This week on HST

The combined UV/X-ray image of M81

M81, the main spiral galaxy in the M81 group, is itself the subject of a separate HST program, also using ACS. The aim here is to characterise the nature of the X-ray sources detected in deep Chandra observations, with the specific aim of separating high-mass and low-mass X-ray binaries (HMXRBs and LMXRBs). As one might expect, HMXRBs are expected to be associated with young stellar populations (high mass stars); LMXRBs are generally believed to consist of either a neutron star or black hole which is accreting matter from a lower-mass companion (main-sequence star, red giant or white dwarf) that fills its Roche lobe. LMXRBs are therefore likely to be present in somewhat older stellar populations than the HMXRBS. This proposal aims to quantify this issue through a detailed survey of M81.

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

HST 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, adding further weight to the measurement of cosmic acceleration.

GO 10870: The Ring Plane Crossings of Uranus in 2007

Images of Uranus spanning 2000 to 2004, showing the rotation of the ring plane

Like the other Solar System gas giants, Uranus not only has an extensive number of satellite moons, but also possesses a ring system. Unlike the other giant planets, Uranus has a polar obliquity of 98o degrees, so its equator is close to perpendicular to the ecliptic plane. Consequently, from our vantage point on Earth, we view the north and south poles alternately during Uranus' 84-year circling of the Sun. Midway between the polar apparitions, of course, we view Uranus' equatorial plane - and see the ring system edge-on. The next ring plane crossing will occur in May and August 2007. At this juncture, the denser and more prominent rings will almost disappear from view, providing an opportunity to search for small satellite "shepherd" moons. These moons are expected to be present, acting as gravitational delineators, defining the radial size of the individual rings. Besides searching for the shepherds, the current HST program will use the High Resolution Camera on ACS to measure the thickness of the rings, and study the colours of the recently discovered fainter rings.

GO 10915: ACS Nearby Galaxy Survey

NGC 3109, one of the galaxies 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 thebarred spiral, NGC 253, are scheduled during the coming week.