This week on HST

HST Programs: July 30 - August 5, 2012

Selected highlights

GO 12448: Towards a Detailed Understanding of T Pyx, Its Outbursts and Shell

Artist's impression of the recurrent nova, RS Oph (by David Hardy)

Recurrent novae are generally agreed to be close binary systems, comprising a white dwarf and a companion main sequence star that is overflowing its Roche lobe, leading to period transfers of mass onto the white dwarf surface. The mass transfer episode triggers nuclear ractions, which lead the star increasing significantly in it luminosity. T Pyxidis is one such system, and it exhibited fairly regular outbursts every 20 years between its discovery, in 1890, and 1966. Since then, however, it has been dormant, a prolonged period of quiescence that led to suggestions, earlier this year, that it might either be headed for hibernation, or in the process of accumulating sufficient mass to trigger a type Ia supernova explosion (in about 1 million years). Perhaps prompted by these suggestions (a la Monty Python Mary Queen of Scots radio sketch), T Pyxidis erupted into activity on or around April 15th. The present HST observations are part of a time series designed to obtain multi-wavelength narrowband images of the illuminated ejecta.

Lab-1, the largest Lyman-alpha blob currently known, at z=3.1

Lyman-alpha blobs are large concentrations of gas that have been detected through their strong emission of Lyman alpha radiation. Most of those found have been foudnt rhough imaging at optical wavelengths, and therefore tend to lie at reshifts exceeding z=2. Some of these form coherent structures, including 3-D ~70-Mpc scale filamentary structures. The present program aims to capitalise on recent discoveries made by the Wide-field infrare Survey Explorer. WISE has identified a number of Ly-alpha emitting blobs that have strikingly different energy distributions at near- and mid-infrared wavelengths, strongly suggestive of the presence of substantial quantities of dust. These objects have radically different energy distributions that the optically-identified LABs at z>2. The present program will use ACS and the WFC3-IR camera to obtain high-resolution images of these unusual systems at red and near-infrared wavelengths, mapping both the overall energy distribution and the detailed morphology in lyman-alpha and the rest-frame UV continuum.

GO 12498: Did galaxies reionize the universe?

The ACS optical/far-red image of the Hubble Ultra Deep Field

Galaxy evolution in the early Universe is a discipline of astronomy that has been transformed by observations with the Hubble Space Telescope. The original Hubble Deep Field, the product of 10 days observation in December 1995 of a single pointing of Wide Field Planetary Camera 2, demonstrated conclusively that galaxy formation was a far from passive process. The images revealed numerous blue disturbed and irregular systems, characteristic of star formation in galaxy collisions and mergers. Building on this initial progam, the Hubble Deep Field South (HDFS) provided matching data for a second southern field, allowing a first assessment of likely effects due to field to field cosmic variance, and the Hubble Ultra-Deep Field (UDF) probed to even fainter magitude with the Advanced Camera for Surveys (ACS). Pushing to larger distances, and greater ages, demands observatons at near-infrared wavelengths, as the characteristics signatures of star formation are driven further redward in the spectrum. The installation of Wide-Field Camera 3 during Servicing Mission 4 opened up this regime, through observations using the WFC3-IR camera in the F105W (J) and F160W (H) filters. Those observations provided the first candidate galaxy at redshift z=10. However, that individual detection - and even the 10s of gaalxies detected at z=7 to 8 - represent only the most luminous galaxies at those redshifts, and provide only limited constraints on the total star formation at those epochs. The present program will add a further 128 orbits centred on the UDF, primarily in the F105W and F140W filters. These observations will push the detection limits to fainter apparent magnitudes, and to lower luminosity galaxies, setting stronger constraints on the total star formation in the early universe.

GO 12504: Bridging the Brown Dwarf/Jupiter Temperature Gap with a Very Cold Brown Dwarf

The very low-mass binary system, CFBDSIR J1458+10AB

Brown dwarfs are objects that form in the same manner as stars, by gravitational collapse within molecular clouds, but which failt to accrete sufficient mass to raise the central temperature above ~2 million Kelvin and ignite hydrogen fusion.In consequence, these objects, which at solar abundances have masses less than 0.075 MSun or ~75 M<\sub>Jup, lack a sustained source of energy, and cool and fade on relatively short astronomical (albeit, long anthropological) timescales. Following their discovery over a decade ago, considerable observational and theoretical attention has focused on the evolution of their intrinsic properties, particularly the details of the atmospheric changes. At their formation, most brown dwarfs have temperatures of ~3,000 to 3,500K, comparable with early-type M dwarfs, but they rapidly cool, with the rate of cooling increasing with decreasing mass. As temperatures drop below ~2,000K, dust condenses within the atmosphere, molecular bands of titanium oxide and vanadium oxide disappear from the spectrum to be replaced by metal hydrides, and the obejcts are characterised as spectral type L. Below 1,300K, strong methane bands appear in the near-infrared, characteristics of spectral type T. At lower temperatures, other species, notably ammonia, are expected to become prominent, and attempts are currently under way to find examples of these "Y" dwarfs. The search is complicated by the fact that such objects are extremely faint instrinsically, so only the nearest will be detectable. Wide-field surveys have been undertaken at infrared wavelengths with both ground-based telescopes (eg UKIDDS) and satellite observatories (eg WISE). However, an alternative approach is to "look uner the lamp-post": both stars and brown dwarfs are often found as binary or multiple systems, so one can take a sample of low-mass obejcts known to be within the Solar Neighbourhood, and look for even lower luminosity companions. That technique served in the past to identify van Biesbroeck 10, the first ultracool dwarf; GD 165B, the first L dwarf; and Gl 229B, the first T dwarf. The prime target of this proposal, CFBDSIR J1458+10B, was found in a similar manner, through deep infrared imaging targetting known nearby ultracool dwarfs. The primary, a ~T9 dwarf, was discovered in the course of the Canada-France Brown Dwarf Survey survey (hence CFBDS), lies at a distance of ~23 parsecs, has a luminosity of ~10-6 LSun and a temperature around 550K. The secondary was uncovered at a separation of 0.11 arcseconds through Keck laser AO imaging, is 2 magnitudes fainter than the primary at H, has a luminosity of ~2 x 10-7 LSun and is the coolest brown dwarf currently known, with a surface temperature estimated as only 370 K, comparable with boiling water. HST will be used to obtain far-red and near-infrared photometry, with the aim of better characterising the spectral energy distribution at those wavelengths.