This week on HST

HST Programs: February 18 - February 24, 2013

Selected highlights

GO 12444: Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey -- GOODS-North Field, Late Visits of SNe Search

Part of the GOODS/Chandra Deep Field South field, as imaged by HST

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 form the final phase of the SNe detection sequence for the GOODS-North field.

GO 12544: Confirming Ultra-cold (Teff < 500K) Brown Dwarf Suspects Identified with WISE

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 12557: Low-Mass Black Holes and CIV in Low-Luminosity AGN

Artist's impression of an accreting black hole

Active galaxies are characterised by bright, compact nuclei that are the source of strong emission lines due highly ionised material. These phenomena are generally believed to arise in hot gas in an accretion disk, centred on a massive (>106 solar mass) hole; indeed, detailed kinematics for a handful of objects have confirmed the presence of a compact, massive object in the core. A wide variety of observations hav e been accumulated over the last decade or more that point to a significant correlation between the host galaxy properties and mass of the central black hole (and vice versa): in brief, larger galaxies seem to harbour more massive black holes. This correlation obviously suggests a causal connection, perhaps reflecting joing growth. However, recent observations suggest that these relations may break down at the smallest scales.in particular, the recent discovery of a high-mass black hole within the relatively mdoest galaxy, NGC 1277, raises questions as to whether this is an isolated object, or whether it is mor representative of an overall trend. The present program aims to tackle this issue through using the Cosmic Origins Spectrograph to obtain UV spectroscopy of the central regions of six low-luminosity AGNS. The width of the C IV 1549 Angstrom line will be used to constrain the velocity distribution in the cenrtal regions, and hence the mass of the central black hole.

GO 13029: A Snapshot Survey of the Sites of Recent, Nearby Supernovae

A recent supernova in M100

Supernovae mark the (spectacular) evolutionary endpoint for a subset of stellar systems. Standard models predict that they originate from massive stars and (probably) close binaries with a compact (WD, neutron star) component, but there are still some questions remaining over whether we fully understand the range of possible progenitors. The last decade has seen the development of a number of large-scale programs, usually using moderate-sized telescopes, that are dedicated to monitoring (relatively nearby galaxies, searching for new supernovae. This program builds on observations taken in several previous cycles, and aims to obtain follow-up multi-waveband images of nearby galaxies, focusing on the sites of recent supernovae. The program concentrates on systems within 20 Mpc of the Milky Way. The observations are taken well after maximum, with the aim of using the unparalleled angular resolution of WFC3 to identify the fading remnant, search for evidence for light echoes,characterise the local stellar population and perhaps determine the nature of the likely progenitor.