This week on HST

HST Programs: October 16 - October 22, 2006

Some selected highlights

GO 10889: The Nature of the Halos and Thick Disks of Spiral Galaxies

Ground-based imaging of the edge-on spiral, NGC 891

The stars in the Milky Way are generally grouped into stellar populations, building blocks that provide insight into the process of galaxy assembly. The traditional populations are the near-spherical, metal-poor Halo, representing the first significant burst of star formation; the Disk, whose constituents have higher metallicities, a flattened density distribution (which defines the Galactic Plane) and significant angular momentum, suggesting a formation history that includes collapse and dissipation; and the central Bulge, which, with a spheroidal distribution and broad metallicity range, may be something of an amalgam of disk and halo. The original models for the Disk envisaged a relatively simple population, with a continuous star formation history and an exponential density distribution, both radially and perpendicular to the Plane. However, in the mid-1980s, starcount analyses revealed more complexity in the vertical density distribution, with evidence for two components with scaleheights ~300 and 1000-1300 pc. Subsequent investigations of the more extended component, dubbed the thick disk, suggest that it probably formed as a result of a merger with a massive satellite early in the Milky Way's history (8-10 Gyrs ago). Ground-based observations suggest that some other spiral galaxies possess a similar component. Clearly, the frequency of such systems and their age distribution offer clues to the merging history of the average spiral galaxy. The present program will use ACS, WFPC2 and NICMOS to image seven edge-on spirals at several locations perpendicular to the Plane, with the aim of resolving the underlying stellar populations and tracing the metallicity distribution and overall morphology of the extended disk components.

GO 10867: SAINTS - Supernova 1987A INTensive Survey

November 2003 HST image of the SN1987A gaseous ring

SN1987A, in the Large Magellanic Cloud, is (as far as we know) the nearest supernova to the Sun since Kepler's supernova of 1604. While its eruption, in January 1987, predated HST's launch by over 3 years, the remnant has been a regular observational target. Those high resolution observations have revealed the development, and evolution, of extensive, intricate structures as the blast wave from SN1987A encounters the surrounding interstellar medium. In particular, a striking circum-remnant ring has developed, with numerous hot spots stimulated by the fastest moving debris. The present HST program continues to monitor the development of those features, using a series of observations that are co-ordinated with the Chandra X-ray Observatory. During the present cycle, the hotspots may fuse, as the shock fully enters the ring, and photons from these regions may excite previously hidden gas outside the ring, illuminating mass lost from the progenitor before the explosion. The inner debris are now well resolved, and clearly asymmetrical. Overall, these observations provide crucial insight into the earliest stages of formation of a supernova remnant.

GO 10915: ACS Nearby Galaxy Survey

NGC 55 in Sculptor, the galaxy currently targeted by the ACS NGS

Colour-magnitude diagrams derived from photometric surveys have proven invaluable in achieving an understanding of the main properties of the galactic stellar populations. Large ground-based telescopes allowed extension of this type of analysis to the principal satellites of the Milky Way and, to a limited extent, the Andromeda spiral. With the advent of HST, particularly following SM3B and the installation of the Advance Camera for Surveys, those fundamental CMD studies can be extended to higher-density star fields, fainter magnitudes and intrinsically lower luminosity stars. Until recently, those studies have concentrated on Local Group galaxies; the ambitious aim of the current program is to conduct a systematic survey of all major star-forming galaxies within ~3.5Mpc of the Milky Way, together with a number of galaxies in the M81 group at a distance of ~4 Mpc. The program includes a total of 45 galaxies, ranging from massive spiral systems to dwarf galaxies. The observations are being made using the wide-field camera on ACS, sampling selected fields in the wide-V (F606W) and I (F814W) passbands. Observations of the dwarf system, DDO 078, are scheduled during the coming week.

GO 10921: Tangential Velocities of Objects in the Orion Nebula and Locating the Embedded Outflow Sources

Sections of the Orion Nebula, from WFPC2 observations

The Orion Nebula, lying at a distance of ~500 pc and with an age of ~5 Myrs, is the nearest example of a large, young star-forming region. The Orion Nebula Cluster, the main site of star formation, has a complement of approaching 10,000 stars and brown dwarfs with masses ranging from a few MJupiter to more than 10 MSun. The most massive stars, including the central O stars of the Trapezium, produce jets, shocks and outflows, powered by winds and photoionisation. HST has been observing the ONC for well over a decade years, and the initial set of WFPC2 observations provide an excellent reference set of images for measuring the relative motions of gaseous knots, shock fronts and the higher-velocity stellar cluster members. The present observational program will use both ACS and WFPC2 to obtain second-epoch images in a variety of wide- and narrow-band filters, coupled with parallel near-infrared imaging with NICMOS.