Title:	Narrow escape: how ionizing photons escape from disc galaxies
Authors:	Roy, Arpita Nath, Biman Sharma, Prateek
Keywords:	ISM: bubbles H ii regions intergalactic medium galaxies: starburst
Issue Date:	5-May-2015
Publisher:	Oxford University Press on behalf of the Royal Astronomical Society
Citation:	Monthly Notices of Royal Astronomical Society, 2015, Vol. 451, p1939
Abstract:	In this paper we calculate the escape fraction (fesc) 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 n0 = 0.15–50 cm −3) and the disc scaleheight (between z0 = 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 fαescn20z30 (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 fesc ∼ 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:	Restricted Access
URI:	http://hdl.handle.net/2289/7917
ISSN:	0035-8711 1365-2966 (Online)
Alternative Location:	https://doi.org/10.1093/mnras/stv1006
Copyright:	2015 The Author(s)
Appears in Collections:	Research Papers (A&A)

Items in RRI Digital Repository are protected by copyright, with all rights reserved, unless otherwise indicated.