This week on HST

HST Programs: March 1, 2010 - March 7, 2010

Selected highlights

GO 11548: Imaging of Protostars in the Orion A Cloud: The Role of Environment in Star Formation

An image of the orion Nebula superimposed on the 13CO map of Orion A (from this link ).

The Orion association is the largest nearby star-forming complex, providing a key laboratory for unlocking the secrets of star formation. As such, it has been subject to intense scrutiny at all wavelengths from both ground and space. Surveys at near-infrared and mid-infrared wavelenths, notably by Spitzer, have identified an extensive number of embedded sources, young stellar objects (YSOs) that are still accreting from the surrounding molecular gas. This proposal focuses on 252 sources within the Orion A molecular cloud, the complex that includes the Orion Nebula Cluster. In the original Cycle 16 incarnation of this program, NICMOS was used to survey a subset of the protostars; the program has since been transferred to the WFC3-IR camera. The observations are an excellent complement to Spitzer since, while HST cannot offer either the same areal coverage or sensitivity at mid-infrared wavelegths, the camera can provide a resolution close to 0.1 arcsecond, an order of magnitude higher than the Spitzer images.

GO 11588: Galaxy-Scale Strong Lenses from the CFHTLS survey

ACS images of galaxy-galaxy Einstein ring lenses

Gravitational lensing is a consequence the theory of general relativity. Its importance as an astrophysical tool first became apparent with the realisation (in 1979) that the quasar pair Q0957+561 actually comprised two lensed images of the same background quasar. In the succeeding years, lensing has been used primarily to probe the mass distribution of galaxy clusters, using theoretical models to analyse the arcs and arclets that are produced by strong lensing of background galaxies, and the large-scale mass distribution, through analysis of weak lensing effects on galaxy morphologies. Gravitational lensing can also be used to investigate the mass distribution of individual galaxies. Until recently, the most common background sources were quasars. Galaxy-galaxy lenses, however, offer a distinct advantage, since the background source is extended, and therefore imposes a stronger constraints on the mass distribution of the lensing galaxy than a point-source QSO. The CFHT Legacy survey was a concerted effort by Canada and France to use almost half of their dark/grey time on the Canada-France-Hawaii telescope to obtain deep images with the MegaCam 1x1 degree imager. In ttoal, about 450 nights were devoted to this project between mid-2003 and early 2009, covering almost 600 square degrees to varying depths. The resultant survey provides a powerful means for identifying candidate galaxy-galaxy lenses. This program is using WFC3 SNAP imaging to verify the nature of those candidates, and provide the angular resolution necessary to model the mass distribution.

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 11711: A Definitive Distance to the Coma Core Elliptical

The Coma cluster of galaxies

The Coma cluster is the nearest rich galaxy cluster, incorporating several thousand member falaxies and lying at a distance of ~100 Mpc. This cluster has been targeted by a number of HST programs in recent cycles, notably the Cycle 15 programs GO 10842, designed to determine the distance through measurement of Cepheids in two spirals, and GO 10861, surveying the cluster members (both programs were interrupted by the ACS failure in January 2007). Determining a reliable distance to Coma remains an important task for the cosmic distance scale. The present program aims to achieve this by using 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 number of giant branch stars; consequently, that galaxy has a "grainier" appearance than a distant high-luminosity galaxy of the same apparent magnitude. The ACS Wide-Field Camera will be used to image the two giant ellipticals in the Coma core, NGC 4874 and NGC 4899, in the F814W filter. The expectation is that the distances can be determined to an accuracy of 3 to 4 percent.