deletions | additions       diff --git a/Hydroxyl_OH_produces_maser_emission__.tex b/Hydroxyl_OH_produces_maser_emission__.tex  index 674b73b..d39b8fc 100644  --- a/Hydroxyl_OH_produces_maser_emission__.tex  +++ b/Hydroxyl_OH_produces_maser_emission__.tex 
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The dominance of 1665 MHz line emission over the 1667 MHz emission from OH mega-masers suggests they form in a different environment than OH galactic masers. Current detections support this, as nearly all OH mega-masers occur in luminous or ultra-luminous infrared galaxies (LIRGs/ULIRGs). These galaxies are unique since they harbour some of the most extreme starburst activity in the universe XXX CITE XXX. The extreme star formation rates lead to widespread heating of the dust and gas in the ISM through many giant HII regions, resulting in elevated emission in the IR. This connection of a dense, heated molecular gas environment to OH maser emission was recognized early on by \cite{Bottinelli_1987}, as such conditions were the observed locations of galactic OH maser emission.   There are two plausible explanations for the production of OH mega-masers, both of which require fairly ordinary conditions \citep{lo2005}. The first occurs from low-gain, unsaturated masers which amplify continuum emission from the heated, diffuse gas around the galactic nucleus \citep[e.g.]{Baan_1985}. There may be some evidence that this type of OH maser emission may be widespread, and only amplified to mega-maser levels in cases of extreme starbursts. Anomalous galactic OH maser emission has been detected within a parsec of warm-IR star-forming regions which match most of the properties (line ratios, unpolarized) seen in extragalactic mega-masers \cite{Mirabel_1989}.  The second explanation  arises from compact regions which have high gains, leading to saturated maser emission \citep[e.g.,]{lonsdale2002}. Each compact source would be an accumulation of many unresolved maser spots, each with narrow linewidths \citet{lo2005}.  Both types of emission have been observed, and \citet{Parra_2005} introduced a model capable of accounting for both types within a single gas phase. XXX ADD PROPS FROM LO 2005 XXX  \begin{itemize}  \item typically almost entirely circularly polarized galactic, not as much for mms (Gray text, Robishaw+08)