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 ( IFS ) data from the CALIFA survey , we derive mass- and light-weighted stellar age gradients ( \mathrm { \nabla } ( t _ { \star,B } ) _ { \cal L,M } ) within the photometrically determined bulge radius ( \mathrm { R } _ { B } ) of a representative sample of local face-on late-type galaxies that span 2.6 dex in stellar mass ( 8.9 \leq log { \cal M } _ { \star, \textrm { T } } \leq 11.5 ) .
Our analysis documents a trend for decreasing \mathrm { \nabla } ( t _ { \star,B } ) _ { \cal L,M } with increasing { \cal M } _ { \star, \textrm { T } } , with high-mass bulges predominantly showing negative age gradients and vice versa .
The inversion from positive to negative \mathrm { \nabla } ( t _ { \star,B } ) _ { \cal L,M } occurs at log { \cal M } _ { \star, \textrm { T } } \simeq 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 simple limiting cases for the origin of radial age gradients in massive LTG bulges .
The first one assumes that the stellar age in the bulge is initially spatially uniform ( \mathrm { \nabla } ( t _ { \star,B } ) _ { \cal L,M } \approx 0 ) , thus the observed age gradients ( \sim –3 Gyr/ \mathrm { R } _ { B } ) arise from an inside-out SF quenching ( ioSFQ ) front that is radially expanding with a mean velocity v _ { q } .
In this case , the age gradients for massive bulges translate into a slow ( v _ { q } \sim 1-2 km s ^ { -1 } ) ioSFQ that lasts until z \sim 2 , suggesting mild negative feedback by SF or an AGN .
If , on the other hand , 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 ( \ll 1 Gyr ) AGN-driven ioSFQ can not be ruled out .
While the { \cal M } _ { \star, \textrm { T } } vs . \mathrm { \nabla } ( t _ { \star,B } ) _ { \cal L,M } relation suggests that the assembly history of bulges is primarily regulated by galaxy mass , its large scatter ( \sim 1.7 Gyr/ \mathrm { R } _ { B } ) reflects a considerable diversity that calls for an in-depth examination of the role of various processes ( e.g. , negative and positive AGN feedback , bar-driven gas inflows ) with higher-quality IFS data in conjunction with advanced spectral modeling codes .