The Origin of the Diffuse Hα in Spirals

Using high-quality H_α images of five spiral galaxies, we have studied the luminosity and distribution of the emission from diffuse ionized gas (DIG). The estimated DIG luminosities account for 25–60%of the total H_α emission in each galaxy and analysis of the distribution has shown that the DIG is highly correlated geometrically with the most luminous HII regions of the galaxies. The power required to ionize the DIG is very high. The mean ionization rates per unit surface area of a galaxy disc are of the order of 10⁷ cm^-2 s^-1. Lyman continuum photons (Lyc) from OB asociations are the most probable sources of this ionization. Here we propose a specific model for these sources: we show that the Lyman photon flux that leaks out of the density-bounded HII regions of the galaxies is more than enough to ionize the measured DIG in the five galaxies analysed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Fossil Starburst in M82

We present high-resolution optical and near-infrared HST observations of two adjacent regions in the fossil starburst region inM82, M82 B1 and B2. The presence of both the active and the fossil starburst in M82 provides a unique physical environment to study the stellar and dynamical evolution of star cluster systems. The cluster population in B2 is more heavily affected by internal extinction than that in B1, amounting to an excess extinction in B2 of A_V,excess≃1.1±0.3 mag. Preliminary age estimates date the cluster population in the fossil starburst between ∼2× 10⁸ and ∼ 10⁹ years. The radial luminosity profiles of the brightest clusters are more closely approximated by power laws than by a Gaussian model, in particular in their wings, which favours a slow star formation scenario. This revised version was published online in July 2006 with corrections to the Cover Date.     

DETERMINANTS OF STRUCTURE IN AQUATIC NON-PELAGIC PROTOZOAN COMMUNITIES

Although microbes have traditionally been used as models for testing ecological theory, research on the organization of these communities has largely been isolated from other areas of community ecology. Early studies indicated that microbial populations can form dynamic, interacting assemblages. Observations of distributional patterns suggest that certain deterministic forces regulate community structure. It appears that protozoans are tolerant of a wide range of environmental conditions although evidence for this is largely based on gross environmental analyses. Laboratory studies have suggested the importance of negative and positive biotic interactions in determining community membership, but little field evidence for this exists. The importance of transport processes in controlling community composition is being increasingly recognized. All three types of processes likely act to regulate the colonization and successional dynamics of these communities. A simple model is presented to promote the devel     

Use of environmental magnetic measurements to validate the vertical extent of ice masses at the Last Glacial Maximum

Analysis of soil samples from above and below trimlines representing the upper limit of glacial erosion at the Last Glacial Maximum demonstrates that soils with prolonged weathering histories above such trimlines yield significantly different mineral magnetic signatures from soils below trimlines. The nature of the contrast is conditioned by lithology. Basalt soils above the trimline yield significantly higher values of concentration‐dependent magnetic parameters (χ, χ_arm, IRM_3T, soft IRM and hard IRM) than those below the trimline, due probably to transformation of non‐magnetic iron‐bearing minerals into magnetic forms. Conversely, for sandstone soils most magnetic parameters yield significantly lower values for above‐trimline samples, probably reflecting loss of ferrimagnetic minerals by dissolution and oxidation to aniferrimagnetic forms. These significant contrasts represent a new approach to validating high‐level weathering limits as periglacial trimlines cut at the Last Glacial Maximum. Copyright © 2002 John Wiley & Sons, Ltd.     

Propagation of magnetoacoustic-gravity waves in a horizontally-stratified medium: IV. kinematics

A model magneto-atmosphere is used to discuss the behavior of the group velocity for magnetoacoustic-gravity (MAG) waves as a function of their angular frequency. The qualitative frequency/time history of a signal generated by a source differs significantly from the case where the magnetic field is zero, for which a clear distinction may be made between the acoustic and gravity components of the signal.