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

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.

5 pages, 2 Figures, published in MNRAS Letters

References in corpus (6)

• Resolving the complex structure of the dust torus in the active nucleus of the Circinus galaxy
• The Early Evolution of Massive Stars: Radio Recombination Line Spectra
• SINFONI adaptive optics integral field spectroscopy of the Circinus Galaxy
• Detection of the H92alpha recombination line from the starbursts in the Circinus galaxy and NGC 1808
• Inverted level populations of hydrogen atoms in ionized gas
• Second Epoch ALMA Observations of 321 GHz Water Maser Emission in NGC 4945 and the Circinus Galaxy

Cited by in corpus (1)

• Star formation in the centre of NGC 1808 as observed by ALMA