deletions | additions       diff --git a/h2o_megamasers.tex b/h2o_megamasers.tex  index 807c064..76e2aef 100644  --- a/h2o_megamasers.tex  +++ b/h2o_megamasers.tex 
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\section{H_2O Megamasers}  \label{sec:h2o_mm}  Multiple H$_2$O transitions have been observed producing maser emission in galactic source \citep{Elitzur_1992}, however the dominant transition detected as stimulated emission is the at  22 GHztransition  (see Table \ref{tab:maser_props}). This transition is relatively easy to cause stimulated emission in since it corresponds at a `backbone' transition (lowest levels at each rotational $J$-value). These transitions have large Einstein $A$-values, leading to trapping of the emitted photons \citep{stahler_palla_2004}. This makes \citep{stahler_palla_2004}, making  collisional excitement and de-excitement more effective than radiation in producing these transitions. The environment needed to produce the 22 GHz H$_2$O emission must pump molecules to fairly high energy levels (640 K, Table \ref{tab:maser_props}). As shown by \citet{stahler_palla_2004}, shocked regions are capable of providing collisional excitement to these levels, and indeed this is where H$_2$O maser emission is observed in both galactic galactic,  and extragalactic possibly extragalactic,  sources \citep{Elitzur_1992, lo2005}. %  The 22 GHz H$_2$O maser emission is typically associated with circumstellar material around late-type stars, and with molecular outflows due to young, embedded stars. These environments suggest a collisional pumping mechanism. H$_2$O mega-maser emitting regions are typically found within a few parsecs of an AGN, coinciding with regions of dense molecular gas heated to suitable temperatures for masers to occur. Approximately 3000 galaxies have been searched for H$_2$O mega-masers, resulting in about 150 detections \citep[][based on compiled information from the Megamaser Cosmology Project, \S\ref{sub:h2o_cosmo}]{tarchi2012}.  \begin{itemize}  \item bennert+09, tarchi+12, tarchi+07, cernicharo+06, baan+06, baan klockner 05, baan+82, dos santos lepine 79 
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\subsection{Extragalactic Properties}  \label{sub:h2o_props}  Approximately 3000 galaxies have been searched for H$_2$O mega-masers, resulting in about 150 detections \citet{tarchi2012}...  \begin{itemize}  \item sales+15, wagner+13, raluy+1998  \item circumnuclear and nuclear jet driven masers and outflow 
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\item Could be due to: lack of molecular gas (henkel+98); instability from tidal disruption in clouds orbiting particularly massive SMBHs (tarchi+07b); insufficient sensitivity of completed surveys  \end{itemize}  \subsection{AGN: Disks \& SMBH masses}  \label{sub:h20_smbh_mass} \subsection{Probing the Unified Model of AGN}  \label{sub:h20_agn}  \begin{itemize}  \item greene+2013, Wardle Yusef-zadeh 12, greene+10, schulz henkel 03, tarchi+12