Line Transfer through Clumpy, Large-Scale Outflows: Lyman Alpha
  Absorption and Halos around Starforming Galaxies
  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.
