This week on HST

HST Programs: June 5 - June 11, 2006

Some selected highlights

GO 10496: Decelerating and Dustfree: Efficient Dark Energy Studies with Supernovae and Clusters

HST images of intermediate redshift supernovae

The last few years of the twentieth century saw a revolution in cosmology, with the measurement of the acceleration term in expansion at high redshifts and the identification of dark energy as a major cosmological component. Type Ia supernovae are the prime yardstick for measuring the rate of expansion at moderate and high redshifts, and Hubble offers almost the only way of obtaining reliable post-maximum photometry of these objects to determine the full shape of the light curve. Most previous HST supernovae programs have concentrated on field galaxies, such as those illustrated here, but applying appropriate corrections for in situ reddening by dust remains an issue in these systems. The present program aims to minimise the uncertainties by searching for supernovae in massive, high-redshift clusters, with the expectation that the majority of detections lie within dust-poor elliptical galaxies. ACS survey observations of eight clusters are scheduled for the coming week, together with follow-up NICMOS observations of a supernova detected in previous ACS images.

GO 10606: Multicolour snapshot observations of 3C Radio Galaxies

Hubble WFPC2 planetary camera images of the central regions of several 3C radio galaxies

The Third Cambridge Catalogue of Radio Sources was compiled originally in 1959 from observations at 159 MHz made by the Cambridge Radio Astronomy Group using the Cambridge interferometer (Edge et al, Mem RAS, 68, 37). The addition of observations at 178 MHz led to the revised catalogue, the 3CR (Bennett, 1962, Mem RAS, 68, 137). This catalogue was the main basis for the Fanaroff-Riley classification scheme (FR-Is are double-lobed sources with small separation between the peaks; FR-IIs have a separation that exceeds half the largest size of the source), and includes many of the brightest known radio galaxies. As such, the 3CR sources have remained extremely important observational targets for understanding the nature and structure these highly energetic sources. Over the past several cycles, many 3CR galaxies have been observed at a variety of wavelengths by HST. Those observations have revealed new optical jets, dust lanes, face-on disks with optical jets, besides point-like nuclei whose properties support FR-I/BL Lac unified schemes. The present programs aim to use NICMOS and the ACS/HRC to extend coverage to near-infrared and ultraviolet wavelengths, searching for dust and regions of star formation in the central regions.

GO 10525: Characterizing the Near-UV Environment of M Dwarfs: Implications for Extrasolar Planetary Searches and Astrobiology

SOHO image of an extremely strong solar flare

M dwarfs - at least, the subset of M dwarfs known as flare stars - are renowned for possessing extremely active chromospheres and coronae. Their discovery as highly variable objects happened largely by chance. Willem Luyten had noticed in 1924 that certain M dwarfs showed spectroscopic variability, with the occasional appearance of emission lines, while in the early 1940s van Maanen commented that two late-type dwarfs, Gl 412B (WX UMa) and Gl 285 (YZ CMi), had brightened by over a magnitude on a handful of parallax plates. The crucial observations came in 1948, when E.F Carpenter noticed that the fainter component of a wide binary system had brightened by more than 3 magnitudes in a matter of minutes. In the succeeding 50 years, these stars have been subjected to extensive observations, particularly at optical and X-ray wavelengths, and the underlying physical processes are relatively well understood. However, most attention has focused on the more active flare stars, and we still have a relatively uncertain grasp on the flare frequency among less active stars. This issue has acquired increased importance as more attention has been devoted to the potential of M dwarfs as planetary hosts. The habitable zones lie much closer to the parent star, and planets are correspondingly vulnerable to detrimental effects from enhanced UV radiation, particularly short-wavelength UV-C. This proposal uses ACS HRC prism to obtain low-resolution near-UV spectra of nearby M dwarfs, providing a broad sampling of the range of activity levels among these low mass dwarfs.

GO 10534 Active Atmospheres on Uranus and Neptune

Nicmos image of aurorae on Uranus

The atmospheres of the gas giant planets in the solar system are dynamic entities that can exhibit dramatic changes over a variety of timescales. Those changes are most apparent in Jovian atmosphere, which displays a wide variety of bands and spots, reflecting complex meteorological phenomena (see, e.g., previous ACS observations of the upper atmosphere and of the new little red spot ). This is not surprising since Jupiter atmosphere receives the highest input of solar energy. However, secular variations are also evident in the atmospheres of the outer planets, albeit usually at a more subtle level. The present program aims to monitor atmospheric changes in the two outermost gas giants, Uranus and Neptune. Both exhibit long-term seasonal variations, whose origins are not yet well understood; both are capable of generating dark spots - phenomena that are presumably related to Jupiter's Great Red Spot and Saturn's Great White Spot. The present observations use a variety of filters on ACS and WFPC2 (notably the 619 nm methane filter) to probe conditions are a variety of levels within the planetary atmospheres.

GO 10626: A Snapshot Survey of Brightest Cluster Galaxies and Strong Lensing to z = 0.9

Lensed arcs in an intermediate redshift galaxy cluster

Gravitational lensing provides a powerful method of tracing the mass distribution in galaxy clusters, while amplifying the light from background galaxies. This proposal will obtain ACS observations of a subset of 150 galaxy clusters, searching for the tell-tale arcs that reveal gravitational lensing. The aim is to use those data to measure the mass distribution in the targeted clusters, and constrain the evolution of both the baryonic and dark mass in cluster cores at redshifts less than z=0.9. These images will also yield a well-defined sample of arcs formed by strong lensing; the frequency and detailed distribution (size, multiplicity, redshifts) of these strong lens systems sets strong constraints on the total mass content (and its structure) in the centers of the targeted clusters.