Radio continuum and OH line emission of high-z OH megamaser galaxies

TL;DR

This study investigates high-redshift OH megamaser galaxies using radio continuum and OH line emission data, revealing their compact emission regions, similarities with low-redshift counterparts, and suggesting a starburst origin over AGN activity.

Contribution

First detailed high-resolution radio study of high-z OH megamaser galaxies, combining VLA and FAST data, highlighting their compact emission and potential variability.

Findings

OH emission is compact and associated with radio continuum.

No significant difference in radio parameters compared to low-z OHMs.

Most sources likely originate from starburst activity, not AGN.

Abstract

We present the study of arcsecond scale radio continuum and OH line emission of a sample of known OH megamaser galaxies with $z \geq$ 0.15 using archival Very Large Array (VLA) data. And also the results of our pilot Five hundred meter aperture spherical radio telescope (FAST) observations of 12 of these OHM galaxies. The arcsecond-scale resolution images show that the OH emission is distributed in one compact structure and spatially associated with radio continuum emission. Furthermore, nearly all the fitted components are likely smaller than the beam size ($\sim$ 1.4"), which indicates that the broad OH line profiles of these sources originated from one masing region or that more components are distributed in sub-arcsec scales. The radio parameters, including brightness temperature, spectral index, and q-index, show no significant differences with the low-redshift OHM galaxies, which have significantly lower OH line luminosities. Because these parameters are indicators of the central power sources (AGN, starburst, or both), our results indicate that the presence of radio AGN in the nuclei may not be essential for the formation of OH emission. Over 1/3 of OHMs in this sample (6/17) show possible variable features likely caused by interstellar scintillation due to small angular sizes. We might underestimate this value because these sources are associated with this sample's highest OH line flux densities. Those with low OH line flux densities might need higher sensitivity observations to study the variabilities. These results support the compact nature of OH maser emission and a starburst origin for the OHMs in our selected sample.