Line transfer through clumpy, large-scale outflows: Ly α absorption and haloes around star-forming galaxies

Abstract

We present constrained radiative transfer calculations of Lyman Alpha (Lya) photons propagating through clumpy, dusty, large scale outflows, and explore whether we can quantitatively explain the Lya halos that have been observed around Lyman Break Galaxies. We construct phenomenological models of large-scale outflows which consist of cold clumps that are in pressure equilibrium with a constant-velocity hot wind. First we consider models in which the cold clumps are distributed symmetrically around the galaxy, and in which the clumps undergo a continuous acceleration in its 'circumgalactic' medium (CGM). We constrain the properties of the cold clumps (radius, velocity, HI column density, & number density) by matching the observed Lya absorption strength of the CGM in the spectra of background galaxies. We then insert a Lya source in the center of this clumpy outflow, which consists of 1e5-1e6 clumps, and compute observable properties of the scattered Lya photons. In these models, the scattered radiation forms halos that are significantly more concentrated than is observed. In order to simultaneously reproduce the observed Lya absorption line strengths and the Lya halos, we require - preferably bipolar - outflows in which the clumps decelerate after their initial acceleration. This deceleration is predicted naturally in 'momentum-driven' wind models of clumpy outflows. In models that simultaneously fit the absorption and emission line data, the predicted linear polarization is ~30-40% at a surface brightness contour of S=1e-18 erg/s/cm^2/arcsec^2. Our work illustrates clearly that Lya emission line halos around starforming galaxies provide valuable constraints on the cold gas distribution & kinematics in their circumgalactic medium, and that these constraints complement those obtained from absorption line studies alone.