A VLBA search for a stimulated recombination line from the accretion region in NGC 1275

Abstract

The radio source 3C 84 in NGC 1275 has a two-sided structure on parsec scales. The northern feature, presumed to be associated with a jet moving away from Earth, shows strong evidence for free-free absorption. The ionized gas responsible for that absorption would be a source of detectable stimulated recombination line emission for a wide range of physical conditions. The VLBA has been used to search for the H65a hydrogen recombination line. The line is expected to be seen only against the northern feature, which contains a small fraction of the total radio flux density. This spatial discrimination significantly aids the search for a weak line. No line was seen, with upper limits of roughly 15% of the continuum over a velocity range of 1486 km s^-1 with resolutions up to 6.6 km s^-1. In the absence of a strong radiation field, this would imply that the free-free absorbing gas has a wide velocity width, is moving rapidly relative to the systemic velocity, or is concentrated in a thin high-density structure. All these possibilities are reasonably likely close to an active galactic nucleus (AGN). However, in the intense radiation environment of the AGN, even considering only the radiation we actually observe passing through the free-free absorbing gas, the nondetection is probably ensured by a combination of saturation and radiation damping.