This week on HST

HST Programs: April 21 - April 27, 2014

Selected highlights

GO 13301: Deep COS Spectra of the Two Brightest Quasars that Probe the He II Post-Reionization Era

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

In astronomy, 'reionisation" usually refers to the period in the early universe (6 < z < 10) when star formation and/or accreting supermassive black holes produced sufficient ultraviolet flux to ionise hydrogen and lift the veil of the cosmic dark ages. However, intergalactic helium remained neutral at that time. The reionisation epoch for intergalactic helium is thought to occur somewhere between redshifts 3 and 4. Tracking the onset of that epoch through analysis of the He II Lyman alpha absorption constrains the evolution of star formation in the univers at those epochs. Observations, however, are complicated by the continued presence of neutral hydrogen, which absorbs radiaton at those wavelengths. The present program is using the Cosmic Origins Spectrograph to obtain deep observations of two of the brightest known He II quasars, HE2347-4342 and HS1700+6416. Both have previous UV spectroscopic observations by HST, as witnessed by the GHRS spectrum, but the new COS observations will be orders of magnitude more sensitive and can resolve individual He II absorption lines for comparison with neutral hydrogen and metal lines (C IV, Si IV). Inded, these observations will set a new benchmark for QSO absorption line studies with COS.

GO 13308: Near-IR spectroscopy of the highly inflated, hottest known Jupiter KOI-13.01

The parent binary system, KOI-13

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.Over the following decade, with the development of additional ground-based survey systems, the number of known systems grew slowly, but the launch of Kepler in March 2009 marked a turning point. In its four yhear survey, kepler has identified more than 960 confirmed transiting exoplanets, with a further ~2500 (highly likely) candidates amd the potential for more discoveries through increasingly detailed analysis of the archival data. Kepler 13 is one of the most interesting such systems. The host star lies at a distance of ~1 kpc and is a member of a triple system that comprises two fast-rotating A stars and a third sub-solar mass (0.4 to 1 MSun) star. The two A stars are separated by ~0.8 arcseconds, and have masses estimated as ~2.05 and ~1.95 times that of the Sun. The observations indicate that the planetary mass companion and the lower-mass star do not orbit the same A star, but the exact attribution remains ambiguous so the planetary mass is estimated as eith 9 or 14 MJ. The aim of the present progam is to use the WFC3 G141 grism to search for characteristic near-infrared spectral features in those systems - and since the stars are well resolved by HST, these observations will eliminate the ambiguity in whichb star owns the planet.

GO 13386: Frontier Field Supernova Search
GO 13389: The Ultraviolet Frontier: Completing the Census of Star Formation at Its Peak Epoch

Pandora's Cluster, Abell 2744: the Chandra X-ray image, tracking hot gas, is plotted in red; the inferred dark matter distribution in blue

The overwhelming majority of galaxies in the universe are found in clusters. As such, these systems offer an important means of tracing the development of large-scale structure through the history of the universe. Moreover, as intense concentrations of mass, galaxy clusters provide highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. Hubble is currently undertaking deep imaging observations of up to 6 galaxy clusters as part of the Frontier Fields Director's Time program (GO 13495/13496). Those observations have provided a basis for several synergistic programs, two of which are highlighted here. Program GO 13386 uses the Frontier Field observations to search for supernovae at high redshifts, z> 1.5, aiming to set further constraints on dark energy and probing the frequency of supernovae as a function of redshift, the delay time and hence the likely progenitors. Program 13389 supplements the visual and near-infrared data in the core Frontier Fields program with deep imaging at near-UV wavelengths using the F275W and F336W filters on WFC3's UVIS camera. At the same time, the ACS-WFC camera is being used to obtain blue (F435W) and red (F606W) data on the associated parallel field from the Frontier Fields program. The UV data will enable investigation of the star formation rates and morphologies of moderate redshift galaxies, 0.5 < z < 3, lying behind the galaxy cluster. Combined with the Frontier Fields photometry, these data will enable more accurate photometric redshift determinations, probe Lyman escape fractions and offer the prospect of mapping the spatial distribution of star formation in lensed systems.

GO 13457: Accurate Mass Determination of the Nearby Old White Dwarf Stein 2051B 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. This program aims to capitalise on this fact by measuring the positional deflection of a background stars introduced by the close passage of the high proper-motion white dwarf, Stein 2051B (also known as Gliese 169.1B, G175-34B or LHS 27; the companion star is an M4 red dwarf at ~40 AU separation). Lying at distance of only 5.5 parsecs, Stein 2051B has a surface temperature of ~7200K and is therefore a DC, too cool to show absorption features due to hydrogen and therefore not accessible to mas measurement techniques such as gravitatiolnal redshifts. The expected signal during the stellar encounter (i.e. the deflection of the background star) is approximately 3 millarcseconds, and therefore well within HST's astrometric capabilities.