We combine the the stellar spectral synthesis code Starburst99, the nebular modelling code MAP-PINGS iii and a 1-D dynamical evolution model of H ii regions around massive clusters of young stars to generate improved models of the spectral energy distribution (SED) of starburst galaxies. We introduce a compactness parameter, C, which characterizes the specific intensity of the radiation field at ionization fronts in H ii regions, and which controls the shape of the far-IR dust re-emission, often referred to loosely as the dust "temperature". We also investigate the effect of metallicity on the overall SED and in particular, on the strength of the PAH features. We provide templates for the mean emission produced by the young compact H ii regions, the older (10 − 100 Myr) stars and for the wavelength-dependent attenuation produced by a foreground screen of the dust used in our model. We demonstrate that these components may be combined to produce a excellent fit to the observed SEDs of star formation dominated galaxies which are often used as templates (Arp 220 and NGC 6240). This fit extends from the Lyman Limit to wavelengths of about one mm. The methods presented in both this paper and in the previous papers of this series allow the extraction of the physical parameters of the starburst region (star formation rates, star formation rate history, mean cluster mass, metallicity, dust attenuation and pressure) from the analysis of the pan-spectral SED.

We present a self-consistent model of the spectral energy distributions (SEDs) of spiral galaxies from the ultraviolet (UV) to the mid-infrared (MIR)/far-infrared (FIR)/submillimeter (submm) based on a full radiative transfer calculation of the propagation of starlight in galaxy disks. This model predicts not only the total integrated energy absorbed in the UV/optical and re-emitted in the infrared/submm, but also the colours of the dust emission based on an explicit calculation of the strength and colour of the UV/optical radiation fields heating the dust, and incorporating a full calculation of the stochastic heating of small dust grains and PAH molecules. The geometry of the translucent components of the model is empirically constrained using the results from the radiation transfer analysis of Xilouris et al. on spirals in the middle range of the Hubble sequence, while the geometry of the optically thick components is constrained from physical considerations with a posteriori checks of the model predictions with observational data. Following the observational constraints, the model has both a distribution of diffuse dust associated with the old and young disk stellar populations as well as a clumpy component arising from dust in the parent molecular clouds in star forming regions. In accordance with the fragmented nature of dense molecular gas in typical star-forming regions, UV light from massive stars is allowed to either freely stream away into the diffuse medium in some fraction of directions or be geometrically blocked and locally absorbed in clumps. These geometrical constraints enable the dust emission to be predicted in terms of a minimum set of free parameters: the central face-on dust opacity in the B-band τ f B , a clumpiness factor F for the star-forming regions, the star-formation rate SFR, the normalised luminosity of the old stellar population old and the bulge-to-disk ratio B/D. We show that these parameters are almost orthogonal in their predicted effect on the colours of the dust/PAH emission. In most practical applications B/D will actually not be a free parameter but (together with the angular size θ gal and inclination i of the disk) act as a constraint derived from morphological decomposition of higher resolution optical images. This also extends the range of applicability of the model along the Hubble sequence. We further show that the dependence of the dust emission SED on the colour of the stellar photon field depends primarily on the ratio between the luminosities of the young and old stellar populations (as specified by the parameters SFR and old) rather than on the detailed colour of the emissions from either of these populations. The model is thereby independent of a priori assumptions of the detailed mathematical form of the dependence of SFR on time, allowing UV/optical SEDs to be dereddened without recourse to population synthesis models. Utilising these findings, we show how the predictive power of radiative transfer calculations can be combined with measurements of θ ...

We build, as far as theory will permit, self consistent model H II regions around central clusters of aging stars. These produce strong emission line diagnostics applicable to either individual H II regions in galaxies, or to the integrated emission line spectra of disk or starburst galaxies. The models assume that the expansion and internal pressure of individual H II regions is driven by the net input of mechanical energy from the central cluster, be it through winds or supernova events. This eliminates the ionization parameter as a free variable, replacing it with a parameter which depends on the ratio of the cluster mass to the pressure in the surrounding interstellar medium. These models explain why H II regions with low abundances have high excitation, and demonstrate that at least part of the warm ionized medium is the result of overlapping faint, old, large, and low pressure H II regions. We present line ratios (at both optical and IR wavelengths) which provide reliable abundance diagnostics for both single H II regions or for integrated galaxy spectra, and we find a number that can be used to estimate the mean age of the cluster stars exciting individual H II regions.

We analyze the physical parameters of interstellar filaments that we describe by an idealized model of isothermal self-gravitating infinite cylinder in pressure equilibrium with the ambient medium. Their gravitational state is characterized by the ratio f cyl of their mass line density to the maximum possible value for a cylinder in a vacuum. Equilibrium solutions exist only for f cyl < 1. This ratio is used in providing analytical expressions for the central density, the radius, the profile of the column density, the column density through the cloud centre, and the FWHM. The dependence of the physical properties on external pressure and temperature is discussed and directly compared to the case of pressure-confined isothermal self-gravitating spheres. Comparison with recent observations of the FWHM and the central column density N H (0) show good agreement and suggest a filament temperature of ∼10 K and an external pressure in the range 1.5 × 10 4 K cm −3 to 5 × 10 4 K cm −3 . Stability considerations indicate that interstellar filaments become increasingly gravitationally unstable with mass line ratio f cyl approaching unity. For intermediate f cyl > 0.5 the instabilities should promote core formation through compression, with a separation of about five times the FWHM. We discuss the nature of filaments with high mass line densities and their relevance to gravitational fragmentation and star formation.