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Abstract

The Mg ii resonance doublet at 2796 & Aring; and 2803 & Aring; is an increasingly important tool to study cold, T similar to 10(4 )K, gas - an observational-driven development requiring theoretical support. We develop a new Monte Carlo radiative transfer code to systematically study the joined Mg ii and Ly alpha escape through homogeneous and 'clumpy' multiphase gas with dust in arbitrary three-dimensional geometries. Our main findings are (i) the Mg ii spectrum differs from Ly alpha due to the large difference in column densities, even though the atomic physics of the two lines are similar. (ii) The Mg ii escape fraction is generally higher than that of Ly alpha because of lower dust optical depths and path lengths - but large variations due to differences in dust models and the clumpiness of the cold medium exist. (iii) Clumpy media possess a 'critical covering factor' above which Mg ii radiative transfer matches a homogeneous medium. The critical covering factors for Mg ii and Ly alpha differ, allowing constraints on the cold gas structure. (iv) The Mg ii doublet ratio R-MgII varies for strong outflows/inflows (greater than or similar to 700kms(-1)), in particular, R-MgII<1 being an unambiguous tracer for powerful galactic winds. (v) Scattering of stellar continuum photons can decrease R-MgII from two to one, allowing constraints on the scattering medium. Notably, we introduce a novel probe of the cold gas column density - the halo doublet ratio - which we show to be a powerful indicator of ionizing photon escape. We discuss our results in the context of interpreting and modelling observations as well as their implications for other resonant doublets.