This week on HST

HST Programs: July 2 - July 8, 2012

Selected highlights

GO 12105, 12109: 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" is conducting a multi-waveband survey of approximately one third of disk and bulge, focusing on the north-east quadrant. Observations extend over Cycles 19, 20 and 21, and will provide a thorough census of upper main-sequence stars, open clusters, associations and star forming regions, matching the stellar distribution against the dust and gas distribution.

The dual AGN in the core of NGC 7358/Markarian 739 (from the galaxy zoo)

Dual AGNs are galaxies that host a pair of actively-accreting supermassive black holes within their central regions. These systems are generally expected to arise through major mergers between large galaxies, with each SMBH representing the core of one of the parent galaxies. Relatively few such systems are known, which is surprising given the expected frequency of these systems within a standard lambda-CDM cosmological model. This scarcity might arise from a number of sources, including basic problems with the formation model, variatioins in the accretion history of the individual SMBHs, and observational difficulties in resolving separate sources. The present prorgam aims to use the high angular resolution provided by HST WFC3 UVIS and IR observations to target AGNs that have double-peaked emission lines (specifically, [OIII] emission). These data offer the poetntial to expand searches to significantly smaller radial separations, likely corresponding to later stages in the evolution of a merger. the distribution of separatiosn can set constraints on the ovall likely frequency of these objects.

A region of massive star formation

Star formation is the key astrophysical process in determining the overall evolution of galactic systems, the generation of heavy elements, and the overall enrichment of interstellar and intergalactic material. Tracing the overall evolution through a wide redshift range is crucial to understanding how gas and stars evolved to form the galaxies that we see around us now. The present program builds on the ability of HST to carry out parallel observations, using more than one instrument. While the Cosmic Origins Spectrograph is focused on obtaining ultraviolet spectra of unparalleled signal-to-noise, this program uses the near-infrared grisms mounted on the Wide-Field Camera 3 infrared channel to obtain low resolution spectra between 1 and 1.6 microns of randomly-selected nearby fields. The goal is to search for emission lines characteristic of star-forming regions. In particular, these observations are capable of detecting Lyman-alpha emission generated by star formation at redshifts z > 5.6. A total of up to 40 "deep" (4-5 orbit) and 20 "shallow" (2-3 orbit) fields will be targeted in the course of this observing campaign.

GO 12801: Hubble Deep Search for Debris and Satellites in the Pluto System in Support of NASA's New Horizons Mission

Hubble Space Telescope images of the Pluto system, including the newly discovered moon, P4

Pluto, one of the largest members of the Kuiper Belt and, until recently, the outermost planet in the solar system, has been in the news over the last year or two. Besides the extended "planet"/"dwarf planet" debate, Pluto is the primary target of the New Horizons Mission. In 1978, James Christy discovered from analyses of photographic plates that Pluto has a relatively large companion moon, Charon, with a diameter of ~1200 km, or almost half that of Pluto itself. In 2005, Hubble observations led to the discovery of two small moons, christened Nix and Hydra. These two new moons are 5,000 fainter than Pluto itself, implying diameters as small as ~30-50 km if the surface composition is similar to Pluto itself. Within the past year, a series of observations were taken in support of the New Horizons mission, using WFC3 to search for faint rings due to dust particles that might jeopardise the space craft and require a course correction. No rings werer detected; however, a fourth small satellite, "P4", was discovered. This object is even fainter than Nix and Hydra, and may well be as small as 13 km in size. The present observations, again in support of new Horizons, will use WFC3 to push to even fainter magnitudes to both better characterise the P4 orbit and constrain the likelihood of substantial debris clouds within the planned mission trajectory. These observations are crucial to determining whether further course corrections are required that might force New Horizons to pass Pluto at larger distances, with a potential impact on the scientific return.