Non-detection of Broad Hydrogen Radio Recombination Lines in Circinus Galaxy

TL;DR

This study used ALMA to search for broad hydrogen radio recombination lines in the Circinus galaxy's active nucleus but only detected narrow lines, suggesting current technology cannot detect broad lines in similar nearby AGNs.

Contribution

The paper provides the first observational limits on broad RRLs in Circinus galaxy, highlighting the challenges and potential of future submillimetre RRL observations for probing BLRs in type II AGNs.

Findings

Narrow RRL components detected, likely from star-forming regions.

Upper limits set for broad RRL emission, non-detection.

Future submillimetre RRLs could enable direct detection of BLRs.

Abstract

The line widths of broad line regions (BLRs) of AGNs are key parameters for understanding the central super massive black holes (SMBHs). However, due to obscuration from dusty torus, optical recombination lines from BLRs in type II AGNs can not be directly detected. Radio recombination lines (RRLs), with low extinction, can be ideal tracers to probe emission from BLRs in type II AGNs. We performed RRL observations for H35$\alpha$ and H36$\alpha$ toward the center of Circinus galaxy with ALMA. Narrow components of H35$\alpha$ and H36$\alpha$, which are thought to be mainly from star forming regions around the nuclear region, are detected. However, only upper limits are obtained for broad H35$\alpha$ and H36$\alpha$. Since Circinus galaxy is one of the nearest AGN, non-detection of broad RRLs in Circinus galaxy at this band tells us that it is hopeless to detect broad RRL emission in local AGNs with current facilities. Submillimetre RRLs, with flux densities that are dozens of times higher than those at the millimetre level, could be the tools to directly detect BLRs in type II AGNs with ALMA, once its backend frequency coverage has been upgraded to several times better than its current capabilities.