HCN(1-0) Maser Emission by IR Pumping from Unresolved AGN Disk/Torus Toward the Seyfert 2 Nucleus of M51?

Paper in proceeding, 2015

We present similar to 1 '' (similar to 34 pc) resolution observations of HCN(1-0) together with CO J=1-0, 2-1, and 3-2 toward the Seyfert 2 nucleus of M51 using IRAM PdBI and SMA. HCN shows a strong emission at the nucleus only at the systemic velocity, where no obvious CO emission. HCN(1-0)/CO(1-0) > 2 at this region. Based on our radiative transfer calculations, we suggest that this strong HCN emission is affected by the IR pumping and possibly weak HCN masing. This suggests the presence of an edge-on rotating circumnuclear dense molecular gas disk or torus, which remains unresolved at our resolution.