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 $\sigma_*$ 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?
