141 research outputs found
The morphology of HII regions during reionization
It is possible that the properties of HII regions during reionization depend
sensitively on many poorly constrained quantities (the nature of the ionizing
sources, the clumpiness of the gas in the IGM, the degree to which
photo-ionizing feedback suppresses the abundance of low mass galaxies, etc.),
making it extremely difficult to interpret upcoming observations of this epoch.
We demonstrate that the actual situation is more encouraging, using a suite of
radiative transfer simulations, post-processed on outputs from a 1024^3, 94 Mpc
N-body simulation. Analytic prescriptions are used to incorporate small-scale
structures that affect reionization, yet remain unresolved in the N-body
simulation. We show that the morphology of the HII regions is most dependent on
the global ionization fraction x_i. This is not to say that the bubble
morphology is completely independent of all parameters besides x_i. The next
most important dependence is that of the nature of the ionizing sources. The
rarer the sources, the larger and more spherical the HII regions become. The
typical bubble size can vary by as much as a factor of 4 at fixed x_i between
different possible source prescriptions. The final relevant factor is the
abundance of minihalos or of Lyman-limit systems. These systems suppress the
largest bubbles from growing, and the magnitude of this suppression depends on
the thermal history of the gas as well as the rate at which minihalos are
photo-evaporated. We find that neither source suppression owing to
photo-heating nor gas clumping significantly affect the large-scale structure
of the HII regions. We discuss how observations of the 21cm line with MWA and
LOFAR can constrain properties of reionization, and we study the effect patchy
reionization has on the statistics of Lyman-alpha emitting galaxies. [abridged]Comment: 23 pages, 18 figure
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