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Narrow escape: how ionizing photons escape from disc galaxies

Roy, Arpita and Nath, Biman B and Sharma, Prateek (2015) Narrow escape: how ionizing photons escape from disc galaxies. In: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 451 (2). pp. 1939-1954.

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Official URL: http://dx.doi.org/10.1093/mnras/stv1006


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 similar to 40 degrees, 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 similar to1 - 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(esc)(alpha)n(0)(2)z(0)(3) (with alpha approximate to 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) similar to 5 per cent, and it increases to approximate 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.

Item Type: Journal Article
Additional Information: Copy right for this article belongs to the OXFORD UNIV PRESS, GREAT CLARENDON ST, OXFORD OX2 6DP, ENGLAND
Keywords: ISM: bubbles; H II regions; intergalactic medium; galaxies: starburst
Department/Centre: Division of Physical & Mathematical Sciences > Joint Astronomy Programme
Division of Physical & Mathematical Sciences > Physics
Date Deposited: 09 Oct 2015 05:02
Last Modified: 09 Oct 2015 05:02
URI: http://eprints.iisc.ac.in/id/eprint/52513

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