deletions | additions       diff --git a/section_OH_Megamasers_label_sec__.tex b/section_OH_Megamasers_label_sec__.tex  index d752768..6c821f8 100644  --- a/section_OH_Megamasers_label_sec__.tex  +++ b/section_OH_Megamasers_label_sec__.tex 
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The evolution of galaxies remains a substantial question in astrophysics. Observationally, it has long been established that major galaxies will undergo interactions and mergers, and many systems have been observed in this process. The massive tidal disruptions give rise to near global instability in the molecular (and maybe atomic) ISM, leading to extreme outbursts. The clear relation between LIRGs and ULIRGs to major mergers, and the ability for OH mega-masers to trace regions near to the nucleus, makes them excellent tools for understanding the evolution of galaxies through mergers. However, of the observed ULIRGs, only $\sim 1/3$ exhibit OH mega-maser emission \citep{darling2002_paperIII}. This drops to $1/5.5$ in the entire sample including LIRGs.  This raises the obvious question put forward by \citet{lo2005}: what properties of these galaxy mergers give rise of OH mega-maser emission? And what does this tell us about the merger process? Significant work has been accomplished in this area within the last few years, and many clear answers to these questions have arisen \citep{darling2012}. The correlation with FIR luminosity is well-established \citep{darling2002_paperIII}, however there is substantial scatter in the correlation which hinders its use for distinguishing between masing and non-masing LIRGs. This is not suprising, since the scales over which each type of emission occurs is vastly different (FIR luminosity is measured over the entire galaxy). \citet{darling2002_paperIII} also explored the correlation of OH maser luminosity to other features of the host galaxy... galaxy, notably the IR `colour' $\log_{10}(f_{100\mu\mathrm{m}}/f_{60\mu\mathrm{m}})$ (see Figure \ref{fig:oh_props}). \citet{darling2002_paperIII} utilize Kapler-Meier (K-M) survival analysis \citep{Feigelson_1985} to explore the significance between the masing and non-masing distributions in their survey. They find that there is a strong rise in FIR colour between $\sim0.4-0$ for OH mega-maser hosts, indicating that LIRGs with `warmer' colours may be preferred. Their K-M analysis confirms this to high confidence. \citet{darling2002_paperIII} also explore the further relations between the FIR and OH maser properties, without a clear result.   \cite{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 XXX FIGURE XXX, suggesting that the hosts of OH mega-masers are undergoing a special triggered star formation event due to the merger.  Further differences between the OH mega-maser hosts and LIRGs without maser emission were found by \citet{willett2011_I, willett_2011_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}.  By also comparing with a dense gas tracer, HCN, \citet{Darling_2007} finds a high dense molecular gas fraction, L$_{\mathrm{HCN}}$/L$_{\mathrm{IR}} \gt 0.07$...  These factors point to the cause OH mega-maser emission being a phase of the merger process where extreme concentrations of gas and dust are evident \citep{darling2012}.  The clear relation between OH mega-masers and major galaxy mergers leads to another potential applications, such as understanding the formation of super-massive black holes (SMBHs) and constraining cosmological parameters (this is already being done for H$_2$O mega-masers, \S\ref{sub:h2o_cosmo}).   \begin{itemize}  \item LIRGS w/ and w/o mm emission (Lo); BGC 6240 examples if ULIRG w/o mm emission (Tacconi 1999)  \item role of IR radiation  \item Arecibo survey (Darling papers)  \item properties + relations determined in Darling paper III  \item relation to H2CO emission - all non-masing merges show 6cm H2CO absorption, all masing show emission (Mangum+08)  \item ~100 pc extents from intense starbursts w/ high concentrations of molecular gas  \item relation to CO luminosity (Darling+07) - OHM hosts break-away from the normal IR-CO correlation in SF galaxies (Gao solomon, 04)