This week on HST

HST Programs: November 28 - December 4, 2016

Selected highlights

GO 14114: A Wide-Field WFC3 Imaging Survey in the COSMOS Field

The full COSMOS field

Hubble has made significant contributions in many science areas, but galaxy formation, assembly and evolution is a topic that has been transformed by the series of deep fields obtained over the past 20 years. The largest area survey to date was conducted in Cycles 12 & 13,when the Advanced Camera for Surveys was used to obtain single filter (F814) images of 2 square degrees in 640 orbits. The resultant dataset, the COSMOS survey, has been surveyed subsequently from both ground and space, with data spanning all wavelengths from X-ray through optical and infrared to the sub-millimetre and millimetre regimes. The HST data are important in providing data with exquisite angular resolution, as well as depth, providing important morphological information on galaxies in the field. Multi-wavelength data at the same resolution can provide important information on star formation history and evolution, and the spatial distribution of dust. Additional HST observations have been obtained for subsets of the field (eg the CANDELs field), but the large areal span has made full coverage overly expensive. That is, until a new observing technique was devised for HST that enables observations of multiple distinct fields in a single orbit. In standard the observing scenario, HST moves to a field, then acquries a guidestar to ensure acurate pointing and stability during the observation; each guide star acquisition takes 4-5 minutes, and HST is generally restricted to 2 pointings at most. In the new mode, HST makes the initial guide-star acquisition, but then offsets to new fields. The telescope drifts without a guide star, but this can be dealt with for near-infrared wavelength imaging; the HST detectors use multiple non-destructive reads, and the telescope drifts by a very limited amount in the time between reads. The net result is that it is possible to cover the COSMOS fields with H-band observations in 57 orbits, adding high angular resolution near-infrared data to compleemnt the ACS I-band data.

GO 14594: QSAGE: QSO Sightline And Galaxy Evolution

An HST GHRS spectrum of the bright quasar, HE 2347-4342

Star formation is the key astrophysical process in determining the overall evolution of galactic systems, the generation of heavy elements, and the overall enrichment of interstellar and intergalactic material. Tracing the overall evolution through a wide redshift range is crucial to understanding how gas and stars evolved to form the galaxies that we see around us now. In particular, the star formation rate appears to have declined significantly between redshifts z~2 and z~1. The present program aims to compile observations that measure the star formation rate in a large sample of galaxies at z~1. This goal will be achieved by using the G141 grism on Wide Field Camera 3 to target fields centred on quasars where spectroscopy indicates the presence of gaseous absorbers (ie galactic halos) at the appropriate redshift.

GO 14762: A UV census of the sites of core-collapse supernovae

Chandra X-ray image of G292.0+1.9, a ~3000-year old supernoa remnant

Supernovae are generally believed to originate through two mechanisms: accretion onto a white dwarf in a close binary system, driving the white dwarf above the Chandrasekhar limit; and the implosion of the core of very massive (> 7 solar masses) stars. Both processes result in explosive nucleosythesis that enriches the interstellar medium, with the ejecta forming a rapidly expanding shell. Supernovae are intrinsically rare: Tycho's star (1604) was the last Galactic supernova identified by contemporary astronomers, although the radio remnant Cas A (identiied as 3C 461 in the 1959 Third Cambridge Catalogue of Radio Stars) may have been recorded, if not recognised as unusual, by Flamsteed in 1680. Understanding their progenitors therefore demands that we expand observations to external galaxies. The present program builds on several from previous cycles, and uses HST to obtain high-resolution imaging of the sites of several Type II supernovae in nearby galaxies. By now, the supernova themselves have faded from view, but the HST data can reveal the stellar population in the immediate environs. detailed analysis of the colour magnitude diagrams can probe the local environment,the likely age of the star forming regions, and hence set limits of the progenitor mass.

GO 14873: Exploratory observations of the TRAPPIST-1 system: essential prelude to an immediate JWST follow-up

An artist's impression of the view from the third planet in the Trappist 1 system

The first exoplanet, 51 Peg b, was discovered through radial velocity measurements in 1995. 51 Pegb was followed by a trickle, and then a flood of other discoveries, as astronomers realised that there were other solar systems radically different from our own, where "hot jupiters" led to short-period, high-amplitude velocity variations. Then, in 1999, came the inevitable discovery that one of those hot jupiters. HD 209458b, was in an orbit aligned with our line of sight to the star, resulting in transits. Transiting systems provide particularly powerful insight into exoplanet characteristics since they provide a direct measurement of size (at least relative to the star) as well as an opportunity to probe the atmospheric composition and temperature structure via the transmission and emissive spectra. Over the following decade the number of known systems grew slowly through extensive ground-based campaigns, but the launch of Kepler in March 2009 marked a turning point. In its four year survey, Kepler has identified more than 1030 confirmed transiting exoplanets, with a further ~4700 candidates and the potential for more discoveries through increasingly detailed analysis of the archival data. Ground-based surveys can still play a major role, however, as illustrated by the recent discovery by the TRAPPIST (Transiting Planets and Planetesimals Small Telescope) team of at least three terrestrial planets orbiting the nearby ultracool dwarf, 2MASS 2306-0502, also known as TRAPPIST 1. This team is one of several that are using relatively small telescopes to monitor nearby M dwarfs for planetary transits. Lower mass and cooler than solar-type stars, M dwarfs constitute approximately 70% of the stars in the Galaxy, and are therefore likely to host most of the planets near the Sun. Since they have substantially smaller diameters than solar-type stars, terrestial planets have a larger covering factor and are therefore easier to detect. Moreover, the cooler stellar temperatures mean that the Habitable Zone lies closer to the parent star, and the orbits are correspondingly shorter. In the case of 2MASS 2306-0502, an M8 dwarf, the three known planets have periods of 1.5, 2.4 and more than 4.5 days, with surface temperatures that have been compared with Venus, Earth and Mars. Spitzer recently undertook a 1500-hour long monitoring campaign, and the present program will use the G141 grism on WFC3 to obtain further follow-up near-infrared spectra.