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Description
A key subject in extragalactic astronomy concerns the chronology and driving mechanisms of bulge formation in late-type galaxies (LTGs). The standard scenario distinguishes between classical bulges and pseudo-bulges (CBs and PBs, respectively), the first thought to form monolithically prior to disks and the second gradually out of disks. These two bulge formation routes obviously yield antipodal predictions on the bulge age and bulge-to-disk age contrast, both expected to be high (low) in CBs (PBs). Our main goal is to explore whether bulges in present-day LTGs segregate into two evolutionary distinct classes, as expected from the standard scenario. Other questions motivating this study center on evolutionary relations between LTG bulges and their hosting disks, and the occurrence of accretion-powered nuclear activity as a function of bulge stellar mass M* and stellar surface density {Sigma}*. In this study we have combined three techniques - surface photometry, spectral modeling of integral field spectroscopy data and suppression of stellar populations younger than an adjustable age cutoff with the code REMOVE YOUNG (RY) - toward a systematic analysis of the physical and evolutionary properties (e.g., M* , {Sigma}* and mass-weighted stellar age <t*>_M_ and metallicity <Z*>_M_, respectively) of a representative sample of 135 nearby (<=130Mpc) LTGs from the CALIFA survey that cover a range between 10^8.9^M_{sun}_ and 10^11.5^M_{sun}_ in total stellar mass M_*,T_. In particular, the analysis here revolves around <{delta}{mu}9G>, a new distance- and formally extinction-independent measure of the contribution by stellar populations of age >=9Gyr to the mean r-band surface brightness of the bulge. We argue that <{delta}{mu}9Gx > offers a handy semi-empirical tracer of the physical and evolutionary properties of LTG bulges and a promising means for their characterization.
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