This week on HST

HST Programs: August 21 - August 27, 2006

Some selected highlights

GO 10554: Globular Cluster Systems of Elliptical Galaxies in Low Density Environments

The giant elliptical, M87, (which is not in a low density environment) has an extensive population of associated globular clusters.

Globular clusters are key remnants of the first major episode of star formation in the Milky Way Galaxy. Other galaxies, both spirals and ellipticals, have their own associated populations of globulars; the clusters associated with M31 are well resolved (HST and, to a lesser extent, ground-based observations have resulted in colour-magnitude diagrams), while hundreds of globulars have been identified in M87, the core galaxy in the Virgo cluster. It is likely that those systems, too, are fossils from the earliest stages of galaxy formation. This proposal focuses on low luminosity elliptical galaxies in low density environments. Previous studies suggest that these clusters show a bimodal colour distribution, perhaps indicative of multiple ages and multiple formation epochs. ACS imaging will be used to identify associated globulars, determine the cluster luminosity function and undertake more detailed investigation of the colour distribution.

GO 10760 Black Hole X-ray Novae in M31

XMM-Newton X-ray image of the central regions of M31; the red dot marks a very active X-ray nova

X-ray novae are generally believed to originate in close binary systems that consist of a red dwarf and a highly compact companion, either a neutron star or a black hole. As in cataclysmic binaries (where the compact object is a white dwarf), orbital energy loss leads to the two components spiralling closer together, prompting Roche lobe overflow from the red dwarf onto the compact object. As that material is accreted onto the neutron star/black hole, it forms an accretion disk, releasing energy at a relatively modest level in the form of UV and X-ray radiation. X-ray outbursts occur when material from the disk accretes directly onto the NS/BH, and those outbursts can achieve luminosities that exceed 1037 erg/s. Consequently, these objects are easily visible in nearby galaxies, including both M33 (Trapezium) and M31 (Andromeda). Our bird's-eye view of those systems allows an assessment of the relative numbers of neutron star and black hole systems. The present program builds on extensive previous Chandra and HST observations of the central regions of M31, and aims to use ACS multi-colour imaging to probe active black hole X-ray novae, identified from near-contemporaneous Chandra-ACIS observations.

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 10816: The Formation History of Andromeda's Extended Metal-Poor Halo

HST ACS image of the outer bulge/inner halo resgions of M31

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 on HST, it has become possible to resolve main sequence late-F and G dwarfs in M31. This permits observations that probe stars with luminosities below the turnoff of the Galactic halo population, and substantial effort has been devoted to this program in recent cycles. The initial results suggested that M31's halo might be very different from our own - specifically, the data indicated the preence of a significant number of stars with both intermediate age (6-8 Gyrs) and intermediate metallicity ([Fe/H] ~ -0.5), rather than the >10 Gyrs and -5 < [Fe/H] < -1.5 values derived for the Galactic halo. However, all previous HST observations were at radial distances of less than 30 kpc from the M31 centre, and it has become clear that those data were contaminated by the M31 Bulge (or "spheroid" component). Recent observations indicate that a more traditional halo component dominates at larger radii. The present program aims to confirm that and derive reliable age, metallicity and density estimates, using the F606W and F814W filters on ACS to image several fields lying at radial distances from 22 to 35 kpc.

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 10882: Emission Line Snapshots of 3CR Radio Galaxies

Extended structure in the radio galaxy, 3C 75

The Third Cambridge Catalogue of Radio Sources was compiled originally in 1959 from observations at 159 MHz made by the Cambridge Radio Astronomy Group using the Cambridge interferometer (Edge et al, Mem RAS, 68, 37). The addition of observations at 178 MHz led to the revised catalogue, the 3CR (Bennett, 1962, Mem RAS, 68, 137). This catalogue was the main basis for the Fanaroff-Riley classification scheme (FR-Is are double-lobed sources with small separation between the peaks; FR-IIs have a separation that exceeds half the largest size of the source), and includes many of the brightest known radio galaxies. As such, the 3CR sources have remained extremely important observational targets for understanding the nature and structure these highly energetic sources. Over the past several cycles, many 3CR galaxies have been observed at a variety of wavelengths by HST. Those observations have revealed new optical jets, dust lanes, face-on disks with optical jets, besides point-like nuclei whose properties support FR-I/BL Lac unified schemes. The present program is using the ramp filters on the ACS/WFC to obtain images covering the H-alpha and O[III]5007 emission lines, probing the star formation characteristics of these radio galaxies.