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 ?