This week on HST

HST Programs: February 24 - March 2, 2014

Selected highlights

GO 13286: Understanding the Progenitor Systems, Explosion Mechanisms, and Cosmological Utility of Type Ia Supernova

Image of the recent supernova in M82, Jan 24th (Katzman Automated Imaging Telescope/LOSS)

Type Ia supernovae are generally believed to be produced by the explosive deflagration of white dwarf star that exceeds the Chandrasekhar due to accretion from a binary companion, either a hydrogen-burning main-sequence/red giant star or another degenerate. Besides providing crucial information on stellar evolution and how stars enrich the interstellar medium, Type Ia supernovae have acquired global importance in recent years through their use as distance indicators. Indeed, these objects played a crucial role in identifying dark energy and the accelerating universe. In that context, it is important to understand the distribution of intrinsic properties of these exploding stars, and whether those properties, particularly lumunisuty, correlate with other parameters, such as metallicity. Relatively nearby supernovae that can be probed in detail are therefore crucial to the large mapping of the cosmic flow. Astronomers were therefore delighted with the discovery of a type Ia supernova in the relatively nearby starburst galaxy, M82. This object, designated SN2014J, was discovered on January 21st by a group of UCL undergraduates and their lecturer in a series of short exposures taken as a quick test as clouds closed in on London's Mill Hill Observatory. The supernoa is expected to reach maximum around February 2nd, at which time the supernova is expected to reach ~10th magnitude. As the second closest Type Ia of recent years (SN 1993J in M81 was at a similar distance), this object has attracted significant attention, despite the substantial line of sight reddening. The current program will use STIS to obtain a time series of spectra in the ultraviolet, probing the intrinsic metallicity and the variation as ejecta permeate the surrounding environment.

GO 13349: Escaping Lyman Continuum in Strongly Lensed Galaxies at z=2.0-2.5

Lyman alpha image of the radio galaxy, 4C41.17

In Big Bang cosmology, the early history of the universe is characterised by three distinct phases: the initial expansion, during which time Big Bang nucleosynthesis occurs, and the universe cools from its initial exceedingly high temperatures; recombination, which occurs at a redshift z~1,100 (or an age of ~400,000 years), when the Universe was cool enough to allow neutral hydrogen to become dominant, leading to high opacity and the cosmic microwave background; and reionisation, when energy sources reionised hydrogen, reducing the opacity of the intergalactic medium and restoring transparency. Reionisation is generally believed to have occurred at a redshift between z~6 and z~20, with the ionising sources either (or both) the first generation of stars (Population III starbursts) and/or proto-quasars. The IGM remains ionised thereafter. A key issue in developing an understanding of this process is assessing how readily starburst-generated Lyman-alpha emission escapes from galaxies, and how starbursts contribute to reionisation at intermediate redshifts. HST has been used probe the Lyman escape fraction in star-forming galaxies spanning a wide range of redshifts, with the goal of using these lower-redshift systems as analogues to the behaviour at high redshift. The present program aims to push observations to redshifts in the range 2.0 < z < 2.5, focusing on half a dozen galaxies at that redshift whose appartent fluxes have been amplfiied by gravitational lensing. The WFC3 UVIS channel will be used to obtain near-UV imaging, probing the restframe far-UV emission in these systems.

GO 13463: Detecting Isolated Black Holes and Neurton Stars through Astrometric Microlensing

A rather spectacular version of black hole lensing.

Gravitational lensing is a consequence of general relativity. Its effects were originally quantified by Einstein himself in the mid-1920s. In the 1930s, Fritz Zwicky suggested that galaxies could serve as lenses, but lower mass objects can also also lens background sources. Bohdan Paczynski pointed out in the mid-1980s that this offered a means of detecting dark, compact objects that might contribute to the dark-matter halo. Paczcynski's suggestion prompted the inception of several large-scale lensing surveys, notably MACHO, OGLE, EROS and DUO. Those wide-field imaging surveys have target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. The duration of each event depends on several factors, including the tangential motion of the lens and its mass. Long-term events are generally associated with a massive lens. Duration alone is not sufficient to identify a lens as a black hole - a source with very low tangential motion relative to the Sun can produce the same effect. However, microlensing not only leads to flux amplification, but also to small astrometric motions, caused by the appearance and disappearance of features in the lensed light. Those motions serve as a mass discriminant - higher mass lenses produce larger amplitude motions. The expected astrometric signal from a black hole lens is > 1.4 millarcseconds, just measureable with HST. This program aims to capitalise on this fact by searching for lensing by black holes and neutron stars in the Galactic field. Over the past three cycles, the program has targeted a number of sources that exhibit long-duration lensing events in the Galactic Bulge. The present set of Cycle 21 observations will complete the search for astrometric signals due to massive foreground objects.

GO 13620: Probing the atmosphere of a transiting ocean world: are there ice fountains on Europa?

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. This DD program follows upon GO 134338, using STIS in time-tag mode to search from transient features in th far-UV, and applying coronagraphy at near-UV wavelengths to look for dust signatures.