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

Maintenance-mode feedback from low-accretion rate AGN, 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$_*$ $\gtrsim$ 3$\times$10$^{10}$ M$_{\odot}$). We demonstrate that at a fixed mass the incidence of radio-loud AGN (L $>$ 10$^{23}$ WHz$^{- 1}$) identified in the FIRST and NVSS radio surveys among a large sample of quiescent (non-star forming) galaxies selected from the SDSS 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 kms$^{-1}$ 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 that this strong trend is due to projection effects in the measured velocity dispersion. The large fraction of radio-loud AGN in massive, round galaxies is consistent with the hypothesis that such AGN deposit energy into their hot gaseous halos, preventing cooling and star-formation. However, the absence of such AGN 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; and, if so, how does that feedback avoid manifesting at least occasionally as a radio-loud galaxy?