This week on HST

HST Programs: February 23 - March 1, 2015

Selected highlights

GO 13679: Europa's Water Vapor Plumes: Systematically Constraining their Abundance and Variability

The HST imaging of a potential water plume around Europa's south pole superimposed on an image of the satellite

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, strongly suggesting that Europa's oceans may be vaporising into space.Follow-up observations on two further occasions earlier in 2014 failed to detect any emission, suggesting that the emission is either sporadic or periodic; in the latter case, the emission might be related to the location of Europa within its orbit and the consequent tidal strain imposed by Jupiter.The present program is conducting a methodical series of observations designed to image Europa at a range of orbital locations. STIS is being used to search for H and O auroral emissions at UV wavelengths and will aim to map the distribution of emission at different phases of the Europan orbit.

GO 13699: Fellowship of the Andromeda Dwarf Galaxies: A Census of their Extended Star Formation Histories

M31, the Andromeda spiral galaxy, and its companiosn - from the PANDAS survey

M31, the Andromeda galaxy, is the nearest large spiral system to the Milky Way (d ~ 700 kpc), and, with the Milky Way, dominates the Local Group. The two galaxies are relatively similar, with M31 likely the larger system; thus, Andromeda provides the best opportunity for a comparative assessment of the structural properties of the Milky Way. Like the Milky Way, Andromeda has a number of satellite galaxy companions. At present, some 31 systems have been identified, the majority from the Pan Andromeda Archaeological Survey (PANDAS), a wide-field survey utilising ground-based imaging data. The four most prominent satellites are the dwarf ellipticals NGC 147, NGC 185, NGC 205 and M32; most of the remaining systems are dwarf spheroidals, with absolute visual magnitudes between MV<\sub> =-10 and -6 and masses less than 108 MSun. The present program aims to probe the star formation history of these systems. The Advanced Camera for Surveys (ACS) will be used to obtain deep V (F606W) and I(F814W) imaging of the 19 dwarf spheroidals that currently lack any HST data; the structure of the red giant branch and horizontal branch in the resultant colour-magnitude diagrams will map the metallicity and age of the component stellar populations.

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

Standard Big Bang cosmology includes an epoch of reionisation, when hydrogen gas in the intergalactic medium is\ionised, reducing the line-of-sight opacity and ending thed ark ages. Among the candidates for carrying out that ionisation are X-ray sources, particularly high-mass X-ray binaries in compact, low-metallicity dwarf galaxies.Studying those galaxies directly is not possible, so the present program aims to investigate the potential of this diea through observations of local analogues. 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 originating from SDSS was the identiication of "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. A subset of those systems ar being targeted in the present program for imaging at red and ultraviolet wavelengths with WFC3 (F606W and F336W filters). The goal is to determine the prevalence of super star clusters, and hence the likelihood for high-mass X-ray binaries in those systems.

GO 13809: The IMF and Internal Kinematics of the Massive Young Star Cluster, Westerlund 1

The massive, reddedened open star cluster, Westerlnd 1

Westerlund 1 is a massive, young star cluster that lies somewhere between 3.5 and 5 kpc from the Sun in the constellation Ara in the general direction of the Galactic Centre. The cluster was discovered just over 50 years ago by Bengt Westerlund, but has only been subject to detailed scrutiny in more recent years since interstellar material along the line of sight leads to foreground absorption of around 14 magnitudes at optical wavelengths. Detailed investigations became possible as near-infrare astronomy came of age in the 1990s. These show that Westerlund 1 has an age of 4-5 Myrs, comparable with the Orion Nebula Cluster but probably singificantly more massive. The cluster includes several O stars within an extremely compact region. As a result, it presents an opportunity to examine how the star formation mioght be influenced by such an extreme environment. In particular, is there any evidence for changes in the IMF? The present proposal is using the WFC3-IR camera to obtain deep, multi-band images at two epochs. The initial observations were obtained in March 2013, during Cycle 20. With a 2 year baseline, the second epoch data will not only permit the identification of cluster members with masses as low as ~0.1 MSun, but will also enable astrometry of sufficient accuracy to constrain the internal cluster kinematics.