting Lyman-alpha emission generated by star formation at redshfits z > 5.6. A total of up to 40 "deep" (4-5 orbit) and 20 "shallow" (2-3 orbit) fields will be targeted in the course of this observing campaign.

GO 12244: Mapping Ganymede's time variable aurora in the search for a subsurface ocean

Hubble ultraviolet image of auroral activity near Jupiter's north magnetic pole

The aurora borealis is a common sight from northern terrestrial latitudes. Planetary aurorae are stimulated by the influx of charged particles from the Sun, which travel along magnetic field lines and funnel into the atmosphere near the magnetic poles. Aurorae therefore require that a planet has both a substantial atmosphere and a magnetic field. They are a common phenomenon on Earth, sometimes visible even at magnetic latitudes more than 40 degrees from the pole, and have also been seen on Jupiter, Saturn, Uranus and Neptune. Jovian auroral activity is also affected by the Galilean satellites, which generate electric currents that can produce bright auroral spots (see figure), and, in some cases, have their own auroral storms. This proposal aims to use that electrical stimulus to probe the sub-surface structure of Ganymede, the largest of the Jovian satellites. That satellite has been postulated as harbouring a saline ocean beneath the icy crust. The presence of such an ocean would affect Ganymede's electrical conductivity, and, in turn, the motions of the auroral loops generated by Ganymede's orbital motion. The present program aims to test this hypothesis by monitoring the auroral motions over a 5-orbit span.