This week on HST

HST Programs: November 13 - November 19, 2006

Some selected highlights

GO 10802: SHOES-Supernovae, HO, for the Equation of State of Dark energy

WFPC2 image of NGC 4639, one of the spirals targeted in this program

The cosmic distance scale and dark energy are two key issues in modern astrophysics, and HST has played a vital role in probing both. On the one hand, HST has been involved in cosmic distance measurements since its inception, largely through the H0 Key Project, which used WFPC2 to identify and photometer Cepheids in 31 spiral galaxies at distances from 60 to 400 Mpc. On the other, HST is the prime instrument for investigating cosmic acceleration by searching for and following Type Ia supernovae at moderate and high redshift. These two cosmological parameters are directly related, and recent years have seen renewed interest in improving the accuracy of H0 with the realization that such measurements, when coupled with the improved constraints from the Cosmic Microwave Background, provide important constraints on cosmic acceleration and the nature of Dark Energy. The present HST program combines observations that are designed to tackle both questions. NICMOS will be used to observe known Cepheids in several Key Project spirals that have hosted Type Ia supernovae; the near-infrared data will provide more accurate distance estimates for those galaxies, tying together the Cepheid and SN Ia distance scales. At the same time, the ACS/WFC will be used for parallel observations designed to search for high-redshift supernovae. Follow-up observations of those discoveries will add further weight to the measurement of cosmic acceleration.

GO 10833: Host Galaxies of Reverberation Mapped AGNs

Artist's impression of an AGN

Active galaxies (AGNs) are generally extremely luminous systems, characterised by the presence of strong emission lines of numerous species, including H, He I, He II, and Fe, Ca, O, C and S over a range of ionisations. These features originate from gas clouds in the nuclear regions, with the energy supplied through accretion onto a central massive black hole. The high-temperature, rapidly-rotating gas clouds nearest the central engine are responsible for producing broad emission lines (hence, the "Broad Line Region"). The structure of the BLR can be discerned using a technique known as reverberation mapping: variations in the accretion rate lead to fluctuations in luminosity; those variations lead, in turn, to variations in the photoionisation of the BLR, and corresponding changes in spectral line strengths; by monitoring those changes, and correlating them with the photometric variability of the central source, we can determine the light travel time from nucleus to BLR gas, and hence map the size of the BLR. In conducting these measurements, it is important to determine the relative contributions made by light from the central source and by the "local" (but uninvoved) stellar population. The high resolution provided by HST is vital for these measurements. The present proposal will use the High Resolution Camera (HRC) on ACS to target 17 AGNs that have been subjected to reverbation mapping.

GO 10915: ACS Nearby Galaxy Survey

M81 and M82 (NOAO 0.9-metre image)

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 M81 and two dwarf systems, DDO 183 and KKH37, are scheduled during the coming week.

GO 10989: Astrometric masses of extrasolar planets and brown dwarfs

Artist's impression of one of the planets circling Epsion eridani (from Nova Celestia )

The overwhelming majority of extrasolar planetary systems have been identified through radial velocity monitoring, and the detection of the reflex motion of the parent star as it orbits the common center of mass of the system. Just as radial velocities measure the stellar "wobble" introduced along the line of sight, so high precision astrometry can be used to measure motion in the plane of the sky. Combining these data gives the full three-dimensional motions of the system, and a direct measure of the mass of the planetary companion. The Fine Guidance Sensors on HST are the only system currently capable of making observations at the required sub-milliarcscond accuracy, and has already been used for astrometry of several systems, including the M dwarf Gl 876 and, most recently, Epsilon Eridani. The current GO program pursues observations of six planetary hosts, and FGS observations of HD 38529, which has at least two planetary companions, are scheduled over the next week.