This week on HST

HST Programs: August 8, 2011 - August 14, 2011

Selected highlights

GO 12163: Structure and Stellar Content of the Nearest Nuclear Clusters in Late-Type Spiral Galaxies

the nearby spiral, NGC 300, one of the galaxies targetted in this program

Extensive observations over the past decade, primarily with HST, have shown that massive, young star clusters are present in the nuclear regions of almost all late-type spiral galaxies. It remains unclear, however, whether those clusters harbour (or generate) the black holes that are commonly found in the central regions of both ellipticals and spiral galaxies with significant bulge components, such as our Milky Way. The present program aims to address this issue through systematic observations of 10 relatively nearby bulgeless spirals. The aim is to combine multi-wavelength, high resolution imaging, using WFC3 on HST, with ground-based spectroscopy of the central regions of these systems. The resultant detailed colour-magnitude diagrams and spectral line strengths will be used in conjunction with stellar population models to constrain the star formation history and stellar content of the central regions, while the kinematics will probe the dynamics of the central regions, and set limits on the likely presence of central black holes in these systems.

GO 12276: Mapping a galaxy filament

Cosmological simulations of structure in the WHIM

Only a small fraction of the baryons in the Universe, perhaps 10%, are thought to reside in visible matter in galaxies. About 30% of the total likely contributes to the ionised gas detected in Lyman-alpha absorption studies. The remainder is generally believed to reside in the WHIM - the Warm-Hot Intergalactic Medium. This material is generally expected to form highly filamentary structures, some of which collapse and condense to for galaxies in the early Universe. Such structures can only be detected through the effect that they have on the light emitted by background sources. As the light passes through the filament, absorption occurs at specific wavelengths that depend on the composition and ionisation of the component materials. The present program focuses on 10-Mpc long filamentary structure that has been identified by mapping the distribution of galaxies in the (relatively) local universe (the measured velocities correspond to a redshift, z~0.01). Ultraviolet observations already exist for four QSOs lying behind this putative structure. The present program aims to use the Cosmic Origins Spectrograph to extend observations to a further 23 QSOs and/or AGN, providing a more detailed map of the density and ionisation structure.

GO 12317: Dynamical Masses of the Coolest Brown Dwarfs

Epsilon Indi Bab, the binary brown dwarf companion of the nearby K dwarf

Brown dwarfs are objects that form like stars, but lack sufficient mass to drive the central temperature above a few million degrees, and therefore never succeed in igniting core hydrogen fusion. Discovered almost 15 years ago, these objects initially have surface temperatures of ~3,500K, but cool rapidly and move through spectral types M, L and T. Following their discovery, considerable theoretical attention has focused on the evolution of their intrinsic properties, particularly the details of the atmospheric changes in the evolution from type L to type T and beyond. This transition marks the emergence of methane as a dominant absorber at near-infrared wavelengths. Current models suggest this transition occurs at ~1400-1200K, and that the spectral changes are at least correlated with, and perhaps driven by, the distribution and properties of dust layers ("clouds"). The overall timescales associated with this process remain unclear. Mass is a crucial factor in mapping those changes, but mass is also the most difficult quantity to measure in a reliable fashion. The present proposal aims to tackle this issue through astrometry of ultracool binary systems, deriving the orbits and hence dynamical masses. This program will use the ACS to obtain observations that supplement data from past cycles obtained as part of program GO 11593.

GO 12322: Detecting Isolated Black Holes through Astrometric Microlensing

A rather spectacular version of black hole lensing.

Gravitational lensing is a consequence of general relativity. Its effects were originally quantified by Einstein himself in the mid-1920s. In the 1930s, Fritz Zwicky suggested that galaxies could serve as lenses, but lower mass objects can also also lens background sources. Bohdan Paczynski pointed out in the mid-1980s that this offered a means of detecting dark, compact objects that might contribute to the dark-matter halo. Paczcynski's suggestion prompted the inception of several large-scale lensing surveys, notably MACHO, OGLE, EROS and DUO. Those wide-field imaging surveys have target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. The duration of each event depends on several factors, including the tangential motion of the lens and its mass. Long-term events are generally associated with a massive lens. Duration alone is not sufficient to identify a lens as a black hole - a source with very low tangential motion relative to the Sun can produce the same effect. However, microlensing not only leads to flux amplification, but also to small astrometric motions, caused by the appearance and disappearance of features in the lensed light. Those motions serve as a mass discriminant - higher mass lenses produce larger amplitude motions. The expected astrometric signal from a black hole lens is > 1.4 millarcseconds, just measureable with HST. This program aims to capitalise on this fact by searching for lensing by black holes in the Galactic field. The observations target long-duration lensing events in the Galactic Bulge.