Nuclear Water Maser Emission in Centaurus A

TL;DR

This paper reports the detection of a water maser in Centaurus A, likely originating from the jet near the supermassive black hole, providing insights into jet composition and surrounding gas interactions.

Contribution

First detection of a water maser in Centaurus A, with evidence suggesting it is a jet maser from shocked gas near the black hole.

Findings

Maser located within <3pc of the black hole

Maser classified as a kilomaser with variable luminosity

Inferred jet density >10cm^-3 indicating substantial gas entrainment

Abstract

We report the detection of a 22GHz water maser line in the nearest (D~3.8Mpc) radio galaxy Centaurus A using the Australia Telescope Compact Array (ATCA). The line is centered at a velocity of ~960kms-1, which is redshifted by about 400kms-1 from the systemic velocity. Such an offset, as well as the width of ~120kms-1, could be consistent with either a nuclear maser arising from an accretion disk of the central supermassive black hole, or for a jet maser that is emitted from the material that is shocked near the base of the jet in Centaurus\,A. The best spatial resolution of our ATCA data constrains the origin of the maser feature within <3pc from the supermassive black hole. The maser exhibits a luminosity of ~1Lo, which classifies it as a kilomaser, and appears to be variable on timescales of months. A kilomaser can also be emitted by shocked gas in star forming regions. Given the small projected distance from the core, the large offset from systemic velocity, as well as the smoothness of the line feature, we conclude that a jet maser line emitted by shocked gas around the base of the AGN is the most likely explanation. For this scenario we can infer a minimum density of the radio jet of ~>10cm-3, which indicates substantial mass entrainment of surrounding gas into the propagating jet material.