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

The majority of galaxies with current star formation rates (SFRs), SFR_o≥10⁻³M⊙yr⁻¹⁠, in the Local Cosmological Volume, where observations should be reliable, have the property that their observed SFR_o is larger than their average SFR. This is in tension with the evolution of galaxies described by delayed-τ models, according to which the opposite would be expected. The tension is apparent in that local galaxies imply the star formation time-scale τ ≈ 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 ∝(t − t₁)η for t₁ = 1.8 Gyr (with no stars forming prior to t₁) implies that η = 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.