Second-epoch ALMA Observations of 321 GHz Water Maser Emission in NGC 4945 and the Circinus Galaxy

We present the results of second-epoch ALMA observations of 321 GHz H2O emission toward two nearby active galactic nuclei, NGC 4945 and the Circinus galaxy, together with Tidbinbilla 70 m monitoring of their 22 GHz H2O masers. The two-epoch ALMA observations show that the strengths of the 321 GHz emission are variable by a factor of at least a few, confirming a maser origin. In the second epoch, 321 GHz maser emission from NGC 4945 was not detected, while for the Circinus galaxy the flux density significantly increased and the velocity gradient and dispersion have been measured. With the velocity gradient spanning ∼110 km s−1, we calculate the disk radius to be ∼28 pc, assuming disk rotation around the nucleus. We also estimate the dynamical mass within the central 28 pc to be 4.3 × 108 M ☉, which is significantly larger than the larger-scale dynamical mass, suggesting the velocity gradient does not trace circular motions on that scale. The overall direction of the velocity gradient and velocity range of the blueshifted features are largely consistent with those of the 22 GHz maser emission in a thin disk with smaller radii of 0.1–0.4 pc and molecular outflows within ∼1 pc from the central engine of the galaxy, implying that the 321 GHz masers could trace part of the circumnuclear disk or the nuclear outflows.

The in-situ formation of molecular and warm ionised gas triggered by hot galactic outflows

Molecular outflows contributing to the matter cycle of star forming galaxies are now observed in small and large systems at low and high redshift. Their physical origin is still unclear. In most theoretical studies only warm ionised/neutral and hot gas outflowing from the interstellar medium is generated by star formation. We investigate an in-situ H2 formation scenario in the outflow using high-resolution simulations, including non-equilibrium chemistry and self-gravity, of turbulent, warm, and atomic clouds with densities 0.1, 0.5 and 1 cm−3 exposed to a magnetised hot wind. For cloud densities ≳ 0.5 cm−3 a magnetised wind triggers H2 formation before cloud dispersal. Up to 3 per cent of the initial cloud mass can become molecular on ∼10 Myr time scales. The effect is stronger for winds with perpendicular B-fields and intermediate density clouds (nc ∼ 0.5 cm−3). Here H2 formation can be boosted by up to one order of magnitude compared to isolated cooling clouds independent of self-gravity. Self-gravity preserves the densest clouds well past their ∼15 Myr cloud crushing time scales. This model could provide a plausible in-situ origin for the observed molecular gas. All simulations form warm ionised gas, which represents an important observable phase. The amount of warm ionised gas is almost independent of the cloud density but solely depends on the magnetic field configuration in the wind. For low density clouds (0.1 cm−3), up to 60 per cent of the initially atomic cloud mass can become warm and ionised.