This week on HST

HST Programs: September 3 - September 9, 2012

Selected highlights

GO 12451: 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 will also use parallel imaging in offset fields to search for high-redshift supernovae. The present observations target the cluster MACS0416-2403 at z=0.42.

GO 12473: An Optical Transmission Spectral Survey of hot-Jupiter Exoplanetary Atmospheres

An artist's impression of the hot Jupiter circling a solar-type star

The first planet orbiting another star was discovered in 1995 around the relatively rbight G dwarf, 51 Pegasi. 51 Pegb confounded expectations as a jovian-mass gas giant in a 3-day orbit, placing it closer to the parent star than Mercury in our Solar System. Over the past nearly 20 years, numerous other systems have been identified, with the most recent flood of identifications coming from the Kepler satellite, which is pushing the detection limit to objects as small as earth in orbits with semi-major axes exceeding 1 AU. Many of these new detections (and all of the Kepler detections) are transiting systems. Transiting systems offer a potential gold-mine for extrasolar planetary studies, since not only is the orbital inclination well defined, but the diameter (and hence the average density) can be measured directly from the eclipse depth, while the atmospheric composition can be probed through line absorption or re-radiated thermal flux. The results from these measurments can be used to test, and improve, theoretical models of extrasolar planets. These observations are best done from space (indeed, the only unequivocally successful atmospheric observations to date have been with HST and Spitzer). The present program targets nine systems, all discovered through ground-based surveys (the WASP and HAT surveys), and all comprising jovian-mass planets in short-period orbits. The program will use STIS to obtain optical spectra, covering the full wavelength range with the G430L and G750L gratings, while WFC3 will be used to target near-IR spectra with the G141 grism.

GO 12498: Did galaxies reionize the universe?

The ACS optical/far-red image of the Hubble Ultra Deep Field

Galaxy evolution in the early Universe is a discipline of astronomy that has been transformed by observations with the Hubble Space Telescope. The original Hubble Deep Field, the product of 10 days observation in December 1995 of a single pointing of Wide Field Planetary Camera 2, demonstrated conclusively that galaxy formation was a far from passive process. The images revealed numerous blue disturbed and irregular systems, characteristic of star formation in galaxy collisions and mergers. Building on this initial progam, the Hubble Deep Field South (HDFS) provided matching data for a second southern field, allowing a first assessment of likely effects due to field to field cosmic variance, and the Hubble Ultra-Deep Field (UDF) probed to even fainter magitude with the Advanced Camera for Surveys (ACS). Pushing to larger distances, and greater ages, demands observatons at near-infrared wavelengths, as the characteristics signatures of star formation are driven further redward in the spectrum. The installation of Wide-Field Camera 3 during Servicing Mission 4 opened up this regime, through observations using the WFC3-IR camera in the F105W (J) and F160W (H) filters. Those observations provided the first candidate galaxy at redshift z=10. However, that individual detection - and even the 10s of gaalxies detected at z=7 to 8 - represent only the most luminous galaxies at those redshifts, and provide only limited constraints on the total star formation at those epochs. The present program will add a further 128 orbits centred on the UDF, primarily in the F105W and F140W filters. These observations will push the detection limits to fainter apparent magnitudes, and to lower luminosity galaxies, setting stronger constraints on the total star formation in the early universe.

GO 12605: Advances in Understanding Multiple Stellar Generations in Globular Clusters

NGC 2808, a globular cluster with multiple stellar populations

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 clusters, including NGC 2808, which shows evidence for three distinct branches to the main sequence. The origin of this feature is remains uncertain, but it may be significant that NGC 2808 is also one of the more massive clusters, and might therefore be able to survive several burst of star formation (or, conversely, be the product of a multi proto-globular merger). Evidence for multiple populations has also been found in other clusters, including NGC 1851, 47 Tucanae and NGC 6752 - all relatively massive clusters. Colours spanning a wide baseline in wavelength offer the best prospects for separating stellar populations in the CMD. The present program aims to extend observations to a wider range of clusters, and probe whether there is any correlation between the multiple population structure and the horizontal branch morphology. WFC3 will be used to obtain uv (F275W), U (F336W) and I-band (F814W) imaging of two classical second-parameter cluster pairs, NGC 288/NGC 362 and M3/M13, and three extreme HB clusters, NGC 2808, M80 and M15.