This week on HST

HST Programs: July 11, 2011 - July 17, 2011

Selected highlights

GO 12071: A Panchromatic Hubble Andromeda Treasury

M31: the Andromeda spiral galaxy

M31, the Andromeda galaxy, is the nearest large spiral system to the Milky Way (d ~ 700 kpc), and, with the Milky Way, dominates the Local Group. The two galaxies are relatively similar, with M31 likely the larger system; thus, Andromeda provides the best opportunity for a comparative assessment of the structural properties of the Milky Way. Moreover, while M31 is (obviously) more distant, our external vantage point can provide crucial global information that complements the detailed data that we can acquire on individual members of the stellar populations of the Milky Way. With the advent on the ACS and, within the last 2 years, WFC3 on HST, it has become possible to resolve main sequence late-F and G dwarfs, permitting observations that extend to sub-solar masses in M31's halo and disk. Initially, most attention focused on the extended halo of M31 (eg the Cycle 15 program GO 10816 ), with deep imaging within a limited number of fields revealing the complex metallicity structure within that population. With the initiation of the present Multi-Cycle Treasury program, attention switches to the M31 disk. "PHAT" will conduct a multi-waveband survey of approximately one third of disk and bulge, focusing on the north-east quadrant. Observations will extend over the next three cycles, and will provide a thorough census of upper main-sequence stars and star forming regions, matching the stellar distribution against the dust and gas distribution.

ACS images of galaxy-galaxy Einstein ring lenses from the Sloan survey

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 present survey aims to target a particular type of galaxy - luminous giant ellipticals. Spectroscopic data from the recently initiatiated Baryon Oscillation Spectroscopic Survey (BOSS). sampling luminous ellipticals at redshifts between z~0.4 and 0.7, has been used to identify candidate strong lens systems. The present program is using HST I-band (F814W) ACS-WFC imaging to verify the nature of those candidates, and, for confirmed sources, provide the angular resolution necessary to model the mass distribution of the lensing system.

GO 12228: Probing for Exoplanets Hiding in Dusty Debris Disks: Inner <10 AU Disk Imaging, Characterization, and Exploration

HST-ACS image of the disk surrounding the nearby M dwarf, AU Mic

Planet formation occurs in circumstellar disks around young stars. Most of the gaseous content of those disks dissipates in less than 10 million years, leaving dusty debris disks that are detectable through reflect light at near-infrared and, to a lesser extent, optical wavelengths. The disk structure is affected by massive bodies (i.e. planets and asteroids), which, through dynamical interactions and resonances, can produce rings and asymmetries. Over the past decade, HST and Spitzer have provided complementary information on this subject, with Spitzer measuring thermal radiation from circumstellar dust and HST providing high-resolution mapping of debris disks in reflected light. Most recently, HST ACS coronagraphic imaging have revealed the presence of a planetary object within the disk of the nearby A star, . Planetary companions to the young (60 Myr-old) F star, HR 8799, have also been imaged by both ground-based telescopes and HST. The ACS coronagraph was associated with the High Resolution Camera, which is no longer functioning; nor is NICMOS. However, coronagraphy is still possible using the occulting bar on the Space Telescope Imaging Spectrograph (STIS). The present program aims to expand the catalogue of imaged exoplanets to other nearby young stars that are known to harbour debris disks. The present set of observations targets the young, nearby M dwarf, AU Mic (or Gliese 803).