This week on HST

HST Programs: June 10 - June 16, 2013

Selected highlights

GO 12789: Through a Lens, Darkly - New Constraints on the Fundamental Components of the Cosmos

The cluster MACS J1206.2-0.47, imaged by HST as part of the CLASH program

The overwhelming majority of galaxies in the universe are found in clusters. As such, these systems offer an important means of tracing the development of large-scale structure through the history of the universe. Moreover, as intense concentrations of mass, galaxy clusters provide highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. The present program aims to capitalise fully on HST's imaging capabilities, utilising the refurbished Advanced Camera for Surveys and the newly-installed Wide-Field Camera 3 to obtain 17-colour imaging of 25 rich clusters. The data will be use to map the mass profiles of the clusters and probe the characteristics of the high-redshift lensed galaxies. Since ACS and WFC3 can be operated in parallel, the program also uses parallel imaging in offset fields to search for high-redshift supernovae. The present observations target the cluster MACS1311-031 at z=0.492.

GO 12903:: The Evolutionary Link Between Type 2 and Type 1 Quasars

Artist's impression of the black hole and surrounding torus in an AGN

This year (2013) is the fiftieth anniversary of the recognition that QSOs (quasars, quasi-stellar objects) were extremely luminous objects lying at substantial redshifts. The central engine powering these luminous objects is now recognised as a supermassive black hole, marking the central regions of a galaxy. As such, QSOs are clearly related to (and more luminous than) active galactic nuclei (AGN). Like AGNs, QSOs have been segregated into two categories based on their spectral properties: systems with broad lines are characterised as Type 1 QSOs; systems with narrow lines are classed as Type 2. As with AGN, the underlying cause of these differences is generally believed to reside more in our perspective than on the sources themselves: heavily obscured systems, where the central accretion disk lies behind a thick veil of dust, are observed as Type 2 systems; they are expected to evolve to form Type 1 systems as the dust is ablated and destroyed. The present SNAP program aims to test this scenario by coupling mid-infrared Herschel observations, probing the dust environment, with HST near-infrared WFC3 imaging of targets drawn from two matched samples of Type 1 and Type 2 QSOs.

GO 13003: Resolving the Star Formation in Distant 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 both to investigate the mass distribution of individual foreground galaxies and to probe the properties of more distant systems. 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. Moreover, the lensed image of the background galaxy is not only amplified, but expanded in angular size, enabling much more detailed investigation of the properties of such systems than in un-lensed field galaxies at the same redshift. The present program is capitalising on galaxy-galaxy lenses as natural telescopes to probe star formation at redshifts z>1. WFC3 is being used to observe 73 galaxy-galaxy lensed systems identified from the Sloan Digital Sky Survey. The systems are imaged in both the UVIS and IR channels, mapping the unrerlying spectral energy distribution, and hence the star formation rate, as a function of location in the background galaxies.

GO 13046: RAISIN: Tracers of cosmic expansion with SN IA in the IR

The first supernova discoevered by the Pan-STARRs survey

Supernovae are the most spectacular form of stellar obituary. In recent years, these celestial explosions have acquired even more significance through the use of Type Ia supernovae as distance indicators in mapping the `dark energy' acceleration term of cosmic expansion. However, while there are well-established models for the two main types of supernovae (runaway fusion on the surface of a white dwarf in a binary system for Type Ia, or detonation of the core in Type II), some uncertainties remain as to the uniformity of the events. Moreover, as the sample of known supernova has grown, so has the range of photometric systems and the methods used to fit the light curve and account for the ever-present uncertainites inroduced by dust absorption. Consequently, the potential remains for systematic bias in distance estimates due both to intrinsic differences and to measurement errors. The persent program aims to minimise these systematics by compiling standard sequences of observations, primarily in the Y, J, and H filters, of supernovae at redshifts between z~0.3 and 0.5. Focusing on those wavelengths minises the effects, and hence the uncertainties, due to dust absorption. The supernovae themselves are drawn from the Pan-STARRS survey, with the WFC3-IR camera on HST employed to obtain the photometry.