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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}. \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, 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.