An Absence of Radio-loud Active Galactic Nuclei in Geometrically Flat Quiescent Galaxies: Implications for Maintenance-mode Feedback Models

Abstract

Maintenance-mode feedback from low-accretion-rate active galactic nuclei (AGNs), manifesting itself observationally through radio-loudness, is invoked in all cosmological galaxy formation models as a mechanism that prevents excessive star formation in massive galaxies ( M * ≳ 3 × 10 10 M ⊙ ). We demonstrate that at a fixed mass the incidence of radio-loud (RL) AGNs ( L > 10 23 W Hz −1 ) identified in the Faint Images of the Radio Sky at Twenty centimeter and NRAO Very Large Array Sky Survey radio surveys among a large sample of quiescent (non-star-forming) galaxies selected from the Sloan Digital Sky Survey is much higher in geometrically round galaxies than in geometrically flat, disk-like galaxies. As found previously, the RL AGN fraction increases steeply with stellar velocity dispersion σ * and stellar mass, but even at a fixed velocity dispersion of 200–250 km s −1 this fraction increases from 0.3% for flat galaxies (projected axis ratio of q < 0.4) to 5% for round galaxies ( q > 0.8). We rule out the hypothesis that this strong trend is due to projection effects in the measured velocity dispersion. The large fraction of RL AGNs in massive, round galaxies is consistent with the hypothesis that such AGNs deposit energy into their hot gaseous halos, preventing cooling and star formation. However, the absence of such AGNs in disk-like quiescent galaxies—most of which are not satellites in massive clusters, raises important questions. Is maintenance-mode feedback a generally valid explanation for quiescence? If so, how does that feedback avoid manifesting at least occasionally as an RL galaxy?