This week on HST

HST Programs: November 8, 2010 - November 14, 2010

Selected highlights

GO 11644: A dynamical-compositional survey of the Kuiper belt: a new window into the formation of the outer solar system

The architecture of the outer Solar System

The Kuiper Belt lies beyond the orbit of Neptune, extending from ~30 AU to ~50 AU from the Sun, and includes at least 70,000 objects with diameters exceeding 100 km. Setting aside Pluto, the first trans-Neptunian objects were discovered in the early 1990s. Most are relatively modest in size, with diameters of a few hundred km and photometric properties that suggested an icy composition, similar to Pluto and its main satellite, Charon. Over the last three years, a handful of substantially larger bodies have been discovered, with diameters of more than 1000 km; one of the objects, 2003 UB313, is comparable in size to Pluto (2320 km.). At the same time, ground-based surveys, such as the Deep Ecliptic Survey, the Canada-France Ecliptic plane Survey and the Palomar Quest Survey, scanned the ecliptic for fainter, lower-mass objects, with the aim of using their properties to assess the likely chemical composition and dynamical history of the early Solar System. The present program will use Wide Field Camera 3 to push up to 2 magnitudes fainter than these ground-based studies, providing reliable estimates of compositions for a representative sample of KBOs.

GO 11675: Stellar Forensics: A post-explosion view of the progenitors of core-collapse supernovae

Chandra X-ray image of G292.0+1.9, a ~3000-year old supernoa remnant

Supernovae are generally believed to originate through two mechanisms: accretion onto a white dwarf in a close binary system, driving the white dwarf above the Chandrasekhar limit; and the implosion of the core of very massive (> 7 solar masses) stars. Both processes result in explosive nucleosythesis that enriches the interstellar medium, with the ejecta forming a rapidly expanding shell. Supernovae are intrinsically rare: Tycho's star (1604) was the last Galactic supernova identified by contemporary astronomers, although the radio remnant Cas A (identiied as 3C 461 in the 1959 Third Cambridge Catalogue of Radio Stars) may have been recorded, if not recognised as unusual, by Flamsteed in 1680. Understanding their progenitors therefore demands that we expand observations to external galaxies. The present program builds on several from previous cycles, and uses HST to obtain high-resolution imaging of the sites of several Type II supernovae in nearby galaxies. By now, the supernova themselves have faded from view, but the HST data can reveal the stellar population in the immediate environs. detailed analysis of the colour magnitude diagrams can probe the likely age of the star forming regions, and hence set limits of the progenitor mass.

GO 12064: Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey -- the UDS fields

Part of the GOODS/Chandra Deep Field South field, as imaged by HST

CANDELS is one of three Multi-Cycle Treasury Program, whose observations will be executed over the next three HST Cycles. It builds on past investment of both space- and ground-based observational resources. In particular, it includes coverage of the two fields of the Great Observatory Origins Deep Survey (GOODS), centred on the northern Hubble Deep Field (HDF) in Ursa Major and the Chandra Deep Field-South in Fornax. In addition to deep HST data at optical and near-infrared wavelengths, the fields have been covered at X-ray wavelengths by Chandra (obviously) and XMM-Newton; at mid-infrared wavelengths with Spitzer; and ground-based imaging and spectroscopy using numerous telescopes, including the Kecks, Surbaru and the ESO VLT. This represents an accumulation of almost 1,000 orbits of HST time, and comparable scale allocations on Chandra, Spitzer and ground-based facilities. The CANDELS program is capitalising on this large investment, with new observations with WFC3 and ACS on both GOODS fields, and on three other fields within the COSMOS, EGS and UDS survey areas (see this link for more details). The prime aims of the program are twofold: reconstructing the history of galaxy formation, star formation and nuclear galactic activity at redshifts between z=8 and z=1.5; and searching for high-redshift supernovae to measure their properties at redshifts between z~1 and z~2. The program incorporates a tiered set of observations that complement, in areal coverage and depth, the deep UDF observations, while the timing of individual observations will be set to permit detection of high erdshift SNe candidates, for subsequent separate follow-up.

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.