This week on HST

HST Programs: January 9 - January 15, 2012

Selected highlights

GO 12210: SLACS for the Masses: Extending Strong Lensing to Lower Masses and Smaller Radii

ACS images of galaxy-galaxy Einstein ring lenses from the Sloan survey

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 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. The Sloan Digital Sky Survey is a powerful tool for identifying candidate galaxy-galaxy lenses, and has provided targets for HST imaging programs in several previous cycles. The presentprogram is using HST-ACS imaging to survey a further 135 strong lens candidates. The HST data will verify the nature of those candidates, and provide the angular resolution necessary to model the mass distribution.

GO 12461: Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey - SNe follow-up

High redshift supernovae from HST observations in previous cycles

CANDELS is one of three Multi-Cycle Treasury Program, whose observations will be executed over the next three HST Cycles. It builds on past investment of both space- and ground-based observational resources. In particular, it includes coverage of the two fields of the Great Observatory Origins Deep Survey (GOODS), centred on the northern Hubble Deep Field (HDF) in Ursa Major and the Chandra Deep Field-South in Fornax. In addition to deep HST data at optical and near-infrared wavelengths, the fields have been covered at X-ray wavelengths by Chandra (obviously) and XMM-Newton; at mid-infrared wavelengths with Spitzer; and ground-based imaging and spectroscopy using numerous telescopes, including the Kecks, Surbaru and the ESO VLT. This represents an accumulation of almost 1,000 orbits of HST time, and comparable scale allocations on Chandra, Spitzer and ground-based facilities. The CANDELS program is capitalising on this large investment, with new observations with WFC3 and ACS on both GOODS fields, and on three other fields within the COSMOS, EGS and UDS survey areas (see this link for more details). The prime aims of the program are twofold: reconstructing the history of galaxy formation, star formation and nuclear galactic activity at redshifts between z=8 and z=1.5; and searching for high-redshift supernovae to measure their properties at redshifts between z~1 and z~2. The program incorporates a tiered set of observations that complement, in areal coverage and depth, the deep UDF observations, while the timing of individual observations will be set to permit detection of high redshift SNe candidates, for subsequent separate follow-up. The present observations target a high-redshift supernova, dubbed Trajan, identified in the course of CLASH observations of the cluster MS2137.3-2353.

GO 12507: The Formation and Fundamental Properties of Wide Planetary-Mass Companions

Artist's impression of activity within a disk in a member of the TW Hya association

Over the past decade, significant effort has been devoted to searching for exoplanet companions through a wide variety of techniques, including radial velocity measurements, photometry (transits), and direct imaging. The latter method is clearly only likely to be sensitive to detecting giant planets and, in order to optimise the detection chances, surveys have tended to focus on young stars, where planetary companions are self-luminous and correspondingly easier to detect. Imaging cannot reach systems at small separations, due to the high background light contributed by the host star, but coronagraphic instruments are starting to produce a number of interesting and intriguing detections. However, considerable ambiguity remains over whether some of these sources are planets or brown dwarfs. While the companions to the A-type star HR 8799 - GO 12281 & GO 12511 are clearly consistent with formation within a circumstellar disk, and therefore almost certainly planetary in nature, the situation is far less clearcut in other cases, For example, the 2MASS1207-3932 is a late-type (spectral type M8) member of the ~10 Myr-old TW Hyadrae Association. It has an L-type companions, 2M1207-3932b, with a likely mass between 3 and 10 times that of Jupiter, and lying at a projected separation of ~40 AU. Similarly, CQ Lupi is young (~10 Myr-old) K7 dwarf lying at a distance of ~150 pc in the Lupus star forming association; it has a 10-30 Jupiter-mass companion at a projected separation of ~100 AU. The present program aims to shed light on this issue by using high-resolution imaging to search for signatures of ongoing accretion (UV excess, H-alpha emission) and potential evidence of binarity. The mlticolour observations will also serve to constrain the temperatures and radii of these very low-mass objects.

GO 12537: Venus observed as an extrasolar planet

An image of the 2004 Venus transit - with a terrestrial interloper (taken by Jerry Zhu

Extrasolar planets that transit their parent star from the vantage point of the Earth are much sought after in exoplanet research. Not only do those systems eliminate the sin(i) uncertainty in estimating planetary masses,they also provide an opportunity to probe atmospheric properties through comparative spectroscopy of the host star during and outside primary transit - differencing the spectra can reveal absorption features introduced as the starlight passes through the exoplanet atmosphere. Earth's inhabitants are also occasionally in a position to view Solar System transits, as bodies interior to the earth's orbit, notably Mercury and Venus, pass across the face of the Sun. The planetary transits occur when inferior conjunction (the planet lies directly between Earth and the Sun) occurs close to the orbital node (where the planetary orbit crosses the ecliptic. In the case of Mercury, this happens relatively frequently, with transits in May or November and occuring at intervals of 7, 13 or 33 years - the most recent events were November 15 1999, May 7 2003 and November 8 2006. Venusian transits occur in June or December, but at a much lower frequency. Transits occur in pairs, 8 years apart, separated by gaps of 121.5 and 105.5 years, with the cycle recurring every 243 years. Thus, the June transits of 1761 and 1769 were followed by December transits in 1874 and 1882, and repeated as June transits in 2004 and 2012. The 18th and 19th century transits were the targets of numerous scientific expeditions, including James Cook's expedition to Tahiti in 1769, since accurate timing could be applied to measuring the astronomical unit, and scaling the solar system. Nowadays, there are much more effective ways of measuring that parameter (that don't have to deal with the uncertainties introduced by the 'black drop"). However, the upcoming transit can be turned to advantage in probing our sensitivity to detecting the atmospheres of terrestrial exoplanets. Direct observations of the Sun are out of the question with HST - indeed, Hubble can only observe Venus itself when the planet is at its furthest elongation from the Sun (see HST's observations in support of Venus express - GO 12433 ). However, HST can observe reflected sunlight by observing the Moon. The present program aims to use the Space Telescope Imaging Spectrograph to observe two regions of the lunar surface, near the craters Hipparchus and Dollond, during the Venusian transit. Those observations will be differenced against reference lunar spectra, with an initial set being taken this week.