Detection of a 6.7 GHz methanol kilomaser toward NGC4945

TL;DR

This paper reports the first detection of 6.7 GHz methanol emission in NGC4945, revealing a diffuse, low-gain maser likely associated with nuclear outflows, and significantly more luminous than Galactic counterparts.

Contribution

It presents the first detection of extragalactic 6.7 GHz methanol emission beyond the local group, suggesting a new class of diffuse masers in galaxy centers.

Findings

Methanol emission is highly luminous, about 10,000 times greater than Galactic masers.

Emission is spatially resolved on scales smaller than 40 pc.

The emission likely originates from a diffuse, low-gain maser amplifying nuclear continuum radiation.

Abstract

We report the detection of emission from the 6.7 GHz 5(1)-6(0)A+ transition of methanol towards the center of the nearby galaxy NGC4945. This is the first detection of emission in this transition beyond the local group. The isotropic luminosity of the integrated 6.7 GHz methanol emission is approximately a factor of 10000 greater than that for 6.7 GHz methanol masers associated with Galactic high-mass star formation regions. The methanol emission is resolved on scales smaller than 40 pc and it appears unlikely that it could be due to a large concentration of Galactic-style star formation masers within a small region. Comparison with observations of other methanol transitions suggests that the 6.7 GHz methanol emission is due to a diffuse, low-gain maser, amplifying the background continuum radiation from the nuclear region. The methanol emission is blueshifted with respect to the the systemic velocity of the galaxy by several hundred kilometers per second and lies outside the velocity range associated with the dense gas and neutral hydrogen in the central region of NGC4945. We speculate that it may be associated with gas entrained in a superwind outflow from the nuclear region.