Stellar age gradients and inside-out star formation quenching in galaxy
  bulges
  Radial age gradients hold the cumulative record of the multitude of physical
processes driving the build-up of stellar populations and the ensuing star
formation (SF) quenching process in galaxy bulges, therefore potentially
sensitive discriminators between competing theoretical concepts on bulge
formation and evolution. Based on spectral modeling of integral field
spectroscopy data from the CALIFA survey, we derive mass- and light-weighted
stellar age gradients ($\nabla$(t,B)L,M) within the photometrically determined
bulge radius (RB) of a representative sample of local face-on late-type
galaxies that span 2.6 dex in stellar mass. Our analysis documents a trend for
decreasing $\nabla$(t,B)L,M with increasing M,T, with high-mass bulges
predominantly showing negative age gradients and vice versa. The inversion from
positive to negative $\nabla$(t,B)L,M occurs at logM,T ~ 10, which roughly
coincides with the transition from lower-mass bulges whose gas excitation is
powered by SF to bulges classified as Composite, LINER or Seyfert. We discuss
two limiting cases for the origin of radial age gradients in massive LTG
bulges. The first assumes that the stellar age in the bulge is initially
spatially uniform, thus the observed age gradients arise from an inside-out SF
quenching (ioSFQ) front that is radially expanding with a mean velocity vq. In
this case, the age gradients translate into a slow ioSFQ that lasts until z~2,
suggesting mild negative feedback by SF or an AGN. If negative age gradients in
massive bulges are not due to ioSFQ but primarily due to their inside-out
formation process, then the standard hypothesis of quasi-monolithic bulge
formation has to be discarded in favor of a scenario that involves gradual
buildup of stellar mass over 2-3 Gyr through, e.g., inside-out SF and inward
migration of SF clumps from the disk. In this case, rapid AGN-driven ioSFQ
cannot be ruled out.
