A single galaxy population? statistical evidence that the Star-Forming Main Sequence might be the tip of the iceberg

TL;DR

This study analyzes galaxy populations using SDSS and GAMA data, suggesting that the traditional bimodal view of star-forming and passive galaxies may be oversimplified, as a single probability distribution can describe the entire population.

Contribution

It demonstrates that a unimodal probability distribution can fit galaxy star formation data as well as the bimodal model, challenging the standard two-population paradigm.

Findings

A single probability distribution fits the galaxy population data.

Both unimodal and bimodal models are statistically comparable.

Additional parameters are needed to fully describe galaxy states.

Abstract

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 $\sSFR(\Mstar)$, 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.