PKS B1718-649: an HI and H2 perspective on the birth of a compact radio source

TL;DR

This study investigates the distribution and kinematics of HI and H2 gas in the young radio galaxy PKS B1718-649 to understand how cold gas influences the triggering of radio activity.

Contribution

It provides new insights into the gas dynamics near a newly active radio galaxy, highlighting the role of inner gas clouds in triggering radio emission.

Findings

Detection of two opposite-moving gas clouds near the nucleus.

Presence of a circum-nuclear H2 disk with a dispersed core.

Evidence of ongoing interaction between the radio source and surrounding ISM.

Abstract

We present neutral hydrogen (HI) and warm molecular hydrogen (H2) observations of the young (10^2 years) radio galaxy PKS B1718-649. We study the morphology and the kinematics of both gas components, focusing, in particular, on their properties in relation to the triggering of the radio activity. The regular kinematics of the large scale HI disk, seen in emission, suggests that an interaction event occurred too long ago to be responsible for the recent triggering of the radio activity. In absorption, we detect two absorption lines along the narrow line of sight of the compact (r<2 pc) radio source. The lines trace two clouds with opposite radial motions. These may represent a population of clouds in the very inner regions of the galaxy, which may be involved in triggering the radio activity. The warm molecular hydrogen (H2 1-0 S(1) ro-vibrational line) in the innermost kilo-parsec of the galaxy appears to be distributed in a circum-nuclear disk following the regular kinematics of the HI and of the stellar component. An exception to this behaviour arises only in the very centre, where a highly dispersed component is detected. These particular HI and H2 features suggest that a strong interplay between the radio source and the surrounding ISM is on-going. The physical properties of the cold gas in the proximity of the radio source may regulate the accretion recently triggered in this AGN.