The Origin and Evolution of Lyman-alpha Blobs in Cosmological Galaxy
  Formation Simulations
  High-redshift Lyman-alpha blobs (LABs) are an enigmatic class of objects that
have been the subject of numerous observational and theoretical investigations.
It is of particular interest to determine the dominant power sources for the
copious luminosity, as direct emission from HII regions, cooling gas, and
fluorescence due to the presence of active galactic nuclei (AGN) can all
contribute significantly. In this paper, we present the first theoretical model
to consider all of these physical processes in an attempt to develop an
evolutionary model for the origin of high-z LABs. This is achieved by combining
a series of high-resolution cosmological zoom-in simulations with ionization
and Lyman-alpha (Lya) radiative transfer models. We find that massive galaxies
display a range of Lya luminosities and spatial extents (which strongly depend
on the limiting surface brightness used) over the course of their lives, though
regularly exhibit luminosities and sizes consistent with observed LABs. The
model LABs are typically powered from a combination of recombination in
star-forming galaxies, as well as cooling emission from gas associated with
accretion. When AGN are included in the model, the fluorescence caused by
AGN-driven ionization can be a significant contributor to the total Lya
luminosity as well. We propose that the presence of an AGN may be predicted
from the Gini coefficient of the blob's surface brightness. Within our modeled
mass range, there are no obvious threshold physical properties that predict
appearance of LABs, and only weak correlations of the luminosity with the
physical properties of the host galaxy. This is because the emergent Lya
luminosity from a system is a complex function of the gas temperature,
ionization state, and Lya escape fraction.
