The specific star formation rate function at different mass scales and quenching: a comparison between cosmological models and SDSS

We present the eddington bias corrected specific star formation rate function (sSFRF) at different stellar mass scales from a sub-sample of the Sloan Digital Sky Survey Data Release DR7 (SDSS), which is considered complete both in terms of stellar mass (M_⋆) and star formation rate (SFR). The above enable us to study qualitatively and quantitatively quenching, the distribution of passive/star-forming galaxies and perform comparisons with the predictions from state-of-the-art cosmological models, within the same M_⋆ and SFR limits. We find that at the low-mass end ( ${M_{\star }} = 10^{9.5}\!-\!10^{10} \, {\rm M_{\odot }}$ ) the sSFRF is mostly dominated by star-forming objects. However, moving to the two more massive bins ( ${M_{\star }} = 10^{10}\!-\!10^{10.5} \, {\rm M_{\odot }}$ and ${M_{\star }} = 10^{10.5}\!-\!10^{11} \, {\rm M_{\odot }}$ ) a bi-modality with two peaks emerges. One peak represents the star-forming population, while the other describes a rising passive population. The bi-modal form of the sSFRFs is not reproduced by a range of cosmological simulations (e.g. Illustris, EAGLE, Mufasa, and IllustrisTNG) which instead generate mostly the star-forming population, while a bi-modality emerges in others (e.g. L-Galaxies, Shark, and Simba). Our findings reflect the need for the employed quenching schemes in state-of-the-art models to be reconsidered, involving prescriptions that allow 'quenched galaxies' to retain a small level of SF activity (sSFR = ${\rm 10^{-11} \, }$ - ${\rm 10^{-12} \, {\rm yr^{-1}}}$ ) and generate an adequate passive population/bi-modality even at intermediate masses ( ${M_{\star }} = 10^{10}\!-\!10^{10.5} \, {\rm M_{\odot }}$ ).