This week on HST

HST Programs: December 11 - December 17, 2006

Some selected highlights

GO 10800: Kuiper Belt Binaries: Probes of Early Solar System Evolution

Composite HST image of the Kuiper Belt binary, WW31

The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun (see David Jewitt's Kuiper Belt page for details). Over 500 KBOs are currently known out of a population of perhaps 70,000 objects with diameters exceeding 100 km. Approximately 2% of the known KBOs are binary (including Pluto, one of the largest known KBOs, regardless of whether one considers it a planet or not). This is a surprisingly high fraction, given the difficulties involved in forming such systems and the relative ease with which they can be disrupted. It remains unclear whether these systems formed from single KBOs (through collisions or 3-body interactions) as the Kuiper Belt and the Solar System have evolved, or whether they represent the final tail of an initial (much larger) population of primordial binaries. This proposal aims to use ACS/HRC images of known KBOs toidentify new binary systems.

GO 10842 A Cepheid Distance to the Coma Cluster

NGC 4911, from DSS scans of POSS II IIIaJ plate material

Cepheid variable stars have been the prime extragalactic distance indicator since Henrietta Leavitt's discovery of the period-luminosity relation described by Cepheids in the Small Magellanic Cloud. It was Hubble's identification of Cepheids in NGC 6822 that finally established that at least some nebulae were island universes. Cepheids and the extragalactic distance scale figure largely in HST's history, notably through the Hubble Constant Program, one of the initial Key Projects. Hubble has accumulated WFPC2 and NICMOS observations of Cepheids in 31 galaxies. All of those galaxies lie within 25 Mpc; thus, both the Key Project's derivation of H0 = 72 +/- 8 km/sec/Mpc and the competing value, H0 = 56 +/- 7 km/sec/Mpc, (an offset of 1.5 sigma), rely on secondary indicators to take measurements to the far-field Hubble flow. The aim of the present project is to use the higher sensitivity and higher resolution of ACS to push the Cepheid measurements to the Coma cluster. The program will target two spiral galaxies in the cluster, NGC 4911 and NGC 4921. If Coma lies at a distance of 100 Mpc ( (m-M)=35.0), then long-period Cepheids (P~50 days) have mean apparent magnitudes of V~29 - challenging observatons even for ACS. The present set of observations is designed to optimise the observing strategy in anticipation of the full obsrving program, which will be executed in Spring 2007.

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.

GO 10909: Exploring the diversity of cosmic explosions: The supernovae of gamma-ray bursts

HST images of GRB-11121, linked with SN2001ke

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected since their discovery in the late 1960s (by military satellites - there were some initial worries that they might be of sinister intent). Analyses of their properties suggest that GRBs can be divided into two basic caregories: short, hard bursts, that are likely due to component mergers in neutron star or black hole binaries; and long, soft bursts, which originate in the collapse of very massive stars. Over the last few years, it has become clear that a subset of GRBs are associated with a particular class of supernova, sometimes termed hypernovae. The first clear example of this type of object was probably GRB 030329; an otpical counterpart to the GRB was identified by ground-based telescopes within ~90 minutes, and monitoring over the next few days by the ESO Very Large Telescope showed the gradual energence of a supernovae spectrum, as the GRB afterglow faded (see this link for further details). The aim of the present program is to use the NIC3 camera on the Near Infrared Camera and Multi Object Spectrograph (NICMOS) to image GRBs, and cleanly separate the declining light curves of the afterglow and the underlying supernova.