This week on HST

HST Programs: September 7 - September 13, 2009

Selected highlights

GO 11202 The Structure of Early-type Galaxies: 0.1-100 Effective Radii

HST16309+8230, a disk galaxy, distorted due to gravitational lensing by a foreground elliptical

Despite their apparently simple appearance, the processes responsible for the formation and evolution of elliptical galaxies remain somewhat obscure. It is clear that most star formationin these systems must occur at early epochs, since these systems are highly gas poor at even moderate redshifts. Grabitational lensing provies one of the more important tools for investigating these systems, since it can probe the detailed form of the mass distribution, and test for the presence of sub-structure in the underlying dark matter, as predicted by some theoretical models. The present program is combining high-resolution, multi-colour HST imaging with ground-based low-resolution VLT/Keck spectroscopic observations of over 50 strong lensing systems. The resultant datasets can be used to investigate the structure of elliptical galaxies over a wide range of radii, and test the predictions of relevant theoretical models.

GO 11599: Distances of Planetary Nebulae from SNAPshots of Resolved Companions

HST image of the Eskimo planetary nebula, NGC 2392

Planetary nebulae are the spectacular results of the penultiumate evoutionary phase of intermediate-mass stars. Towards the conclusion of the second, or asymptotic, giant branch, ~1 to ~7 solar mass stars have achieved radii exceeding 100 RSun and their extensive envelopes are subject to long-period, opacity-driven pulsations and substantial mass loss. AGB evolution terminates with the ejection of the stellar envelope, revealing the bare core, with an initial surface temperature approaching 100,000K. The core cools to become a white dwarf but, during the initial phase, its luminosity is sufficient to excite gases in the expanding envelope, producing a planetary nebula. These objects are extremely spectacular, but also, in Galactic terms, rare. As a result, we only know reliable distances to a handful of such systems. The present program aims to tackle this issue through observations of planetary nebulae with known or suspected lower-mass companions. Once confirmed as binaries, distances can be estimated using the companions, which still reside on the main sequence.

GO 11670: The Host Environments of Type Ia Supernovae in the SDSS Survey

SN 2007uy and 2008D in NGC 2770

Supernovae have long attracted the attention of both amateur and professional astronomers in their own right, as a means of studying the violent eruption and death of massive stars and degenerates. However, in the last decade they have also acquired considerable importance as distance indicators, tracing the expansion of the universe to redshifts well beyond the reach of more conventional yardsticks, such as cepheids, and providing a key underpinning for the hypothesised existcen of dark energy. Understanding the supernovae themselves, and, in particular, their progenitors, is key to accurately interpreting their luminosities and distances. The present program aims to tackle that aspect of the problem by using ACS to obtain deep, high resolution images of galaxies that have harboured recent type Ia supernovae. The targets are all drawn from the Sloan Digital Sky Survey, which has uncovered more than than 500 type Ia supernovae,. The supernovae themselves are long gone from view, but the ACS data will be used to probe the stellar populations in the immediate vicinity of the explosion, and hence gain a better understanding of the likely progenitor.

GO 12003: The Impact Event on Jupiter in 2009

HST WFC3 observations of the comet scar on Jupiter

One of HST's most cited set of observations was the series of images taken in July 1994 as the dishevelled remnants of Comet Shoemaker-Levy 9 crashed into Jupiter. That event highlighted the catastrophic consequences of cosmic impacts, and sparked considerable interest in identifying earth-crossing asteroids and comets. Fifteen years later, on 19 July 2009, an Australian amateur astronomer, Anthony Wesley, noticed a strange dark 'scar" near Jupiter's southern pole. The feature was quickly confirmed by other astronomers, both amateur and professional, and was quickly judged as likely to be the result of another, completely unexpected, cometary impact. The SL9 impact was thought to be a once-in-a-lifetime (if not more) event; clearly, the impact rate is higher than previously suspected. At the time of the new event, HST was deeply immersed in the initial on-orbit instrument performance tests, but the science staff were able to interrupt those procedures for a few orbits to obtain images with Wide-Field Camera 3. Further observatio9ns will be obtained at a later date to track the evolution of the feature, as Jovian winds disperse the detritus through the atmosphere.