deletions | additions       diff --git a/section_OH_Megamasers_label_sec__.tex b/section_OH_Megamasers_label_sec__.tex  index b686226..2123745 100644  --- a/section_OH_Megamasers_label_sec__.tex  +++ b/section_OH_Megamasers_label_sec__.tex 
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\subsection{Extragalactic Properties}  \label{sub:oh_gal_props}  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, universe,  and are almost exclusively caused by major galaxy mergers \citep{clements1996}. In fact, LIRGs and ULIRGs are classified based on their far infrared (FIR) luminosities alone ($L_{\mathrm{FIR}} \ge 10^{11.4}$ L$_{\odot}$ for LIRGs, $L_{\mathrm{FIR}} \ge 10^{12}$ L$_{\odot}$). The star-formation activity is centered around at least one of the nuclei in the system. OH mega-masers tend to be found within $\sim 100$ pc of this nucleus, where there is an abundance of heated dense molecular gas \citep{lo2005}. The extreme star formation rates near the nucleus lead to widespread heating of the dust and gas in the ISM through many giant HII regions, resulting in elevated emission in the IR. \citet{Elitzur_1992} note that IR radiation is the only pumping agent capable of permeating the large galactic volumes required. This connection of a dense, heated molecular gas environment to OH maser emission was recognized early on by \citet{Bottinelli_1987}, as such conditions are the observed locations of galactic OH maser emission. The relation between the far-infrared (FIR) luminosity and the isotropic OH maser luminosity was rigorously established using 95 detections by \citet{darling2002_paperIII}. They find a power-law relation of $L_{\mathrm{OH}} \propto L_{\mathrm{FIR}}^{1.2\pm0.1}$, after thoroughly correcting for the biases in their sample. This updated the initial relationship found by \cite{Baan_1989}, which had an index of 2. This index fits within the expectation for a mix of saturated and unsaturated masers, which is observationally confirmed based on multi-velocity components components in some spectra . \citet{darling2002_paperIII} show that high-gain, unsaturated masers are expected to have an index 2, while an index of 1 should result for low-gain saturated masers. This is related to the source of the stimulated emission: the unsaturated case stimulates the background radio continuum, which itself is correlated with the FIR radiation \citep{Yun_2001}. Furthermore, this result establishes that, despite differing from galactic OH maser emission, OH mega-masers arise due to radiative pumping. This relationship suggests that if even brighter galaxies than the known ULIRGs are discovered, one may find a class of {\it giga}-masers, particularly if mega-maser detections out to $z\sim 2$ can be detected. Two such systems were detected by \citet{darling2002_paperIII}. The properties of OH mega-maser hosts are more thoroughly explored in \S\ref{sub:oh_mergers}. 
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\citet{Darling_2007} extended this search by exploring the connection between star formation rate (calculated from IR luminosity) and the line luminosity of CO. A linear relationship exists between the IR and CO luminosities for star-forming galaxies over many orders of magnitude \citep{Gao_2004}. \citet{Darling_2007} finds the OH mega-maser hosts break this linear trend as shown in Figure \ref{fig:ohm_IR_CO}. This suggests that the hosts of OH mega-masers are undergoing a special triggered star formation event due to the merger. Since the IR-CO relationship describes such a large-range of star-forming galaxies, the observed break from this trend also suggests the star formation events associated with OH mega-masers are relatively short-lived events, as the elevated star-formation rates cannot be maintained \citep{Darling_2007}. However, these triggers may occur multiple times within a merger event as the galactic nuclei execute close interactions.   Further differences between the OH mega-maser hosts and LIRGs without maser emission were found by \citet{willett2011_I,willett2011_II}. In these two papers, the authors use Spitzer IRS spectroscopy to study LIRGs and ULIRGs that were part of the \citet{darling2002_paperIII} survey, both detections and non-detections. The medianed spectra of all sources is shown in Figure \ref{fig:oh_ir_spectra}. There is a clear difference between the galaxies with and without OH mega-masers. Notably, hosts of OH mega-masers show deeper absorption features near 10 and 18 $\mu$m, and the slope at longer wavelengths is steeper. \citet{ivezic1997} show that these are features due to an increase in dust opacity. This also shifts the peak of the IR emission to a maximum between 35-53$\mu$m, which is likely the pumping mechanism for the OH masers \citep{darling2012}. \citet{willett2011_II} then derive the dust temperature and optical depths in their selection of sources. \citep{darling2012}. The model of \citet{lockett2008} predicts that OH mega-masing galaxies should fall in a locus on the dust temperature-optical depth plane, well-separated from non-masing galaxies. The observations of \citet{willett2011_I} do show this separation well XXX FIG?? 9 XXX, (Figure \ref{fig:oh_ir_spectra}),  however the inferred optical depths for the OH maser hosts are an order of magnitude larger than the predicted values. This result is key for determining targets in future surveys \citep{darling2012}.% XXX ??? From McBride+14 XXXThe spectra collected in this survey allowed \citet{willett2011_II} to conclude that the majority of OH mega-maser host galaxies are starbursts, and not AGN.  A further piece of evidence comes from the 6.3 cm H$_{2}$CO line. \citet{Mangum_2008} surveyed nearby star-forming galaxies for this line, and found a clear separation between non-masing galaxies and OH mega-maser hosts: all non-masing galaxies show the line in absorption, while maser hosts show it in emission. This line is collisionally driven to low excitation temperatures and flips between emission and absorption when the excitation temperature crosses the CMB temperature \citep{darling2012}. This suggests that OH mega-maser emission may have a density threshold. % XXX darling 2006?? XXX  These Combined, these  factors point to the cause OH mega-maser emission being suggest  a phase of the merger process is the cause of OH mega-maser emission,  where extreme concentrations of gas and dust are evident. The clear relation between OH mega-masers and major galaxy mergers leads to another potential applications, such as studying super-massive black hole binaries, highly-obscured star formation, merger rates of galaxies \citep{darling_2001_merger} and constraining the gravitational wave background. These are the applications proposed by \citet{darlin2002_lumfunc}, where the authors perform an excruciatingly careful and thorough analysis to derive the OH mega-maser luminosity function:  \begin{equation}