This week on HST

HST Programs: August 4 - August 10, 2014

Selected highlights

GO 12893: Study of Small and Cool Kepler Planet Candidates with High Resolution Imaging

The Kepler satellite

Kepler is a NASA Discovery-class mission, designed to search for extrasolar planets by using high-precision photometric observations to detect transits. Launched on 7 March 2009, Kepler continuously monitored ~100,000 (mainly) solar-type stare within a ~100 square degree region in Cygnus for more than 4 years. Routine observations ceased on May 11 2013 when a second reaction wheel failed; efforts are currently under way to examine the options for restoring observations. Regardless, the mission has been an astounding success. Ground-based observations have successfully detected a couple of dozen transiting planets (e.g. HD 209458); almost all are "hot jupiters", gas giants on short-period orbits which produce a photometric dip of ~10-2 with a period of a few days, with a smattering of neptune-sized "super-Earths". Kepler, in contrast, has identified more than 2,700 exoplanet candidates around over 2,000 candidate host stars. More significantly, the exquisite precision of Kepler's photometric observations enables it to detect the 0.01% transit signature of earth analogues in these systems. A subset of stellar binaries provide one of the main sources of confusion in searching for planetary transits, since "grazing" transits can mimic the planetary signature. This is particularly an issue with Kepler, since the optical system is designed to provide a broad psf, spreading the stellar flux over a large area on the detector to allow high photometric accuracy. As a result, faint eclipsing stellar binaries will contribute to the source counts. Moreover, since the target field is (intentionally) within the Milky Way, there is a significant potential for unresolved stars within the (relatively broad) Kepler psf to increase the total signal, and hence dilute the depth of transits, giving the appearance of a smaller diameter exoplanet. This program is using the high spatial resolution imaging provided by HST to study a subset of the Kepler Earth-like candidates to assess the potential of this effect.

GO 13401: A 3D view of the SN 1987A Ejecta

November 2003 HST image of the SN1987A gaseous ring

SN1987A, in the Large Magellanic Cloud, is (as far as we know) the nearest supernova to the Sun since Kepler's supernova of 1604. While its eruption, in January 1987, predated HST's launch by over 3 years, the remnant has been a regular observational target since the installation of COSTAR at the first servicing mission. Those high resolution observations have revealed the development, and evolution, of extensive, intricate structures as the blast wave from SN1987A encounters the surrounding interstellar medium. In particular, a striking circum-remnant ring has developed, with numerous hot spots stimulated by the fastest moving debris. HST has been monitoring developments in the supernova remnant since Cycle 1, obtaining optical and near-infrared images with almost every imaging instrument installed over the apst 2 decades. The most recent such program, GO 13405, uses the UVIS and IR channels on WFC3 to continue monitoring the development of those features, using a series of observations that are co-ordinated with the Chandra X-ray Observatory. The present program complements those imaging observations with optical spectroscopy of the ejecta with the Space Telescope Imaging Spectrograph, STIS. Alignng the STIS slit along filaments will allow measurement of the line of sight velocity structure, mapping the spatial distribution of the debris and probing the presence and structure of shocks.

GO 13412: An ACS Snapshot Survey of the Most Massive Distant Galaxy Clusters in the South Pole Telescope Sunyaev-Zel'dovich Survey

The South Pole Telescope at the Amundsen-Scott South Pole Station

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 is using the Wide Field Camera on the Advanced Camera for Surveys to obtain SNAPshot images of up to 120 of the most massive clusters so far detected by the SPT. The HST data will be analysed for weak lensing signatures that can be combined with X-ray, IR and optical observations to constrain the mass estimates for these clusters.

GO 13496: HST Frontier Fields - Observations of MACSJ0416.1-2403

The Frontier Fields cluster, MACSJ0416.1-2403

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 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. in addition, the observations identified several lensed galaxies at redshifts that enter the JWST domaine, with the most distant object lying at a redshift z~11, within a few hundred million years of the Big Bang. The Frontier Fields program is a large-scale Director's Discretionary program that capitalises on the latter characteristic by targeting 4-6 strong-lensing galaxy clusters for very deep optical and near-infrared imaging. WFC3 and ACS will be used to observe the clusters, with simultaneous imaging obtained in parallel of a nearby "blank" field. Since the observations need to made at a specific orientation, they are being taken in two sets, ~6 months apart, alternating between detectors. MACSJ0416.1-2403 at z=0.396 is the second target: at the first epoch, the cluster was imaged with WFC3-IR, with ACS obtaining optical data on the nearby blank field; the second epoch observations switch cameras, with ACS on the cluster and WFC3-IR on the parallel field.