An Objective Definition for the Main Sequence of Star-Forming Galaxies

TL;DR

This paper introduces an objective, selection-independent method to define the main sequence of star-forming galaxies by identifying the ridge line of the star-forming peak in a 3D SFR-Mass distribution, improving consistency across studies.

Contribution

It proposes a novel, objective definition of the galaxy main sequence that does not rely on pre-selected star-forming galaxy samples, enabling better comparison and understanding of galaxy evolution.

Findings

Defines MS as the ridge line of the star-forming peak in 3D SFR-Mass-Number plot

Facilitates comparison of results across different diagnostics and studies

Provides a more objective criterion for identifying quenching galaxies

Abstract

The Main Sequence (MS) of star-forming galaxies plays a fundamental role in driving galaxy evolution and in our efforts to understand it. However, different studies find significant differences in the normalization, slope and shape of the MS. These discrepancies arise mainly from the different selection criteria adopted to isolate star-forming galaxies, that may include or exclude galaxies with specific star formation rate (SFR) substantially below the MS value. To obviate this limitation of all current criteria, we propose an objective definition of the MS that does not rely at all on a pre-selection of star-forming galaxies. Constructing the 3D SFR-Mass-Number plot, the MS is then defined as the ridge line of the star-forming peak, as illustrated with various figures. The advantages of such definition are manifold. If generally adopted it will facilitate the inter-comparison of results from different groups using the same star formation rate (SFR) and stellar mass diagnostics, or to highlight the relative systematics of different diagnostics. All this could help understanding MS galaxies as systems in a quasi-steady state equilibrium and would also provide a more objective criterion for identifying quenching galaxies.