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

We present the first search for the 5.29 GHz methanimine (CH2NH) 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. CH2NH emission is detected toward six CONs. The brightness temperatures measured toward Arp 220 indicate maser emission. Isotropic luminosities of the CH2NH transition, from all sources where it is detected, exceed 1 Lpdbl and thus may be considered megamasers. We also detect formaldehyde (H2CO) emission toward three CONs. The isotropic CH2NH 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 CH2NH megamasers are linked to the nuclear processes within 100 pc of the Compton-thick nucleus within CONs