This week on HST

HST Programs: December 17 - December 23, 2012

Selected highlights

GO 12442: Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey -- GOODS-North Field, Non-SNe searched visits

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 redshift SNe candidates, for subsequent separate follow-up. The present observations form part of the survey of the GOODS-North field.

GO 12488: SNAPshot observations of gravitational lens systems discovered via wide-field Herschel imaging

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 that were being detected and investigates 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. HST has carried out a number of programs following up candidate lenses identified from the Sloan Digital Sky Survey (eg GO 10886 , GO 11289 , GO 12210 ). The present program is using WFCE on HST to obtain follow-up near-infrared (F110W) images of up to 200 candidate lenses selected from the Herschel Astrophysical Terahertz Large Area (H-ATLAS) and the Herschel Multi-tiered Extra-galactic (HerMES) surveys. The HST data will verify the nature of those candidates, and provide the angular resolution necessary to model the mass distribution.

GO 12555:On the Triggering of Quasars During First Passage

HST images of the merger pair, NGC 4676, The Mice

Galaxy mergers are generally held to be one of the processes that can give rise to the formation of super-massive black holes (SMBH), with the combining galaxies supporting formation within the nucleus of the final system. As the SMBH grows, it is anticipated that it will display the high excitation radiation, sharp velocity curves and jets that are characteristic of quasars. However, while the overall conceptual scenario is relatively well described by models, many of the details remain to be filled in. In particular, one outstanding issue centres on how fuel can continue to be delivered to the burgeoning SMBH throughout the process. Both theoretical models and observations suggest that the evolution is governed by the bulge; as an example, there is a strong correlation between the velocity dispersions measured for bulges and the masses estiametd for the central black hole in those systems. The present program aims to test this hypothesis through osbervations of 10 pairs of galaxies that are believed to be undergoing the early stages of a merger, where one of the galaxies in the pair has strong quasar activity and the other is quiescent. The goal is to examine the overall morphology with the aim of determining whether that ha any bearing on the triggering of the QSO characteristics.

GO 12918: Origin of UCDs in the Coma Cluster

Wide-field iamge of the cluster (KPNO 4-metre)

The Coma cluster is the nearest rich galaxy cluster, lying at a distance of ~100 Mpc from the Milky Way. The cluster includes well over 1000 major galaxies, centred on two giant ellipticals, NGC 4874 and NGC 4889. Chandra observations show that the galaxies are embedded in very hot intracluster gas (see this site ). Cluster galaxies have also been surveyed at mid-infrared wavelengths by Spitzer, and in the ultraviolet by GALEX. Individual galaxies have been studied in the past using HST, and a Cycle 15 Treasury program aimed to obtain systematic imaging with ACS (and parallel observations with NICMOS) of the cluster core ( a 7x6 mosaic, covering approximately 400 sq. arcmin.), together with 40 fields at larger radii, sampling infalling galaxies in the outer cluster. That program was cut short by the failure of ACS in January, 2007. However, before that unfortunate event ACS had succeeded in capturing images of a subset of the central regions, allowing the detection of significant numbers of ultra-compact dwarf (UCD) galaxies. The present program aims to use the refurbished ACS to complete coverage of the core regions, providing a detailed census of the low luminosity tail of the galaxian mass function. The observations will be used to probe colour gradients and internal chemical evolution in these systems, and match the UCD luminosity function against the luminosity of globular cluster systems in these galaxies.