This week on HST

HST Programs: March 7 - March 13, 2016

Selected highlights

GO 14096: RELICS: Reionization Lensing Cluster Survey

Hubble image and mass map for the cluster ACT-CL J0102-4915, one of the clusters included in the RELICS program

The overwhelming majority of galaxies in the universe are found in clusters. As such, those 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 builds on the highly successful CLASH program,which used 17-colour ACS/WFC3 images to map 25 galaxy clusters, tracing the mas profile and the dark matter distribution, and the Frontier Fields program, targeting six clusters for deep multi-colour imaging. RELICS is focused on using massive galaxy clusters as gravitational telescopes, searching for strongly lensed background galaxies drawn from the high redshift universe. Imaging 46 fields in 41 galaxy clusters, this program aims to identify galaxies with redshifts in the range 9 < z < 12. By targeting strongly-lensing clusters, standard models for galaxy evolution suggest that the program can deliver ~100 galaxies in that redshift range, together with more than 150 galaxies at z~8. A significant number of these galaxies should be brighter than H~25.5, and therefore accessible to more detailed follow-up observations. Conversely, the actual number of galaxies detected will set constraints on the galaxy number-redshift distribution, and the overall formation and assembly history.

GO 14147: Opening a New Window towards the Nuclear Star Cluster in the Milky Way

A multi-wavelength composite image of the Galactic Centre (red - 90 cm radio data; green, mid-IR data; blue, X-ray)

The Galactic Centre lies in the heart of the constellation of Sagittarius, and at a distance of ~8 kiloparsecs from the Sun. Galactic nuclei are the likely end-point for mass accretion, and are generally the site of highly energetic activity; the Galactic Centre is no exception. AO near-infrared imaging has been brought to bear on this issue, resolving a number of stars within the core, close to the compact radio source Sagittarius A*. Monitoring of those sources over the last decade has shown that they are in rapid orbital motion around a very massive central object, now clearly identified as a ~3 million solar-mass black hole. Moving beyond the core, to distances of tens of parsecs from centre, observations have revealed extensive molecular gas and young star forming regions. The present proposal focuses on the stars in the immediate vicinity of the black hole. Highly obscured by foreground dust, the program will use the IR channel on Wide-Field Camera 3 to obtain deep images in the F105W filter. Coupled with existing near-infared data extending through the J, H and K apssbands, these observations will probe the exteinction law, and hence the dust properties near the Galactic Centre, as well as mapping the mass distribution of stars in the nuclear cluster.

GO 14212: A Snapshot Imaging Survey of Spitzer-selected Young Stellar Objects in Nearby Star Formation Regions

HST image of the face-on debris disk in the G2 dwarf, HD 107146

Planet formation occurs in circumstellar disks around young stars. Most of the gaseous content of those disks dissipates in less than 10 million years, leaving dusty debris disks that are detectable through reflect light at near-infrared and, to a lesser extent, optical wavelengths. The structure of those disks is affected by massive bodies (i.e. planets and asteroids), which, through dynamical interactions and resonances, can produce rings and asymmetries. Analysis of the rangle of morphological structure in these systems provides insight into the distribution of properties of planetary systems. HST currently provides almost the only means of achieving the high-contrast required for the detection of scattered light from these disks in the presence of the bright parent stars. While many such systems have been observed, only a relatively small number of disks have been imaged successfully at visual or near-infrared wavelengths. The present SNAPSHOT program aims to expand the sample by targetting sources within nearby star-forming regions that have past Spitzer observations that indicate a significant infra-red excess, suggesting the presence of a circumstellar disk.The Advanced Camera for Surveys will be used to obtain images in the V and I-bands (F606W and F814W filter), providing high resolution data that can be analysed for direct evidence of disks or companions.

GO 14260: A Metallicity and Cloud Survey of Exoplanetary Atmospheres Prior to JWST

Artist's impression of the GJ 1214 system

The first exoplanet, 51 Peg b, was discovered only a little more than two decades ago. In the intervening years, the catalogue has expanded rapidly. Transiting planets are particularly valuable, since they provide an opportunity to probe the atmospheric properties through differencing spectra taken when the planet is in or out of transit and/or exclipse. The vast majority of transiting systems have been discovered by the Kepler satellite, which currently accounts for 1039 confirmed planets with an additional ~3550 candidates.Kepler's initial observations were focused on a single ~105 square degree region in Cygnus; the subsequent K2 observations cover similarly sized fields around the ecliptic. Consequently, the majority of planet host stars are relatively faint. Small, ground-based telescopes have also contributed to the census, complementing Kepler by identifying brighter host stars from all-sky surveys; those brighter stars are more accessble to detailed follow-up observations, and offer the best prospect for probing the atmospheric content. Observations of an increasing number of such systems show diverse atmospheric properties. In some cases, features due to water and methane are discernible; in others, the spectra are essentially flat. The latter results strongly suggest an extensive cloud layer, masking the lower atmosphere. The present program targets 16 transiting planets with masses from 2 to 300 times that of the Earth, aiming to map the presence of clouds in those systems with the aim of discerning trends and correlations that will provide a guide to selecting the prime targets for future observations with the James Webb Space Telescope.