Exploring the genesis of spiral galaxies

Abstract

Context. A tantalizing enigma in extragalactic astronomy concerns the chronology and driving mechanisms of the build-up of latetype galaxies (LTGs). The standard scenario envisages two formation routes, with classical bulges (CBs) assembling first in a quick and violent quasi-monolithic episode followed by gradual disk assembly, and pseudo-bulge (PB) formation occurring gradually over the gigayear-long secular evolution of LTGs through gentle gas inflow from the disk and in situ star formation at its centre. An expectation from this antagonistic rationale is the segregation of present-day LTG bulges into two evolutionary distinct groups, which is in sharp contrast with recent observations. Aims. The present study aims for a thorough investigation of the star formation history (SFH) of LTGs with its ultimate goal being to outline a coherent framework for the formation and evolution of spiral galaxies and their main stellar components. Methods. Using population spectral synthesis models, we analyse the spatially resolved SFH of the bulge and the disk for 135 LTGs from the CALIFA survey that adequately cover the relevant range in LTG mass. Complementarily, characteristic physical properties of bulges and disks, such as mean colours, mass-and light-weighted stellar age and metallicity, and EW(Hα), were contrasted with predictions from evolutionary synthesis models. The latter adopted exponentially declining SFHs, with an e-folding time τ between 0.1 and 20 Gyr. Results. Analysis of the SFH of approximately half a million spaxels consistently reveals that the main physical and evolutionary properties of both bulges and disks show a continuous distribution across present-day total stellar mass M ,T . The e-folding time τ in spiral galaxies with log(M ,T ) > 10 increases from the centre to the periphery, suggesting that these systems grow in an inside-out fashion. Quite importantly, the radial gradient of τ in gigayear over kiloparsec in an individual galaxy increases with increasing M ,T , which is consistent with a high bulge-to-disk age contrast in high-mass spirals, while lower-mass LTGs display roughly the same τ throughout their entire radial extent, with intermediate mass galaxies in between. Predictions obtained through evolutionary synthesis are overall consistent with observed properties, such as mean colours, stellar ages, and Hα equivalent widths. Finally, bulges and disks of higher mass galaxies exhibit shorter formation timescales as compared to their lower mass counterparts. Conclusions. 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 mass sequence. This analysis is consistent with the framework where bulges are assembled jointly with their parent disks by gradual inside-out growth, at a pace that is regulated by the depth of the galactic potential. In accordance with this is the fact that the revealed correlations are entirely devoid of a bimodality, which would be expected if CBs and...