High-Resolution Imaging of Water Maser Emission in the active galaxies NGC 6240 and M51

TL;DR

This study uses high-resolution radio observations to detect water maser emissions in NGC 6240 and M51, linking maser activity to active galactic nuclei rather than star formation, and providing new insights into galaxy nuclear activity.

Contribution

First detection of nuclear radio continuum emission in M51 and detailed analysis of maser origins in both galaxies, emphasizing AGN activity over star formation.

Findings

Maser features in NGC 6240 are redshifted by ~300 km/s.

Maser activity correlates with radio continuum emission, indicating AGN-related excitation.

First detection of nuclear radio continuum in M51 co-located with maser emission.

Abstract

We present the results of observations of 22GHz H2O maser emission in NGC 6240 and M51 made with the Karl G. Jansky Very Large Array. Two major H2O maser features and several minor features are detected toward the southern nucleus of NGC 6240. These features are redshifted by about 300 km/s from the galaxy's systemic velocity and remain unresolved at the synthesized beam size. A combination of our two-epoch observations and published data reveals an apparent correlation between the strength of the maser and the 22GHz radio continuum emission, implying that the maser excitation relates to the activity of an active galactic nucleus in the southern nucleus rather than star-forming activity. The star-forming galaxy M51 hosts H2O maser emission in the center of the galaxy; however, the origin of the maser has been an open question. We report the first detection of 22GHz nuclear radio continuum emission in M51. The continuum emission is co-located with the maser position, which indicates that the maser arises from active galactic nucleus-activity and not from star-forming activity in the galaxy.