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

TL;DR

This study reports second epoch ALMA observations of 321 GHz water masers in NGC 4945 and the Circinus galaxy, revealing variability, velocity gradients, and potential links to circumnuclear disks or outflows, enhancing understanding of AGN environments.

Contribution

First detection of variability and velocity structure in 321 GHz water masers in two AGNs, linking them to nuclear disks and outflows, with estimates of disk size and dynamical mass.

Findings

321 GHz masers are variable, confirming their maser nature.

Velocity gradient suggests a disk radius of ~28 pc and a dynamical mass of 4.3×10^8 M_sun.

Masers may trace circumnuclear disks or nuclear outflows.

Abstract

We present the results of second epoch ALMA observations of 321 GHz H$_2$O emission toward two nearby active galactic nuclei, NGC 4945 and the Circinus galaxy, together with Tidbinbilla 70-m monitoring of their 22 GHz H$_2$O 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 $\sim$110 km/s, we calculate the disk radius to be $\sim$28 pc, assuming disk rotation around the nucleus. We also estimate the dynamical mass within the central 28 pc to be 4.3 $\times$ 10$^8$ M$_{\odot}$, 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 $\sim$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.