This week on HST

HST Programs: November 19 - November 25, 2012

Selected highlights

GO 12533: Escape of lyman-Alpha Photons from Dusty Starbursts

HST NICMOS image of the interacting Luminous IR Galaxy, NGC 6090

Ultraluminous infrared galaxies (LIRGs) are systems that are characterised as having luminosities that exceed 1012 LSun, with most of the energy emitted at wavelengths longward of 10 microns. Many (perhaps most) of these galaxies are interacting or merging disk galaxies, with the excess infrared luminosity generated by warm dust associated with the extensive star formation regions. Many systems also exhibit an active nucleus, and may be in the process of evolving towards an S0 or elliptical merger remnant. One of the surprising discoveries in recent years has been the extent to which Lyman-alpha ionising emission can be detected escaping from these dusty systems. The present program looks to quantify the distribution of these properties through COS observations of sixteen ULIRGs in the local universe (z~0.1). These relatively nearby systems can provide insight into the structure of these systems, and give clues to the likely behaviour at higher redshifts.

GO 12662: Hypervelocity Stars as Unique Probes of the Galactic Center and Outer Halo

Artist's impression of a hypervelocity star

Hypervelocity stars are stars that have velocities that exceed the escape velocity of the Milky Way by a very substantial margin. Stars can only achieve such velocities through violent gravitational interactions with other bodies. Indeed, to achieve velocities of 1,000 km/sec or more, the proposed mechanism involves binary stars interacting with the black hole at the Galactic Centre: one of the binary components is lost to the black hole, and the other ejected at from the core. The existence of such objects was hypothesised over 20 years ago, but the first candidate was only identified in 2005: SDSS J090745.0+024507, an apparently non-descript sunlike star, lying at a distance of 71 kpc from the Sun in the Galactic, and moving at a velocity of 850 km/sec, or more than twice the escape velocity. Since then, a number of other candidates have been identified. HST was used to image 11 stars in Cycle 17 (GO 11589); the present program will obtain follow-up imaging, permitting the determination of precise proper motions, and hence space velocities, for these unusual stars.

GO 12970: Completing the Census of Ultracool Brown Dwarfs in the Solar Neighborhood using HST/WFC3

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, and all are too faint to be characterised with any degree of certainty using ground-based observations. The current program will use WFC3 G102 grism spectroscopy to verify the nature of a further 20 candidates.

GO 12982: Are the Milky Way's High Velocity Clouds Fuel for Star Formation or for the Galactic Corona?

A map of the high velocity cloud systems surrounding the Milky Way (B. Wakker, U. Wisconsin).

The stellar components of the Milky Way Galaxy are well known: the disk, the central bulge and the old, metal-poor stellar halo. However, the Milky Way is also surrounded by a halo of hot, gas that is itself embedded within a much more tenuous corona of even hotter, ionised gas. Within that structure lie high velocity clouds. Originally discovered in the 1930s as absorption features in stellar spectra, these clouds have velocities that differ significantly from the rotational velocity along that line of sight, and they are generally believed to be undergoing infall into the Galaxy. The origin and nature of these systems remains uncertain, with some favouring a Galactic origin, driven by star formation and feedback between disk and halo, and others supporting their origin within the warm-hot intergalactic medium. HVCs are not self luminous, so indirect methods need to be applied to examine their characteristics. The most effective is to identify stars that lie behind individual systems and, as with their discovery in the 1930s, search the stellar spectra for signature absorption lines produced by material within the cloud. Many, indeed most, of the key absorption features lie at ultraviolet wavelengths, a spectral region that has been opened up with the installation of the Cosmic Origins Spectrograph on HST. The present observations build on a Cycle 17 program that used COS to obtain spectra of distant halo stars aligned with a subset of the known HVCs within the Milky Way. The results indicate that HVCs are streams of gas in the lower halo, ruling out several models for their formation and evolution. The current observations will probe several known features for structure closer to the Disk by using COS to target stars significantly closer to the Sun.