ALBERT

Description: In this paper we calculate the escape fraction ( f esc ) of ionizing photons from starburst galaxies. Using 2D axisymmetric hydrodynamic simulations, we study superbubbles created by overlapping supernovae in OB associations. We calculate the escape fraction of ionizing photons from the centre of the disc along different angles through the superbubble and the gas disc. After convolving with the luminosity function of OB associations, we show that the ionizing photons escape within a cone of ~40°, consistent with observations of nearby galaxies. The evolution of the escape fraction with time shows that it falls initially as cold gas is accumulated in a dense shell. After the shell crosses a few scaleheights and fragments, the escape fraction through the polar regions rises again. The angle-averaged escape fraction cannot exceed ~[1 – cos (1 rad)] = 0.5 from geometrical considerations (using the emission cone opening angle). We calculate the dependence of the time- and angle-averaged escape fraction on the mid-plane disc gas density (in the range n 0 = 0.15–50 cm –3 ) and the disc scaleheight (between z 0 = 10 and 600 pc). We find that the escape fraction is related to the disc parameters (the mid-plane disc density and scaleheight) roughly so that $f_{\rm esc}^\alpha n_0^2 z_0^3$ (with α 2.2) is a constant. For discs with a given warm neutral medium temperature, massive discs have lower escape fraction than low-mass galaxies. For Milky Way ISM parameters, we find f esc ~ 5 per cent, and it increases to 10 per cent for a galaxy 10 times less massive. We discuss the possible effects of clumpiness of the ISM on the estimate of the escape fraction and the implications of our results for the reionization of the Universe.

Description: We study the conditions for disc galaxies to produce superbubbles that can break out of the disc and produce a galactic wind. We argue that the threshold surface density of supernovae rate for seeding a wind depends on the ability of superbubble energetics to compensate for radiative cooling. We first adapt Kompaneets formalism for expanding bubbles in a stratified medium to the case of continuous energy injection and include the effects of radiative cooling in the shell. With the help of hydrodynamic simulations, we then study the evolution of superbubbles evolving in stratified discs with typical disc parameters. We identify two crucial energy injection rates that differ in their effects, the corresponding breakout ranging from being gentle to a vigorous one. (a) Superbubbles that break out of the disc with a Mach number of the order of 2–3 correspond to an energy injection rate of the order of 10 –4 erg cm –2  s –1 , which is relevant for disc galaxies with synchrotron emitting gas in the extra-planar regions. (b) A larger energy injection threshold, of the order of 10 –3 erg cm –2  s –1 , or equivalently, a star formation surface density of ~0.1 M yr –1 kpc –2 , corresponds to superbubbles with a Mach number ~5–10. While the milder superbubbles can be produced by large OB associations, the latter kind requires super-starclusters. These derived conditions compare well with observations of disc galaxies with winds and the existence of multiphase halo gas. Furthermore, we find that contrary to the general belief that superbubbles fragment through Rayleigh–Taylor (RT) instability when they reach a vertical height of the order of the scaleheight, the superbubbles are first affected by thermal instability for typical disc parameters and that RT instability takes over when the shells reach a distance of approximately twice the scaleheight.

Description: Recent observations have detected molecular outflows in a few nearby starburst nuclei. We discuss the physical processes at work in such an environment in order to outline a scenario that can explain the observed parameters of the phenomenon, such as the molecular mass, speed and size of the outflows. We show that outflows triggered by OB associations, with N OB ≥ 10 5 (corresponding to a star formation rate (SFR)≥1 M  yr –1 in the nuclear region), in a stratified disc with mid-plane density n 0  ~ 200–1000 cm –3 and scaleheight z 0 ≥ 200( n 0 /10 2 cm –3 ) –3/5  pc, can form molecules in a cool dense and expanding shell. The associated molecular mass is ≥10 7 M at a distance of a few hundred pc, with a speed of several tens of km s –1 . We show that an SFR surface density of 10 ≤ SFR ≤ 50 M  yr –1 kpc –2 favours the production of molecular outflows, consistent with observed values.