This week on HST

HST Programs: December 7 - December 13, 2009

Selected highlights

GO 11153: The Physical Nature and Age of Lyman Alpha Galaxies

Composite optical/X-ray/IR imaging of a nigh-redshift Lyman-alpha blob

Simple galaxy formation models envisage a period of extensive star formation at the earliest epochs. That activity is likely to be accompanied by substantial Lyman alpha emission, generated by ionising photons from short-lived, high-mass stars. The degree of such activity, however, and its duration remain matters of some conjecture, which can be addressed through deep, high resolution investigations of the underlying morphology of high-redshfit Lyman-alpha emitters. The higher angular resolution and increased sensitivity of HST offer significant advantages in characterising these systems. The present proposal is using HST to obtain follow-up F110W/F160W (J/H) images of 35 spectroscopically confirmed 4.5 < z < 6.5 Ly-alpha emitters. Initial observations were with NICMOS, and the progrma has moved to using WFC3 following SM4. These systems also have mid-IR data, taken using the IRAC camera on Spitzer. The observations will be used to fully characterise the spectral energy distributions and morphologies of these sources.

GO 11589: Hypervelocity Stars as Unique Probes of the Galactic Center and Outer Halo

Artistic view 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, and the present HST program aims to measure precise proper motions that will quantify their space motions.

GO 11593: 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 initialy have surface temperatures of ~3,500K, but cool rapidly and move through spcetral 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 remains 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. Initially designed for ACS, the current observations are being made with WFPC2, and the binary system SDSSJ092615.38+584720.9 will be imaged in the coming week.

GO 11712: Calibration of Surface Brightness Fluctuations for WFC3/IR

Simulations of a nearby dwarf galaxy, a nearby giant galaxy and a distant giant galaxy; note that the last is similar in angular size to the dwarf, but has a much smoother brightness distribution (simulations from Ned Wright's ABC of distances

The determination of the Cosmic Distance Scale remains one of the major goals of cosmological programs in the early 21st century. Achieving this goal requires a reliable distance indicator. While observing programs continue to pursue conventional primary distance indicators (such as RR Lyraes and Cepheids) and secondary distance indicators (such as the RGB tip and the Tully-Fisher relation), attention is also being given to the method of surface brightness fluctuations. This method rests primarily on the hypothesis that the stellar populations in most galaxies have similar colour-magnitude diagrams. Thus, the total luminosity of the galaxy is generated by similar stars - mainly red giants. In a nearby low-luminosity galaxy, most of the light comes from a relatively small numebr of giant branch stars; consequently, that galaxy has a "grainier" appearance than a distant high-luminosity galaxy of the same apparent magnitude (see figure). The degree of granularity can therefore serve as a distance indicator. The present program will use the IR channel of Wide-Field Camera 3 (F110W and F160W filters) to observe seventeen galaxies in the Fornax and Virgo clusters to provide a reliable calibration of this technique.