Discovery of Methanimine Megamasers Toward Compact Obscured Galaxy Nuclei

Abstract

We present the first search for the 5.29 GHz methanimine(${\mathrm{CH}}_2\mathrm{NH}$) $1_{10}-1_{11}$ 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. ${\mathrm{CH}}_2\mathrm{NH}$ emission is detected toward six CONs. The brightness temperatures measured toward Arp220 indicate maser emission. Isotropic luminosities of the ${\mathrm{CH}}_2\mathrm{NH}$ transition, from all sources where it is detected, exceed 1 L${}_{\odot }$ and thus may be considered megamasers. We also detect formaldehyde (${\mathrm{H}}_2\mathrm{CO}$) emission toward three CONs. The isotropic ${\mathrm{CH}}_2\mathrm{NH}$ luminosities are weakly correlated with the infrared luminosity of the host galaxy and strongly correlated with OH megamaser luminosities from the same galaxies. Non-LTE radiative transfer models suggest that the maser is pumped by the intense mm/submm radiation field of the CONs. Our study suggests that ${\mathrm{CH}}_2\mathrm{NH}$ megamasers are linked to the nuclear processes within 100 pc of the Compton Thick nucleus within CONs.

13 pages, 8 figures, to be published in Astronomy and Astrophysics

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