This week on HST

HST Programs: May 14 - May 20, 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 12450: Understanding a new class of mid-IR transient

SN 1999bw in NGC 3198, one of the transients targeted by this proposal

Searching for transient objects, particularly supernovae, novae, cataclysmic variables and flare stars, has factored prominently in astronomy since at least the cataloguing of "guest stars" by the Chinese over 2000 years ago. For most of the 19th and 20th centuries, this field was largely the pursuit of amateur astronomers, coupling visual scans of the heavens with pesonal encyclopaedic knowledge of star patterns. However, the development of large-format, highly-sensitive digital imaging devices oevr the past decade or more has opened the subject for investment by professional observatories. Several transient surveys are currently underway, notably the Berekeley Automated Supernova Survey, using the Leischner Observatory in Lafeyette, california; the Palomar Transient Factory, using the 48-inch Oschin Schmidt of Palomar Sky Survey fame, now equipped with multiple CCDs; and Pan-STARRS, a dedicated survey telescope operating on Haleakala. Further down the road lies LSST, the Large Synoptic Survey Telescope, the highest ground-based priority from the Astro2010 Decadal Survey. In the meantime, the increasingly extensive catalogues generated by the ongoing surveys have turned up a variety of unusual transients. The present program focuses on one such example, a set of luminous spernova-like transients, detected in external galaxies, where pre-outburst observations show no evidence for a progenitor. The hypothesis is that these originate in highly obscured AGB stars. This program couples WFC3-IR J and H observations with Chandra and Spitzer measurements to probe the underlying nature of these sources.

Preliminary orbital determination for the KBO WW31, based on C. Veillet's analysis of CFHT observations; the linked image shows the improved orbital derivation, following the addition of HST imaging

The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun (see David Jewitt's Kuiper Belt page for details). Over 500 KBOs (or trans-Neptunian objects, TNOs) are currently known out of a population of perhaps 70,000 objects with diameters exceeding 100 km. Approximately 2% of the known TNOs are binary (including Pluto, one of the largest known TNOs, regardless of whether one considers it a planet or not). TNOs are grouped within three broad classes: resonant objects, whose orbits are in m,ean motion resonance with Neptune, indicating capture; scattered objects, whose current orbits have evolved through gravitational interactions with Neptune or other giant planets; and classical TNOs, which are on low eccentricity orbits beyond Neptune, with no orbital resonance with any giant planet. The latter clas are further sub-divided into "hot" and "cold" objects, depending on whether the orbits have high or low inclinations with respect to the ecliptic. Cold, classical TNOs show relatively uniform characteristcis, including red colours, high albedos and an extremely high binary fraction (>30%). They are believed to have formed in situ, and were therefore in place to experience the range of gravitational interactions as the giant planets migrated to their present location. As that migration occurred, subsets are expected to have been trapped in transitory resonance orbits. The present proposal aims to use HST to complete a photometric survey of all known resonant TNOs, with the goal of identifying the proportion of cold classical TNOs that have been captured. The relative number of such objects can be used to constrain models for Neptune's orbital migration in the early Solar System.

GO 12532: The Scale Sizes of Globular Clusters: Tidal Limits, Evolution, and the Outer Halo

The giant elliptical, M87, and its extensive population of associated globular clusters.

Globular clusters are key remnants of the first major episode of star formation in the Milky Way Galaxy. Other galaxies, both spirals and ellipticals, have their own populations of globulars. The clusters associated with M31 are well resolved (both HST and, to a lesser extent, ground-based observations permit the derivation of colour-magnitude diagrams), but in most cases the clusters are only identifiable based on their slightly extended profiles, or through statistical analysis of starcounts in the immediate vicinity of target galaxies. Despite years of study, significant undertanties still remain regarding the physical properties of golbulars, and how those properties vary with either extrinsic or intrinsic parameters. The present program aims to probe the physical size of globular clusters, which have long been thought to have been governed largly by tidal stripping within the gravitational field of the parent galaxy. However, the recent discovory of "anomalous extended clusters" in M31 and other systems has cast some doubt on that proposition. Hundreds of globulars have been identified in M87, the core galaxy in the Virgo cluster. This program aims to map the entire halo of this system, using WFC3 and ACS to obtain deep, high angular resolution images capable of measuring the average size, and the distributionin size, of clusters as a function of radial separation from the parent galaxy.