Discovery of methanimine (CH2NH) megamasers toward compact obscured galaxy nuclei

We present the first search for the 5.29 GHz methanimine (CH2 NH) 110  − 111 transition toward a sample of galaxy nuclei. We target seven galaxies that host compact obscured nuclei (CONs) with the Karl G. Jansky Very Large Array. These galaxies are characterized by Compton-thick cores. CH2 NH emission is detected toward six CONs. The brightness temperatures measured toward Arp 220 indicate maser emission. Isotropic luminosities of the CH2 NH transition, from all sources where it is detected, exceed 1 L ⊙ and thus may be considered megamasers. We also detect formaldehyde (H2 CO) emission toward three CONs. The isotropic CH2 NH luminosities are weakly correlated with the infrared luminosity of the host galaxy and strongly correlated with OH megamaser luminosities from the same galaxies. Non-local thermodynamic equilibrium radiative transfer models suggest that the maser is pumped by the intense millimeter-to-submillimeter Our study suggests that CH2 NH megamasers are linked to the nuclear processes within 100 pc of the Compton-thick nucleus within CONs.