This week on HST

HST Programs: April 23 - April 29, 2012

Selected highlights

GO 12210: SLACS for the Masses: Extending Strong Lensing to Lower Masses and Smaller Radii

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 Sloan Digital Sky Survey is a powerful tool for identifying candidate galaxy-galaxy lenses, and has provided targets for HST imaging programs in several previous cycles. The presentprogram is using HST-ACS imaging to survey a further 135 strong lens candidates. The HST data will verify the nature of those candidates, and provide the angular resolution necessary to model the mass distribution.

GO 12478: The Mysterious Redness of Saturn's Rings

Cassini's measurements of fine structure in the Saturnian ring system

The most prominent feature in the Saturnian system is undoubtedly the multiple set of rings that encircle the planet. Originally noted by Galileo in his first telescopic observations, albeit characterised more as horns, the rings were first characterised as such by Christian Huyghens in 1655. Since then, telescopic observations of increasing power and resolution have revealed their segregation into many radial systems. The close-up imaging carried out by the Cassini satellite since its arrival in the Saturnian system in 2004 has revealed increasingly complex structures, many driven by the presence of small shepherding satellites. The ring system has long been known to be made up from numerous small icy fragments. However, the detailed composition of those fragements remains uncertain. The present program aims to probe this issue by using the Space Telescope imagign Spectrograph (STIS) to map the spectrum of the reflected light at wavelengths between 1500 and 3000 Angstroms. This region includes features due to carbon-based materials (including graphite), ammonia, water, ozone and nanophase metals, and can therefore set constraints on the underlying composition and hence the possible origin of the materials.

GO 12512: Debris Disk Chemistry from Spatially Resolved Spectroscopy

HST image of the debris disk around two nearby A stars, including HR 4796, one of the stars targeted by this program

Planet formation occurs in circumstellar disks around young stars. Most of the gaseous content of those disks dissipates in less than 10 million years, leaving dusty debris disks that are detectable through reflect light at near-infrared and, to a lesser extent, optical wavelengths. The structure of those disks is affected by massive bodies (i.e. planets and asteroids), which, through dynamical interactions and resonances, can produce rings and asymmetries. Analysis of the rangle of morphological structure in these systems provides insight into the distribution of properties of planetary systems. HST currently provides the most effective (in some cases, the only) means of achieving the high-contrast required for the detection of scattered light from these disks in the presence of the bright parent stars, and a number of HST post-SM4 programs are being devoted to obtaining high-resolution images for newly discovered systems (eg GO 12228, 12291, 12573, 12576). The present program takes a different tack, aiming to obtain spectroscopic imaging of disks around three bright nearby stars. The aim is to use the Space Telescope Imaging Spectrograph in coronagraphic mode, where an occulting bar is placed over the central star. The observations will be made with the G430L and G750L low-resoltuion gratings, covering the near-UV, optical and far-red regions of the spectrum. The resulting spectra will provide insight on the composition of the dust grains in these relatively young systems.

GO 12283/12568: WISP - A 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 redshifts 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.