Related papers: The Diffuse Nature of Stromgren Spheres
The expansion of ionization fronts in uniform and spherically symmetric power-law density distributions is a well-studied topic. However, in many situations, such as a star formed at the edge of a molecular cloud core, an offset power-law…
It is possible that the properties of HII regions during reionization depend sensitively on many poorly constrained quantities (the nature of the ionizing sources, the clumpiness of the gas in the IGM, the degree to which photo-ionizing…
Deep H$\alpha$ images of a sample of nearby late-type spiral galaxies have been analyzed to characterize the morphology and energetic significance of the ``Diffuse Ionized Medium'' (DIM). We find that the DIM properties can be reasonably…
The accepted explanation of the observed dichotomy of two orders of magnitude between in situ measurements of electron density in HII regions, derived from emission line ratios, and average measurements based on integrated emission measure,…
Aims: We systematically explore a plausible subset of the parameter space involving effective temperatures and metallicities of the ionizing stellar sources, the effects of the hardening of their radiation by surrounding leaky HII regions…
In this article we first outline the mounting evidence that a significant fraction of the ionizing photons emitted by OB stars within HII regions escape from their immediate surroundings and explain how an HII region structure containing…
We present numerical radiation-hydrodynamic simulations of the evolution of HII regions formed in an inhomogeneous medium resulting from turbulence simulations. We find that the filamentary structure of the underlying density distribution…
We present the results of radiation-magnetohydrodynamic simulations of the expansion of HII regions and surrounding photodissociation regions in turbulent, magnetised, molecular clouds on scales of up to 4 parsecs, including the effects of…
We discuss the theory of coupled ionization--dissociation fronts produced when molecular clouds are exposed to $\lambda < 1110$\AA\ radiation from hot stars. A steady, composite structure is developed, which generally includes an ionized…
We develop an analytic approach to study inhomogeneous reionization on large scales by solving the equations of ionization balance and radiative transfer to first order in perturbations. Given the spatial distribution and spectrum of the…
(Abridged) We present evidence that the HII regions of high luminosity in disk galaxies may be density bounded, so that a significant fraction of the ionizing photons emitted by their exciting OB stars escape from the regions. The key piece…
By comparing radial velocities of radio bright compact HII regions with their HI absorption profiles, we discovered expanding shells of neutral hydrogen around them. These shells are revealed by absorption of the radio continuum emission…
We investigate the resolution dependence of HII regions expanding past their Str\"{o}mgren spheres. We find that their structure and size, and the radial momentum that they attain at a given time, is in good agreement with analytical…
As part of a large study to map the distribution of star formation across galactic disks, we have obtained deep Halpha images of the nearby Sculptor Group spirals NGC 247 and NGC 7793. These images are of sufficiently high quality that they…
The emission line ratios [OIII]5007/H-beta and [NII]6584/H-alpha have been adopted as an empirical way to distinguish between the fundamentally different mechanisms of ionization in emission-line galaxies. However, detailed interpretation…
Recently, there has been a great deal of interest in understanding the reionization of hydrogen in the intergalactic medium (IGM). One of the major outstanding questions is how this event proceeds on large scales. Motivated by numerical…
We carry out a study of the hardness of the radiation of ionizing clusters in HII regions. Firstly, we explore the applicability of the softness parameter, originally defined in the optical for pairs of consecutive ionization stages of the…
(Abridged) From a new H alpha mosaic image of the complete disc of the spiral galaxy M100, a catalogue is composed listing 1948 individual HII regions. I give details of the data collection and reduction procedure, and of the production of…
We examine, from a theoretical viewpoint, how the physical parameters of HII regions are controlled in both normal galaxies and in starburst environments. These parameters are the HII region luminosity function, the time-dependent size, the…
The drift and diffusion of a cloud of ions in a fluid are distorted by an inhomogeneous electric field. If the electric field carries the center of the distribution in a straight line and the field configuration is suitably symmetric, the…