This week on HST

HST Programs: February 27 - March 4, 2012

Selected highlights

GO 12330: Spitzer Verification of the Coldest WISE-selected Brown Dwarfs

The stellar menagerie: Sun to Jupiter, via brown dwarfs

Brown dwarfs are objects that form in the same manner as stars, by gravitational collapse within molecular clouds, but which do not accrete sufficient mass to raise the central temperature above ~2 million Kelvin and ignite hydrogen fusion. As a result, these objects, which have masses less than 0.075 MSun or ~75 M<\sub>Jup, lack a sustained source of energy, and they fade and cool 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 objects are characterised as spectral type L. Below 1,300K, strong methane bands appear in the near-infrared, characteristics of spectral type T. At present, the coolest T dwarfs known have temperatures of ~650 to 700K. At lower temperatures, other species, notably ammonia, are expected to become prominent, and a number of efforts have been undertaken recently 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. Identifying such ultra-ultracool dwarfs was a goal of the WISE satellite mission, which recently completed its all-sky survey. WISE has succeeded in identifying a number of extremely interesting sources, including at least 4 objects that have been confirmed as dwarfs with temperatures lower than 350K. These are among the first examples of Y dwarfs. The current program is combining WFC3-grism imaging with warm-Spitzer photometry to verify the nature of further candidates.

GO 12468: How Fast Did Neptune Migrate? A Search for Cold Red Resonant Binaries

Preliminary orbital determination for the KBO WW31, based on C. Veillet's analysis of CFHT observations; the linked image shows the improved orbital derivation, following the addition of HST imaging

The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun (see David Jewitt's Kuiper Belt page for details). Over 500 KBOs (or trans-Neptunian objects, TNOs) are currently known out of a population of perhaps 70,000 objects with diameters exceeding 100 km. Approximately 2% of the known TNOs are binary (including Pluto, one of the largest known TNOs, regardless of whether one considers it a planet or not). TNOs are grouped within three broad classes: resonant objects, whose orbits are in m,ean motion resonance with Neptune, indicating capture; scattered objects, whose current orbits have evolved through gravitational interactions with Neptune or other giant planets; and classical TNOs, which are on low eccentricity orbits beyond Neptune, with no orbital resonance with any giant planet. The latter clas are further sub-divided into "hot" and "cold" objects, depending on whether the orbits have high or low inclinations with respect to the ecliptic. Cold, classical TNOs show relatively uniform characteristcis, including red colours, high albedos and an extremely high binary fraction (>30%). They are believed to have formed in situ, and were therefore in place to experience the range of gravitational interactions as the giant planets migrated to their present location. As that migration occurred, subsets are expected to have been trapped in transitory resonance orbits. The present proposal aims to use HST to complete a photometric survey of all known resonant TNOs, with the goal of identifying the proportion of cold classical TNOs that have been captured. The relative number of such objects can be used to constrain models for Neptune's orbital migration in the early Solar System.

GO 12534: The Panchromatic Hubble Ultra Deep Field: Ultraviolet Coverage

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). The original UDF program comprised 412 orbits directed at a single ACS field within the Chandra Deep Field-South (CDF-S) GOODS area. Those 412 orbits were divided among four filters - F435W (broad B-band: 56 orbits), F606W (broad V/R-band: 56 orbits), F775W (I-band: 150 orbits), and F850LP (z-band: 150 orbits). A further 144 orbits were devoted to a 3x3 grid of F110W (J) and F160W (H) NICMOS images covering the same field.(GO program 9803). Immediately following Servicing Mission 4, the Wide-Field Camera 3 infrared camera was used to obtain obtain deep F850LP (Y), F105W (J) and F160W (H) images centred on the UDF and two flanking fields (GO 11563), resulting in the identification of significant numbers of galaxies at redshifts z~7 and 8, and even the identification of a z~10 galaxy candidate. The present program aims to fill in details at lower redshifts. Deep images will be obtained with the WFC3-UVIS camera at near-UV waveelngths using the F225W, F275W and F336W filters. Those data will extendn to 29th magnitude (AB), and provide invaluable information on the level of star forming activity in galaxies at redshifts in the range 1 < z < 2.5,

GO 12591: A Chandra/HST census of accreting black holes and nuclear star clusters in the local universe

SDSS image of NGC 3457, one of the galaxies targeted by this proposal

Black holes are now recognised as likely to be present in many, perhaps most, spiral and elliptcal galaxies that are above modest size. The central black hole manifests its presence as a bright, compact nucleus that is the source of strong emission lines due highly ionised material stemming from the accretion of hot gas. Observations of a number of active galactic nuclei have shown that central kinematisc clearly require the presence of a massive (>106 solar mass) central black hole. In some cases, the non-thermal emission is accompanied by stellar light due to a nuclear star cluster. Most active galactic nuclei (AGNs) are found in spiral galaxies that possess at least a moderately prominent bulge. The present program aims to extend coverage to lower luminosity spheroidal galaxies. These 31 systems are targeted by Chandra in a search for central X-ray emission that might idnicate the rpesence of an accreting black hole. The HST observations will use the Advanced Camera for Surveys to obtain blue (F475W) and far-red (F850LP) images to search for photometric signatures of central star clusters.