Ageing and quenching through the ageing diagram: predictions from simulations and observational constraints

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 similar to 20 Myr through the equivalent width of the H alpha line and similar to 1 -3 Gyr through the dust-corrected optical colour ( g - r)(0)or the Balmer break. We provide a physical characterization by using PIPE3D estimates of the SFH of Calar Alto Le gac y Inte gral Field Area and Mapping Nearby Galaxies at Apache Point Observatory 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 20 Myr and 3 Gyr: ageing systems, whose star formation rate changes on scales of several Gyr, account for 70 - 80 per cent of the galaxy population. Objects whose SFH was abruptly truncated in the last similar to 1 Gyr arrange along a detached quenched sequence that represents similar to 5 - 10 per cent by (volume-corrected) number for 10(9) < M (*)/M-circle dot < 10(12) . 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 equivalent to ((SF R)) /(SF R) , we find that galaxies transit from the ageing to quenched sequences on scales similar to 500 Myr. We conclude that the AD provides a useful tool to discern recently quenched galaxies from the dominant ageing population.