This week on HST

HST Programs: July 31 - August 6, 2006

Some selected highlights

GO 10496: Decelerating and Dustfree: Efficient Dark Energy Studies with Supernovae and Clusters

Recent HST/ACS images of a supernova in a galazy at z=1.2

The last few years of the twentieth century saw a revolution in cosmology, with the measurement of the acceleration term in expansion at high redshifts and the identification of dark energy as a major cosmological component. Type Ia supernovae are the prime yardstick for measuring the rate of expansion at moderate and high redshifts, and Hubble offers almost the only way of obtaining reliable post-maximum photometry of these objects to determine the full shape of the light curve. Most previous HST supernovae programs have concentrated on field galaxies, such as those illustrated here, but applying appropriate corrections for in situ reddening by dust remains an issue in these systems. The present program aims to minimise the uncertainties by searching for supernovae in massive, high-redshift clusters, with the expectation that the majority of detections lie within dust-poor elliptical galaxies. ACS survey observations of eight clusters are scheduled for the coming week, together with follow-up NICMOS observations of a supernova detected in previous ACS images.

GO 10546: The filaments of NGC 1275

A composite image of NGC 1275

NGC 1275 is the central galaxy in the relatively nearby (~80 Mpc) Perseus galaxy cluster. It has long been known as an unusual system: it is an original Seyfert galaxy (from Carl Seyfert's 1943 paper); in 1954, Baade & Minkowski identified it as the optical counterpart of Perseus A, one of the first extragalactic radio sources (it is also 3C 84); and it is the brightest extraglactic X-ray source, originally detected by Uhuru in 1971. Optically, NGC 1275 has a complex structure, with dust lanes superimposed on an elliptical-like structure. Baade & Minkowski originally suggested that we might be witnessing a galaxy collision, and subsequent observations generally support this hypothesis. There is evidence for substantial star formation, as well as a central black hole that is probably responsible for the strong radio and X-ray emission. Most recently, narrowband images have revealed an extensive network of gaseous filaments that extend more than 100 kpc from the nucleus (see the Hubble Heritage site ). The present proposal aims to us ACS on HST to image NGC 1275 and its immediate surroundings using several filters that sample both continuum radiation and line emission from H-alpha and O II. The observations will be combined with a deep (1 Megasecond) Chandra image, probing the physical mechanisms present in the filamentary structure.

GO 10775: An ACS Survey of Galactic Globular Clusters

Hubble image of the globular cluster M15

Globular clusters are members of the Galactic halo population, which formed during the first extensive period of star formation in the Milky Way. As such, the properties of the 106 to 107 stellar constituents can provide crucial insight into the earliest stages of galaxy formation. The present HST program will obtain two-colour (F606W, F814W) ACS images of the central regions of 66 of the ~150 Galactic clusters, with the goals of determining cluster ages and distances; studying the main sequence luminosity function, mass function and mass segregation within the cluster; investigating the internal motions and dynamical evolution; and using absolute cluster motions to determine cluster orbits and probe the galactic potential. Observations of the cluster NGC 4833 are scheduled over the next week.

GO 10911: Calibration of ACS F814W Surface Brightness Fluctuations

Simulations of a nearby dwarf galaxy, a nearby giant galaxy and a distant giant galaxy; note that the last is similar in angular size to the dwarf, but has a much smoother brightness distribution (simulations from Ned Wright's ABC of distances

The determination of the Cosmic Distance Scale remains one of the major goals of cosmological programs in the early 21st century. Achieving this goal requires a reliable distance indicator. While observing programs continue to pursue conventional primary distance indicators (such as RR Lyraes and Cepheids) and secondary distance indicators (such as the RGB tip and the Tully-Fisher relation), attention is also being given to the method of surface brightness fluctuations. This method rests primarily on the hypothesis that the stellar populations in most galaxies have similar colour-magnitude diagrams. Thus, the total luminosity of the galaxy is generated by similar stars - mainly red giants. In a nearby low-luminosity galaxy, most of the light comes from a relatively small numebr of giant branch stars; consequently, that galaxy has a "grainier" appearance than a distant high-luminosity galaxy of the same apparent magnitude (see figure). The degree of granularity can therefore serve as a distance indicator. The present program will use ACS (F814W filter) to observe half a dozen galaxies in the Fornax cluster to provide a reliable calibration of this technique.