This week on HST

HST Programs: November 30 - December 6, 2015

Selected highlights

GO 13665: Exploring the Diversity of Exoplanet Atmospheres in the Super-Earth Regime

Artist's impression of the GJ 1214 system

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. Since that date, the number of known transiting exoplanet systems has grown to more than 100 from ground-based observations, most detected through wide-field photometric surveys, while the high-sensitivity data provided by Kepler has added a further 1000+ confirmed systems and ~2000 additional candidates. With the added numbers, observations have pushed detections to lower and lower masses, and it is now clear that the most common type of planet is the "super-Earth" - planets with masses that are several (3-6) times that of Earth and radii 2-4 times larger than Earth. One of the earliest examples is the planet circling the M dwarf, GJ 1214. Such planets have no obvious analogue in the Solar System, and the measured masses and diameters might reflect a range of interior structurees: large rocky bodies with relatively thin atmospheres; dense cores surrounded by a steam atmosphere; or "mini-Neptunes", with rock or ice cores surrounded by extended hydrogen or helium atmospheres. The present program aims to probe the diversity of these systems by using the G141 grism on Wide-Field Camera 3 to obtain time-resolved scanning observations of five transiting systems. The goal is to obtain data that will clearky distinguish between large scaleheight, hydrogen-dominated atmospheres and a more compact, steam-dominated systems.

GO 13794: Seasonal Dependence of the Escape of Water from the Martian Atmosphere

HST imaging of Mars - high clouds are visible in the upper atmosphere

Mars has been in the news recently for several reasons. The rover Curiousity continus to explore the surface; it is seeing the close passage of the Oort cloud comet, Comet 2013A Siding Spring, discovered by Robert McNaught last year; and this week sees the arrival of MAVEN, the Mars Atmosphere and Volatile EvolutioN planetary probe launched in 2013 and scheduled for orbit insertion on September 21. MAVEN's mission is to study the tenuous Martian atmosphere, using the onboard instruments to probe the upper atmosphere and determine its rate of escape. Hubble offers some limited opportunities to tackle the same questions, and the current progam aims to complement MAVEN's in situ measurements through ultraviolet imaging and spectroscopy with the Advanced Camera for Surveys' Solar Blind channel (ACS/SBC) and the Space Telescope Imagign Spectrograph. Those observations can detect emission from hydrogen and oxygen, and previous observations have revealed significant changes with the martian seasons. The present observations are part of a series that aim to characterise the atmospheric structure as Mars passes through its seasonal changes. In addition, these observations, taken while most of the Martian satellites have been powered down for the Comet Siding Spring encounter, offer the prospect of testing whether the comet's tail might have some detectable impact on the extended Martian atmosphere.

GO 13868: Are Compton-Thick AGN the Missing Link Between Mergers and Black Hole Growth?

Composite optical/radio image of CenA, the elltipical merger that harbours the nearest AGN

AGN (Active Galactic Nuclei) systems are galaxies that exhibit strong non-thermal emission within their core regions. The energy is generally believed to stem from gas accretion onto a central supermassive black hole, with M > 107 MSun - similar in form, but less extreme in luminosity, to the energy source in QSOs. One of the key issues in such systems is understanding where the fuel comes from, since AGN can be found within galaxies that are morphologically similar to elliptical galaxies, which are predominantly gas poor at the present epoch. One possibility is through mergers, with the host galaxy assimilating smaller, gas-rich neighbours. Many nearby ellipticals are known to exhibit characteristics signatures of mergers - tidal tails, dust lanes and shells. Cen A is the classic example, which also happens to support a weak AGN and mild star formation within the dust lane. However, most previous surveys of AGN at moderate redshift (z < 2) have failed to find strong evidence for ongoing mergers. One possibility is that the very act of merging can hide AGN activity, as gas from the accreted satellite galaxy obscures the central black hole. The present program aims to test that hypothesis by using WFC3 to obtain near-infrared (F160W - H-band) imaging of 25 "Compton-thick" AGN systems at z~2 - systems that show evidence for significant line-of-sight obscuration by dust and gas. By working at near-infrared wavelengths, WFC3 will obtain detailed images of the galaxy morphology at rest-frame optical wavelengths, and will probe for the presence of merger signatures.

GO 14148: Near-IR Imaging of Three Spectacular Lensed Submillimeter Galaxies Discovered by the Herschel Lensing Survey

An ALMA/HST composite image of the lensed galaxy, SDP 81

Gravitational lensing is a consequence the theory of general relativity. Its importance as an astrophysical tool first became apparent with the realisation (in 1979) that the quasar pair Q0957+561 actually comprised two lensed images of the same background quasar. In the succeeding years, lensing has been used primarily to probe the mass distribution of galaxy clusters, using theoretical models to analyse the arcs and arclets that are produced by strong lensing of background galaxies, and the large-scale mass distribution, through analysis of weak lensing effects on galaxy morphologies. Gravitational lensing can also be used to investigate the mass distribution of individual galaxies. Until recently, the most common background sources that were being detected and investigates were quasars. Galaxy-galaxy lenses, however, offer a distinct advantage, since the background source is extended, and therefore imposes a stronger constraints on the mass distribution of the lensing galaxy than a point-source QSO. HST has carried out a number of programs following up candidate lenses identified from the Sloan Digital Sky Survey (eg GO 10886 , GO 11289 , GO 12210 ). The present program is using WFC3 on HST to obtain follow-up near-infrared (F110W/F160W) images of three exceptionally bright gravitationally lensed sub-millimeter galaxies. The systems lie at redshifts of 2.04, 4.69 and 5.04, and have been extensively studied through continuum and line emission mapping at sub-millimeter wavelengths. Observations will soon be obtained by ALMA. The HST observations will probe the underlying stellar populations as well as providing the angular resolution necessary to model the mass distribution.