This week on HST

HST Programs: January 11, 2010 - January 17, 2010

Selected highlights

GO 11570: Narrowing in on the Hubble Constant and Dark Energy

A GALEX image of Messier 106 (NGC 4258), one of the galaxies targeted in this program

The Hubble constant remains a key parameter in understanding cosmology and the evolution of the Universe. Refining measurements of H0 therefore still represents a vital means of probing the nature of dark energy. The present program aims to tackle this question by laying a firmer foundation to the SNe Ia distance scale. The WFC3 IR camera will be used to identify and characterise Cepheid variables in eight relatively nearby galaxies that have hosted Type Ia SNe. Cepheids have signficantly lower amplitude at near-infrared wavelengths, and the measured magnitudes are less subject to uncertainties due to foreground reddening and variations in metallicity. As a consequence, determining the mean apparent magnitude, and hence the period/apparent magnitude relation, is substantially more straightforward than at optical wavelengths. Matching the observed relation against reference stars from the LMC allows a more reliable determination of the distance to the parent galaxy, and hence a firmer zeropoint for the SNe Ia distance scale. The aim is to reduce the level of systematics in determinations oif H0 to the 3 percent level.

GO 11592: Testing the Origin(s) of the Highly Ionized High-Velocity Clouds: A Survey of Galactic Halo Stars at z>3 kpc

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 extremely hot, 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 spetral regioon that has been opened up with the installation of the Cosmic Origins Spectrograph on HST. The present program aims to use distant halo stars to probe a subset of the known HVCs within the Milky Way.

GO 11677: Is 47 Tuc Young? Measuring its White Dwarf Cooling Age and Completing a Hubble Legacy

Hubble image of the globular cluster, 47 Tucanae

Globular clusters are members of the Galactic halo population, which formed during the first extensive period of star formation in the Milky Way. As such, the properties of the 106 to 107 stellar constituents can provide crucial insight into the earliest stages of galaxy formation. Hubble has conducted a significant number of observing programs targeting these systems, with the majority designed to obtain moderately deep, multicolour imaging data of a range of clusters. Those programs probw evolved stars, on the red giant and horizotal branch, and generally extend no more than a few magnitudes below the main-sequence turnoff. A few clusters, however, have been studied in detail - specifically, the two nearest clusters, NGC 6397, an extremely metal-poor cluster, and M4, a moderately metal-rich systems; Omega Centauri, one of the most massive clusters, perhaps even the remnant core of a dwarf galaxy; and 47 Tucanae, one of the higher metallicity systems, lying in the foreground of the Small Magellanic Cloud. Deep imaging of NGC 6397 and M4 has succeeded in clear detecion of the white dwarf cooling sequence in those clusters, and those data have been used to derive age estimates. The present observation aims to obtain similar data for 47 Tucanae, permitting an estimate of the relative age of these three, disparate clusters.

GO 11724: Direct Age Determination of the Local Group dE Galaxies NGC 147 and NGC 185

Star formation in the central regions of NGC 185, taken by the 1.5-metre Kapteyn telescope on La Palma

The Andromeda galaxy, M31, is the nearest large spiral to the Milky Way system, and, like the Milky Way, it is acccompanied by a large number of much smaller, satellite galaxies. The majority of those systems are dwarf spheroidals, similar to Milky Way satellites like Sextans, Draco and Leo. M31 lacks any moderately massive irregular galaxies as companions, like the LMC and SMC, but it does posses the four nearest examples of dwarf elliptical systems: NGC 147, NGC 185, NGC 205 (M110) and NGC 221 (M32). The latter two systems lie too close to the body of M31 itself to allow detailed investigation of its constituent populations, but NGC 147 and NGC 185 lie at large projected distances. Both of these galaxies have been studied extensively by ground- and space-based telescopes. Those observations strongly suggest that both galaxies are dominated by old stellar populations, although there is some evidence for continuing star formation in the cores of some systems. It remains unclear whether there is a significant contribution from intermediate age stars, as are present in some dwarf spheroidal satellites of the Milky Way. The present proposal will use ACS and WFC3 to obtain deep imaging of fields in both galaxies, with the aim of deriving deep, accurate colour-magnitude diagrams that will reach below the main-sequence turnoff and can be used to probe the detailed star formation history.