This week on HST

HST Programs: December 23 - December 29, 2013

Selected highlights

GO 12880: The Hubble Constant: Completing HST's Legacy with WFC3

NGC 7541 (upper left), one of the spiral galaxies targeted in this program

The Hubble constant remains a key parameter in understanding cosmology and the evolution of the Universe. Refining measurements of H0 therefore still represents a vital means of probing the nature of dark energy. The present program aims to tackle this question by laying a firmer foundation to the SNe Ia distance scale. The WFC3 IR camera will be used to identify and characterise Cepheid variables in eight relatively nearby galaxies that have hosted Type Ia SNe. Cepheids have signficantly lower amplitude at near-infrared wavelengths, and the measured magnitudes are less subject to uncertainties due to foreground reddening and variations in metallicity. As a consequence, determining the mean apparent magnitude, and hence the period/apparent magnitude relation, is substantially more straightforward than at optical wavelengths. WFC3 has revolutionised this field by providing substantial greater areal coverage and higher precision photometry than NICMOS. Past observational program have targeted the Cepheids in the maser galaxy, NGC 4258, as well as Galactic Cepheids. The present program targets eight galaxies that have hosted Type Ia supernovae, offering the prospect of tying the SNe Ia scale directly to the Galaxy and to NGC 4258, avoiding the many intermediate steps of previous analyses. The aim is to reduce the level of systematics in determinations of H0 to the 1-2 percent level, setting signficantly stronger constraints on dark energy.

GO 13293: Green Pea Galaxies: Extreme, Optically-Thin Starbursts?

A montage of green pea galaxies discovered by the Galaxy Zoo project

Understanding the galaxy formation and galaxy evolution has been a strong focus of astronomical research since thne 1970s and remains a key issue for 21st century astrophysics. Since we cannot follow an individual galaxy through time, tackling these questions requires a statistical approach, and, as a result, large-scale surveys have played a crucial role in the field. In particular, the Sloan Digital Sky Survey, conducted from Apache point Observatory from the 1990s through the early years of this century, has provided a treasure trove of information of galaxies at low and moderate redshifts. One of the interesting discoveries that originated from SDSS was the discovery of so-called "green pea" galaxies - compact objects whose combined images have a greenish hue, originally uncovered by citizen scientists as part of the Galaxy Zoo project. Closer inspection shows that these are gas-rich galaxies lying at relatiVEly low redshifts, 0.1 < z < 0.36, with the green-tinged hue partly stemming from the presence of strong oxygen ([O III]) emission. These characteristics indicate that the galaxies are undergoing strong star-forming episodes. Teh present program aims to take advantage of HST's unparalleled resoltuion and extraordinary sensitivity at ultraviolet wavelengths, and will use the Cosmic Origins Spectrograph to obtain spectra are far-UV wavelengths and probe the physical structure of the underlying star forming regions.

GO 13438: Probing the atmosphere of a transiting ocean world

An image of europa taken by Voyage 2 in 1979

Europa is the smallest, and the most intriguing, of the four Galilean satellites of Jupiter. With a diameter of 3139 km, Europa is almost twice the size of Earth's moon and significantly larger than Mercury. In 1957, Gerard Kuiper commented that both infrared spectroscopy and the optical colours and albedo suggested that Jovian satellite II (Europa) is covered "by H2O snow". Images taken by the Voyager space probes in the late 1970s (see left) reveal a smooth surface, with only a handful of craters larger than a few kilometres. These features are consistent with a relatively young, icy surface. Subsequent detailed investigations by the Galileo satellite strongly suggest that a substantial body of liquid water, heated by tidal friction, underlies a 5 to 50 km thick icy crust. The presence of this subterranean (subglacial?) ocean clearly makes Europa one of the two most interesting astrobiology targets in the Solar System. Most recently, analysis of observations taken by the Space Telescope imaging Spectrograph (STIS) on Hubble indicated the presence of an extended cloud of Lyman-alpha emission near the polar regions while Europa was furthest in its orbit from Jupiter, stongly suggesting that Europa's oceans may be vaporising into space. The present HST program also aims to search for outgassing, but in this by looking for absorption features against the smooth background light of Juptier while Europa is in transit. The prorgam will use STIS in time-tag mode to search from transient features in th far-UV, and use coronagraphy at near-UV wavelengths to look for dust signatures.

GO 13449: A Non-Universal Initial Mass Function in the Ultra-Faint Galaxy Coma Berenice

SDSS imaging opf the Bootes I ultra-faint dwarf

The Milky Way possesses at least 15 satellite galaxies. The most prominent are the Large and Small Magellanic Clouds, irregular galaxies which have been known since at least the tenth century. The remaining systems are gas-poor dwarf spheroidal systems, with luminosities less than 108 LSun and masses less than 108 MSun. All of these dwarfs were discovered from wide field imaging surveys. The two brightest systems, Scuptor and Fornax, were found on plates taken in the 1930s by Harvard's Boyden Observatory, in Bloemfontein, South Africa. The brighter systems in the northern hemisphere, including Ursa Minor and Draco, were identified from photographic plates taken as part of the first Palomar Sky Survey in the 1950s, and the brighter southern systems, including Carina and Sextans (L ~ 105 LSun, were uncovered in 80s and 90s on plates taken with UK Schmidt telescope, Siding Spring, Australia. These systems have such low densities that they are barely discernible to the eye; in fact, Sextans was discovered based from analysis of automated plate scans. The advent of CCD surveys and improved analysis techniques have allowed astronomers to push to even fainter, more diffuse systems. In the past fwe years, SDSS has yielded a new class of systems, dubbed ultra-faint dwarfs, with lminosities less than 104 LSun and mass-to-light ratios exceeding 100. The faintest of these systems, Bootes I, has a total luminosity of only ~300 LSun, or only ten times more luminous than Vega. The theoretical expectatio is that these systems represent dark-matter dominated satellites predicted by Lambda-CDM models.Hubble has obtained past observations of a number of these systems, notably ACS data for Bootes I, Canes Venatici, Hercules, Leo IV, Ursa Major I and Coma Berenices. Analysis of the resulting colour-magnitude diagram strongly suggests that these systems have shallower initial mass functions (fewer low amss stars) than the standard distribution observed within the Milky Way. The present series of observations aims to confirm that through deeper imaging of Coma berenices system