This week on HST

HST Programs: April 23 - April 29, 2007

Some selected highlights

GO 10474: Shooting Stars: Looking for Direct Evidence of Massive Central Black Holes in Globular Clusters

The formation, and subsequent evolution, of intermediate-mass black holes (IMBHs) is a process of considerable importance for understanding th development of active galactic nuclei at high redshifts. IMBHs are generally expected to originate in globular-cluster-mass structures; hence, one might expect such objects to be present in some present-day Galactic clusters, particularly those that have undergone dynamical core collapse. The present program targets the bulge globular clusters, NGC 6388 and NGC 6441. These are both fairly massive globular clusters, and therefore good candidates to have central black holes. This program will obtain second-epoch ACS images of the central regions of these clusters. The proper motions derived by combining those data with observations from Cycle 12 will have an accuracy of ~6 km/sec, and therefore should be sufficient to detect the kinematic signature of a central massive object.

GO 10798: Dark Halos and Substructure from Arcs & Einstein Rings

Hubble imanges of Einstein ring gravitational lenses

Gravitational lensing provides a powerful method of tracing the mass distribution of individual galaxies and galaxy clusters; at the same time, the amplified the light from background galaxies provides a means of probing the early stages of galaxy formation. The Sloan Digital Sky Survey (SDSS) has proven a powerful tool for identifying new lens candidates, although high angular resolution HST observations are essential for confirming those candidates as genuine gravitational lensing systems (see Program GO 10886). The present program focuses on that sample, targeting confirmed lenses for deep, multicolour imaging that aims to measure the overall mass distribution of the lens galaxies, and quantify the existence of small-scale sub-structure. The current cosmological paradigm, cold dark matter, predicts that sub-clustering within galactic halos should scale with increasing redshift; observations of lenses at different redshifts will serve to test this prediction.

Recent supernova in M100

Supernovae mark the (spectacular) evolutionary endpoint for a subset of stellar systems. Standard models predict that they originate from massive stars and (probably) close binaries with a compact (WD, neutron star) component, but there are still some questions remaining over whether we fully understand the range of possible progenitors. The last decade has seen the development of a number of large-scale programs, usually using moderate-sized telescopes, that are dedicated to monitoring (relatively nearby galaxies, searching for new supernovae. This program obtains follow-up images of recent supernovae, concentrating on systems within 20 Mpc of the Milky Way. The observations are taken well after maximum, with the aim of using the high spatial resolution of WFPC2 to identify the fading remnant and perhaps determine its origin.

GO 11085: Europa in Eclipse: Tenuous Atmosphere, Electromagnetic Activity and Surface Luminescence

Image of Europa taken by Voyager 2 in 1979

Europa is the smallest, and the most intriguing, of the four Galilean satellites of Jupiter. With a diameter of 3139 km, Europa is almost twice the size of Earth's moon and significantly larger than Mercury. In 1957, Gerard Kuiper commented that both infrared spectroscopy and the optical colours and albedo suggested that Jovian satellite II (Europa) is covered "by H2O snow". Images taken by the Voyager space probes in the late 1970s (see left) reveal a smooth surface, with only a handful of craters larger than a few kilometres. These features are consistent with a relatively young, icy surface. Subsequent detailed investigations by the Galileo satellite strongly suggest that a substantial body of liquid water, heated by tidal friction, underlies a 5 to 50 km thick icy crust. The presence of this subterranean (subglacial?) ocean clearly makes Europa one of the two most interesting astrobiology targets in the Solar System. The aim of the present HST program is to search for optical and UV fluorescence on the Europa by imaging the satellite as it disappears into eclipse in Jupiter's shadow. That fluorescence might arise from a number of sources - auroral, mineralogical or, as an extreme longshot, biological.