According to their specific star formation rate (sSFR), galaxies are often divided into ‘star-forming’ and ‘passive’ populations. It is argued that the former define a narrow ‘Main Sequence of Star-Forming Galaxies’ (MSSF) of the form sSFR(M*), whereas ‘passive’ galaxies feature negligible levels of star formation activity. Here we use data from the Sloan Digital Sky Survey and the Galaxy and Mass Assembly survey at z < 0.1 to constrain the conditional probability of the specific star formation rate at a given stellar mass. We show that the whole population of galaxies in the local Universe is consistent with a simple probability distribution with only one maximum (roughly corresponding to the MSSF) and relatively shallow power-law tails that fully account for the ‘passive’ population. We compare the quality of the fits provided by such unimodal ansatz against those coming from a double log-normal fit (illustrating the bimodal paradigm), finding that both descriptions are roughly equally compatible with the current data. In addition, we study the physical interpretation of the bidimensional distribution across the M* − sSFR plane and discuss potential implications from a theoretical and observational point of view. We also investigate correlations with metallicity, morphology and environment, highlighting the need to consider at least an additional parameter in order to fully specify the physical state of a galaxy.
We study recent changes on the star formation history (SFH) of galaxies by means of the ageing diagram (AD), tracing the fraction of stars formed during the last ∼20 Myr through the equivalent width of the $\rm H\alpha$line and ∼1 − 3 Gyr through the dust-corrected optical colour (g − r)0or the Balmer break. We provide a physical characterization by using Pipe3Destimates of the SFH of CALIFA and MaNGA galaxies, in combination with the predictions from IllustrisTNG-100. Our results show that the AD may be divided into four domains that correlate with the stellar mass fractions formed in the last 20Myr and 3 Gyr: Ageing systems, whose SFR changes on scales of several Gyr, account for $70-80{{\%}}$ of the galaxy population. Objects whose SFH was abruptly truncated in the last ∼1 Gyr arrange along a detached Quenched sequence that represents $\sim 5-10{{\%}}$ by (volume-corrected) number for 109 < M*/M⊙ < 1012. Undetermined systems represent an intermediate population between the Ageing and Quenched regimes. Finally, Retired galaxies, dominated by old stellar populations, are located at the region in the AD where the Ageing and Quenched sequences converge. Defining different star formation activity levels in terms of the birth rate parameter $b\equiv \frac{SFR}{\langle SFR \rangle }$, we find that galaxies transit from the Ageing to Quenched sequences on scales ∼500 Myr. We conclude that the ageing diagram provides a useful tool to discern recently Quenched galaxies from the dominant Ageing population.
This work investigates the fundamental mechanism(s) that drive galaxy evolution in the Local Universe. By comparing two proxies of star-formation sensitive to different timescales, such as EW($\rm H\alpha$) and colours like g − r, one may distinguish between smooth secular evolution (ageing) and sudden changes (quenching) on the recent star formation history of galaxies. Building upon the results obtained from a former study based on 80.000 SDSS single-fibre measurements, we now focus on spatially-resolved (on kpc scales) galaxies, comparing with a sample of 637 nearby objects observed by the CALIFA survey. In general, galaxies cannot be characterised in terms of a single ‘evolutionary stage’. Individual regions within galaxies arrange along a relatively narrow ageing sequence, with some intrinsic scatter possibly due to their different evolutionary paths. These sequences, though, differ from one galaxy to another, although they are broadly consistent with the overall distribution found for the (central) SDSS spectra. We find evidence of recent quenching episodes (relatively blue colours and strong $\rm H\alpha$ absorption) in a small fraction of galaxies (most notably, low-mass ellipticals), on global scales and individual regions (particularly at high metallicity). However, we argue that most of the systems, over their entire extent, are compatible with a secular inside-out scenario, where the evolutionary stage correlates with both global (mass, morphology, and environment) as well as local (surface brightness and metallicity) properties.
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