Galaxy Threshing and Ultra-Compact Dwarfs in the Fornax Cluster

We have discovered a new type of galaxy in the Fornax Cluster: `ultra-compact' dwarfs (UCDs). The UCDs are unresolved in ground-based imaging and have spectra typical of old stellar systems. Although the UCDs resemble overgrown globular clusters, based on VLT UVES echelle spectroscopy, they appear to be dynamically distinct systems with higher internal velocity dispersions and M/L ratios for a given luminosity than Milky Way or M31 globulars. Our preferred explanation for their origin is that they are the remnant nuclei of dwarf elliptical galaxies which have been tidally stripped, or `threshed' by repeated encounters with the central cluster galaxy, NGC1399. If correct, then tidal stripping of nucleated dwarfs to form UCDs may, over a Hubble time, be an important source of the plentiful globular cluster population in the halo of NGC1399, and, by implication, other cD galaxies. In this picture, the dwarf elliptical halo contents, up to 99% of the original dwarf luminosity, contribute a significant fraction of the populations of intergalactic stars, globulars, and gas in galaxy clusters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Plasma clouds associated with Comet P/Borrelly dust impacts

The NASA DS1 spacecraft encountered Comet P/Borrelly on September 22, 2001 at a distance of ∼2171 km on the sunward side of the comet. The flyby speed was ∼16.5 km s⁻¹. Using high temporal resolution (50 μs) absolute electric field amplitude measurements from a ∼1 m dipole antenna, new features of plasma clouds created by cometary dust impacts have been detected. The pulses have 1/e exponential decays of ∼650 μs duration, exponentially shaped overshoots with rise times of ∼2 ms, and exponential-shaped overshoot decay times of ∼10 ms. Assuming a plasma temperature of 10⁴ K, these pulse features have been explained as plasma cloud space charge effects from the electron, proton and heavy ion portions of the clouds passing the antenna. Complex pulse shapes were also detected. These are believed to be due to either plasma cloud scattering off of the spacecraft, or to secondary impacts. Small electric pulses of duration 10-15 ms of cometary origin were detected but are presently unexplained. The electric component of the plasma wave spectra at closest approach had an f^−2.4 power law shape from 10 Hz to 1 kHz. The electron cyclotron frequency was approximately 1 kHz. One possible explanation of the wave spectrum is that whistler mode waves associated with phase steepened cometary plasma waves are dispersed, leading to the broad spectrum. Finally, based on the present results, a new type of low-cost, large-area dust detector is proposed.     

Radio Investigations Of 19p/Borrelly In Support To The Deep Space 1 Flyby

NASA's Deep Space 1 mission flew by Comet 19P/Borrelly on September 22, 2001.We present observations of molecular species obtained with the 30-m telescope of theInstitut de Radioastronomie Millimétrique (IRAM) and the Nançay radio telescopeat and near the time of this flyby. OH, HCN, and CS production rates were measured,while upper limits were deduced for CO, H₂CO and H₂S.     

Estimation of bottom scattering strength from measured and modeledmid-frequency sonar reverberation levels

Hamilton-type geoacoustic models were developed for Area Foxtrot, a shallow water test bed south of Long Island, for emerging active sonar systems where the surface sediment type is highly spatially variable. Reverberation levels (RL) were modeled using the finite-element parabolic equation (FEPE) propagation model to augment the generic sonar model (GSM) propagation model because the bottom loss model in GSM did not estimate transmission loss (TL) accurately in shallow water. FEPE estimates reveal that there is a greater than 15 dB difference between TL for sand and that for silt-day sediments in Area Foxtrot. The comparison between modeled RL and measured RL (from a 1991 active sonar exercise) enabled bottom scattering strength kernels to be developed for Area Foxtrot. Bottom scattering strength was found to be a function of sediment type. Hard sand sediment has a bottom scattering strength which obeys Lambert's law (sin² &thetas;) while that of silt-clay sediment is consistent with sub-bottom volume scattering (sine). The RLs in Area Foxtrot are azimuth-dependent and are a function of TL and bottom scattering strength (and hence bottom sediment type). Sonar beams steered towards the hard sand show higher RLs than for silt-clay, and knowledge of the sediment type and its spatial variation must be known to model RL accurately. A method to determine sediment type using measured RLs and RL slopes is given     

10.7-cm Solar radio flux and the magnetic complexity of active regions

During sunspot cycles 20 and 21, the maximum in smoothed 10.7-cm solar radio flux occurred about 1.5 yr after the maximum smoothed sunspot number, whereas during cycles 18 and 19 no lag was observed. Thus, although 10.7-cm radio flux and Zürich suspot number are highly correlated, they are not interchangeable, especially near solar maximum. The 10.7-cm flux more closely follows the number of sunspots visible on the solar disk, while the Zürich sunspot number more closely follows the number of sunspot groups. The number of sunspots in an active region is one measure of the complexity of the magnetic structure of the region, and the coincidence in the maxima of radio flux and number of sunspots apparently reflects higher radio emission from active regions of greater magnetic complexity. The presence of a lag between sunspot-number maximum and radio-flux maximum in some cycles but not in others argues that some aspect of the average magnetic complexity near solar maximum must vary from cycle to cycle. A speculative possibility is that the radio-flux lag discriminates between long-period and short-period cycles, being another indicator that the solar cycle switches between long-period and short-period modes.Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.     

The cold-water coral Lophelia pertusa (Scleractinia) and enigmatic seabed mounds along the north-east Atlantic margin: are they related?

In this study, an updated distribution of Lophelia pertusa between the Porcupine Seabight and Norwegian shelf is presented. It seems unlikely that enigmatic mound structures observed at water depths of more than 570 m during acoustic seabed surveys, particularly to the west of the Shetland Islands, are related to the occurrence of L. pertusa. At these depths in the Faroe-Shetland Channel, the predominant influence of cold Arctic water precludes its growth. Iceberg dumpsites are also considered unlikely explanations for the origin of these mounds, and they are interpreted as most likely to be related to the release of fluids at the seabed. When mound structures were investigated, no scleractinian corals were recovered at water depths >500 m. This study shows the importance of seabed temperature as an environmental control on cold-water coral distribution. The significance of cold-water coral habitats in sustaining high levels of local-scale biodiversity is now becoming apparent in parallel with increased hydrocarbon extraction and fishing activity beyond the shelf edge. There is growing evidence that these areas have been marked by the passage of deep-water trawls. It seems likely that trawling activity has already reduced the extent of cold-water coral distribution in this region of the north-east Atlantic.     

Acoustic characteristics of fish bombing: potential to develop an automated blast detector

The use of explosives to catch fish has caused extensive damage to coral reefs throughout Southeast Asia, but the frequency with which they are used is largely unknown. The aim of this work is to develop a detection system capable of distinguishing underwater explosions from background noise, and locating their origin by triangulation. Blast signals have been recorded over a range of distances and the key features that differentiate them from background noise have been determined. For small charges the effective range of such a detector is more than 12 km and may extend up to 50 km depending on the mass of the charges being used. Such a system would help to determine the scale of the problem, identify areas at greatest risk and quantify the effectiveness of management intervention designed to control destructive fishing practices. It may assist with fisheries enforcement in some areas.