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 sSFR 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 lognormal 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.
Bibliographical noteThis article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 499, Issue 1, November 2020, Pages 573–586, https://doi.org/10.1093/mnras/staa2818. Copyright 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
- Galaxies: evolution
- Galaxies: fundamental parameters
- Galaxies: general
- Galaxies: star formation