Cold molecular gas outflow encasing the ionized one in the Seyfert galaxy NGC 3281

Abstract

We present ALMA CO (2-1) observations of the Seyfert 2 galaxy NGC 3281 at ∼ 100 pc spatial resolution. This galaxy was previously known to present a bi-conical ionised gas outflow extending to 2 kpc from the nucleus. The analysis of the CO moment and channel maps, as well as kinematic modelling reveals two main components in the molecular gas: one rotating in the galaxy plane and another outflowing and extending up to ∼ 1.8 – 2.6 kpc from the nucleus, partially encasing the ionised component. The mass of the outflowing molecular gas component is Mmol, out = (2.5 ± 1.6) × 106 $\rm {M_{\odot }}$, representing ∼ 1.7 – 2 % of the total molecular gas seen in emission within the inner 2.3 kpc. The corresponding mass outflow rate and power are $\dot{M}_{\mathrm{out},\mathrm{mol}}$ = 0.12 – 0.72 $\rm {M_{\odot }\, yr^{-1}}$ and $\dot{E}_{\mathrm{out},\mathrm{mol}}$ = (0.045 – 1.6) × 1040 $\rm {erg\, s^{-1}}$, which translates to a kinetic coupling efficiency with the AGN power of only 10−4 – 0.02 %. This value reaches up to 0.1 % when including both the feedback in the ionised and molecular gas, as well as considering that only part of the energy couples kinetically with the gas. Some of the non-rotating CO emission can also be attributed to inflow in the galaxy plane towards the nucleus. The similarity of the CO outflow – encasing the ionised gas one and the X-ray emission – to those seen in other sources, suggests that this may be a common property of galactic outflows.