Constraints on the star formation histories of galaxies in the Local Cosmological Volume

TL;DR

This paper investigates the star formation histories of local galaxies, revealing a near-constant SFR over time that challenges traditional delayed-tau models and has implications for galaxy evolution theories.

Contribution

It introduces a power-law model for galaxy SFRs with a nearly constant rate, contrasting with standard models and providing new insights into galaxy formation timescales.

Findings

Most local galaxies have current SFRs exceeding their average over time.

The star formation timescale tau is about 6.7 Gyr, longer than previous estimates.

Galaxy SFRs follow a power law with eta approximately 0.18, indicating near-constant star formation.

Abstract

The majority of galaxies with current star-formation rates (SFRs), SFRo >= 10^-3 Msun/yr, in the Local Cosmological Volume where observations should be reliable, have the property that their observed SFRo is larger than their average star formation rate. This is in tension with the evolution of galaxies described by delayed-tau models, according to which the opposite would be expected. The tension is apparent in that local galaxies imply the star formation timescale tau approx 6.7 Gyr, much longer than the 3.5-4.5 Gyr obtained using an empirically determined main sequence at several redshifts. Using models where the SFR is a power law in time of the form propto (t - t1)^eta for t1 = 1.8 Gyr (with no stars forming prior to t1) implies that eta = 0.18 +- 0.03. This suggested near-constancy of a galaxy's SFR over time raises non-trivial problems for the evolution and formation time of galaxies, but is broadly consistent with the observed decreasing main sequence with increasing age of the Universe.