This week on HST

HST Programs: October 10, 2011 - October 16, 2011

Selected highlights

GO 12257: The Nature of Multiple Main Sequence Turn-offs and Dual Red Clumps in Magellanic Cloud Star Clusters

NGC 1818, a globular cluster in the Large Magellanic Cloud

Globular clusters are remnants of the first substantial burst of star formation in the Milky Way. With typical masses of a few x 105 solar masses, distributed among several x 106 stars, the standard picture holds that these are simple systems, where all the stars formed in a single starburst and, as a consequence, have the same age and metallicity. Until recently, the only known exception to this rule was the cluster Omega Centauri, which is significantly more massive than most clusters and has both double main sequence and a range of metallicities among the evolved stars. Omega Cen has been joined by several additional Galactic clusters, including NGC 2808, NGC 1851 and 47 Tucanae. In addition, there is growing evidence for similarly complex stellar populations within at least some of the globular clusters associated with the Large and Small Magellanic Clouds. Several of the latter clusters show clear evidence for bimodality in the red clump stars, which lie towards the base of the red giant branch. The origin of this feature is remains uncertain, but it may be significant that the Galactic clusters are amongst the most massive within the Milky Way's population. The present program focuses on the Magellanic clusters, and will use WFC3 to obtain deep F475W (B-band) and F814W (I-band) imaging for a sample of 17 clusters.

GO 12477: Weak lensing masses of the highest redshift galaxy clusters from the South Pole Telescope SZ survey

The ACS optical/far-red image of the galaxy cluster, Abell 2218, including an extensive number of lensed arcs

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. Galaxy clusters can be identified at moderate redshifts by searching for signatures of the Sunyaev-Zeldovich effect: high energy electrons in the hot intercluster medium interact with radiation from the cosmic microwave background to distort the microwave spectrum. The South Pole Telescope is a 10-metre microwave/millimetre telescope located at Amundsen-Scott South Pole Station on the Antarctiva high plateau, close to the geographic South Pole. That telescope has been used to search for galaxy clusters. As intense mass concentrations, these systems are 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 use the Wide Field Camera on the Advanced Camera for Surveys to obtain observations of seven clusters with masses between 3 x 1014 and 1015 MSun, and with redshifts in the range 0.93 < z < 1.2. The HST data will be used to measure weak lensing, and combined with ground-based and space-based observations (Chandra, XMM, Spitzer) to refine the mass estimates for these clusters.

GO 12502: From the Location to the Origins of Short Gamma-Ray Bursts

An artist's impression of a gamma-ray burst

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected over the last 30+ years, and analyses indicate that they can be divided into two classes with durations longer or shorter than 2 seconds. The short bursts appear to release more high energy radiation, so the two subsets are known as long/soft and short/hard bursts. The long/soft bursts appear to originate in the collapse of very massive stars, while the short/hard bursts are coalescing binary systems (probably pairs of netron stars or black holes). The first optical counterpart to a gamma ray burst was identified in 1998, allowing confirmation of their extragalactic nature, and, since then, more than 60 bursts have been detected at X-ray wavelengths, and half that number detected at either optical or radio wavelengths; all of these detections are long/soft bursts. The present program uses a two-pronged approach to probe the nature of soft GRB progenitors by gaining a better understanding of the nature of the surrounding stellar population. One approach is statistical, with high angular resolution WFC3 imaging used to examine the colours of stellar populations in a sample of galaxies that have hosted past GRBs - are they characteristic of young, star-forming regions, or of older, more mature environments? The drawback with this study lies in the modest accuracies associated with the GRB positions in those galaxies. Complementing that analysis, the program will trigger ToO observations of a newly-discovered discovered GRBs in an elliptical. The HST imaging will lead to a more precise position - in particular, a test for coincidence a globular cluster in the targeted galaxy - and hence a clearer idea of the local stellar environment.

GO 12314: Mapping Brown Dwarfs: The Evolution of Cloud Properties Through the L/T Transition

Brown dwarfs are likely to have complex atmospheric structures that resemble Jupiter

Brown dwarfs are failed stars - objects that form like stars, by gravitational collapse within giant molecular clouds, but which have insufficient mass to raise the central temperature above 107 K, and which therefore are unable to ignite hydrogran fusion and maintain a long-lived central energy source. As such, these objects reach a maximum surface temperature of perhaps 3,000K some tens of millions of years after their formation, and subsequently cool and fade into oblivion. As they cool, they move through spectral types M, L and T, with the oldest brown dwarfs now likely to have temperatures close to 300K and emergent spectra characterised by water and ammonia bands, the putative signatures of the spectral class Y. As these dwarfs cool from L to T (~1500 to ~1200K), the atmospheres undergo significant changes, with heavier elements condensing to form dust. That dust can form clouds, perhaps giving the dwarf's surface a banded appearance, similar to Jupiter. The clouds themselves may appear and disappear over relatively short timescales, leading to photometric variations at particular wavelengths. The current program focuses on a handful of brown dwarfs with spectral types near the L/T transition, and uses the WFC3 grism to obtain high-accuracy monitoring of their spectral behaviour. Two of these sources will also be observed with Spitzer.