On the genesis of spiral galaxies -- Classical and pseudo bulges as extremities of a continuous sequence

Abstract

A tantalizing enigma in extragalactic astronomy concerns the chronology and driving mechanisms of the build-up of late-type galaxies (LTGs). The standard scenario envisages two formation routes, with classical bulges (CBs) assembling first in a quick quasi-monolithic episode followed by gradual disk assembly, and pseudo-bulges (PBs) forming over the Gyr-long secular evolution of LTGs. The expectation is, therefore, the segregation of present-day LTG bulges into two distinct groups. Here we analyse the star formation histories (SFHs) of bulges and disks for 135 LTGs from the CALIFA survey covering the relevant range in LTG mass. In addition, their physical properties were contrasted with predictions from evolutionary synthesis models, adopting exponentially declining SFHs, with an e-folding time 0.1 < $\tau$ < 20 Gyr. Analysis of the SFHs of ~ half-million spaxels consistently reveals that the main properties of bulges and disks show a continuous distribution across total stellar mass. Moreover, the $\tau$ in high-mass LTGs radially increases, suggesting that these grow in an inside-out fashion, while lower-mass LTGs display roughly the same $\tau$ throughout their entire radial extent. Evolutionary synthesis predictions are consistent with observations. Finally, bulges and disks of higher mass LTGs exhibit shorter formation timescales as compared to their lower mass counterparts. Collectively, the obtained results evince a coherent and unified picture for the formation and evolution of LTGs, in which PBs and CBs denote extremities of a continuous sequence. This analysis is consistent with the framework where bulges are assembled with their parent disks by gradual inside-out growth, at a pace that is regulated by the depth of the galactic potential. In accordance is the utter absence of bimodal correlations, as expected if CBs and PBs were to emerge from two distinct formation routes.