An Elusive Population of Massive Disk Galaxies Hosting Double-lobed Radio-loud AGNs

TL;DR

This study identifies a rare class of massive, disk-shaped galaxies hosting double-lobed radio-loud AGNs, challenging the conventional view that such phenomena occur only in elliptical galaxies, and suggests stellar mass influences jet formation.

Contribution

It provides the first systematic evidence of massive, disk-like galaxies hosting extended radio-loud AGNs with detailed morphological and stellar mass analysis.

Findings

18 confirmed host galaxies with disk morphologies and double-lobed radio structures.

Host galaxies are massive, red, and have pseudo bulges, contrary to typical elliptical hosts.

Radio jets are mostly FR II type with lower power than elliptical-hosted sources.

Abstract

It is commonly accepted that radio-loud active galactic nuclei are hosted exclusively by giant elliptical galaxies. We analyze high-resolution optical Hubble Space Telescope images of a sample of radio galaxies with extended double-lobed structures associated with disk-like optical counterparts. After systematically evaluating the probability of chance alignment between the radio lobes and the optical counterparts, we obtain a sample of 18 objects likely to have genuine associations. The host galaxies have unambiguous late-type morphologies, including spiral arms, large-scale dust lanes among the edge-on systems, and exceptionally weak bulges, as judged by the low global concentrations, small global S\'{e}rsic indices, and low bulge-to-total light ratios (median $B/T = 0.13$). With a median S\'{e}rsic index of 1.4 and low effective surface brightnesses, the bulges are consistent with being pseudo bulges. The majority of the hosts have unusually large stellar masses (median $M_* = 1.3\times 10^{11}\, M_\odot$) and red optical colors (median $g-r = 0.69\,$mag), consistent with massive, quiescent galaxies on the red sequence. We suggest that black hole mass (stellar mass) plays a fundamental role in launching large-scale radio jets, and that the rarity of extended radio lobes in late-type galaxies is the consequence of the steep stellar mass function at the high-mass end. The disk radio galaxies have mostly Fanaroff-Riley type II morphologies yet lower radio power than sources of a similar type traditionally hosted by ellipticals. The radio jets show no preferential alignment with the minor axis of the galactic bulge or disk, apart from a possible mild tendency for alignment among the most disk-dominated systems.