This week on HST

HST Programs: February 29 - March 6, 2016

Selected highlights

GO 13820: Search for an evaporating ocean on the super-Earth HD 97658b

Comparison of Earth and HD97658b in transit

HD 97658 is an early-type K dwarf (spectral type K1) with a mass approximately 80% that of the Sun lying at a distance of ~21 parsecs. In 2011, the NASA-UC ETA-Earth search team announced the discovery of a ~8 Earth-mass planet, a "super-Earth", in a 9.49 day orbit around this star. The orbit is very close to circular with a semi-major axis of 0.083 AU. Subsequently, and after a little controversy, the system was found to be transiting, with observations by the Canadian MOST satellite indicating a radius only 2.34 times that of the Earth.Combined with the mass measurementy, this indicates an average density of ~3.44 g/cc; that, in trun, implies that there is a strong probability that the planet has a relatively small rocky core surrounded by an extensive atmosphere of volatile elements (eg H & O). The present program aims to test that hypothesis: at its small separation, HD 19658b has a "surface" temperature of ~760K, sufficiently high that a volatile atmosphere will evaporate, leading to a substantial cloud of hydrogen surrounding the planet. The present program aims to use the Space Telescope Imaging Spectrograph to search for the Lyman alpha absorption that would be associated with any such cloud.

GO 14178: WISP - A Survey of Star Formation Across Cosmic Time

A region of massive star formation

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. The present program builds on the ability of HST to carry out parallel observations, using more than one instrument. While the Cosmic Origins Spectrograph is focused on obtaining ultraviolet spectra of unparalleled signal-to-noise, this program uses the near-infrared grisms mounted on the Wide-Field Camera 3 infrared channel to obtain low resolution spectra between 1 and 1.6 microns of randomly-selected nearby fields. The goal is to search for emission lines characteristic of star-forming regions. In particular, these observations are capable of detecting Lyman-alpha emission generated by star formation at redshifts z > 5.6. A total of up to 40 "deep" (4-5 orbit) and 20 "shallow" (2-3 orbit) fields will be targeted in the course of this observing campaign.

GO 14182: The Coma Cluster Core Project

Hubble image covering part of the central regions of the Coma cluster

The Coma cluster is the nearest rich galaxy cluster, lying at a distance of ~100 Mpc from the Milky Way. The cluster includes well over 1000 major galaxies, centred on two giant ellipticals, NGC 4874 and NGC 4889. Chandra observations show that the galaxies are embedded in very hot intracluster gas (see this site ). Cluster galaxies have also been surveyed at mid-infrared wavelengths by Spitzer, and in the ultraviolet by GALEX. Individual galaxies have been studied in the past using HST. In Cycle 15 the Advanced Camera for Surveys started a systematic imaging with ACS (with parallel observations with NICMOS) of the cluster core ( a 7x6 mosaic, covering approximately 400 sq. arcmin.), together with 40 fields at larger radii.That program was curtailed by the failure of ACS in January 27th. The present program builds on those observations, using Wide-Field Camera 3 to obtain complementary UV and IR data for the existing ACS images. ACS itself is being used in parallel to cover other regions with the cluster. Those observations will be capable of detecting dwarf galaxies with absolute magnitudes as faint as MB ~ -8, fainter than most of the Milky Way's dwarf spheroidal companions. Besides completing a detailed census of the low luminosity tail of the galaxian mass function, the observations will be used to probe colour gradients and internal chemical evolution.

GO 13033: Project AMIGA: Mapping the Circumgalactic Medium of Andromeda

The extent of Andromeda's gaseous halo, as sampled by COS

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution therefore depends on how gas is accreted, recycled, circulated through the halo and, perhaps, ejected back into the intergalactic medium. Tracing that evolutionary history is difficult, since gas passes through many different phases, some of which are easier to detect than others. During accretion and, probably, subsequent recycling, the gas is expected to be reside predominantly at high temperatures. The most effective means of detecting such gas is through ultraviolet spectroscopy, where gas within nearby systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. Extensive observations of galaxies at modest redshift (0.15 < z < 0.35) have shown that material extends to radii of hundreds of kpc, with a total mass in metals that is at least comparable with the mass in the central galaxy. Andromeda, the nearest large spiral to the milky Way,provides an unparalleled opportunity to probe