deletions | additions       diff --git a/h2o_megamasers.tex b/h2o_megamasers.tex  index 8dee95a..0ced6b1 100644  --- a/h2o_megamasers.tex  +++ b/h2o_megamasers.tex 
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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 at 22 GHz (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}, 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 galactic, and 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\footnote{based on information compiled from the Megamaser Cosmology Project, \S\ref{sub:h2o_cosmo}} \citep{tarchi2012}. Nearly all of the detections come from active galaxies, specifically radio-quiet AGN classified as Seyfert 2 (Sy2) or low-ionization nuclear emission-line regions (LINERs) with redshifts up to 0.05 (\S\ref{sub:h20_agn}). These galaxies tend to all  host the `circumnuclear' H$_2$O mega-masers associated with compact disks within parsec scales of the AGN \citep{lo2005}. Two other types of H$_2$O mega-masers are observed corresponding to interactions with a nuclear jet and molecular outflows \citep{lo2005, tarchi2012}. There are few fewer  detections of masers from these mechanisms, and so I focus on circumnuclear masers in this section. In galactic sources, 22 GHz H$_2$O masers are associated with circumstellar material around late-type stars, and with molecular outflows due to young, embedded stars. These environments, and the reasons stated above, highly suggest a collisional pumping mechanism \citep{Elitzur_1992}. Circumnuclear H$_2$O mega-masers appear to arise from a related mechanism to the circumstellar sources, except that the ultimate energy source of the pump is the AGN itself \citep{lo2005}. The energy output of the AGN is significantly greater than that from late-type stars, which possibly explains why these maser sources are much brighter.  Numerous surveys have been undertaken to find H$_2$O mega-maser detections, notably by \citet{Braatz_1996}, who surveyed {\it all} nearby galaxies with AGN characteristics (see also \citet{Braatz_1997}). The Megamaser Cosmology Project is an on-going H$_2$O mega-maser survey of known sources that combines high- and low-resolution observations over long time periods to obtain geometric distances to galaxies beyond $\gt 100$ Mpc, and infer H$_0$ to a high precision, independent of other methods (\S\ref{sub:h2o_cosmo}) \citep{reid2009_mmproject_I}. Given the applications discussed in the preceding sections, many more H$_2$O mega-maser will be conducted as more sensitive instrumentation becomes available. 
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\subsection{Circumnuclear H$_2$O mega-masers}  \label{sub:h20_agn}  Circumnuclear H$_2$O mega-masers exhibit emission at three peaks in the velocity dimension: one centered at the systematic velocity of the AGN, and two red- and blue-shifted from the center by hundreds of km/s. Through high-resolution observations with the VLBI, the maser sources are resolved out to show that these components are distributed in a near planar distribution, where each velocity component arises from different regions in a parsec-scale disk around the nucleus. Assuming the disk is seen edge-on (as expected for Sy2 galaxies), the shifted velocity components correspond to components moving toward and away from the observer. This is direct proof of the expected circumnuclear disks predicted by the Unified Model \citep{lo2005}. An example observation is shown in Figure XXX for NGC 4258 \citep{Bragg_2000}. Other H$_2$O mega-maser spectra show similar properties, along with the further evidence for a rotating disk from the velocity components increasing in time \citep{lo2005}.  \begin{itemize}  \item greene+2013, Wardle Yusef-zadeh 12, greene+10, schulz henkel 03, tarchi+12  \item these are circumnuclear masers  \item Unified model of AGN (antonucci 93, urry+padovani 95) predicts an accretion disk about AGN surrounded by torus or thick disk  \item Type 1 AGN - view accretion disk and BH through hole in torus; type 2 - direct view of nucleus obscured by torus  \item masers are unique tool to probe about pc regions around AGN -- other bands are highly obscured  \item characteristically have triple peak in velocity -- center at systematic vel of galaxy, w shifted by hundreds of km/s -- rotation of disk  \item NGC 4258 best studied for this (Miyoshi+95, Herrnstein+99) -- MCP has greatly extended this