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The majority of galaxies with current star-formation rates (SFRs), SFRo >=10^-3 Msun/yr, in the Local Cosmological Volume where observations should bereliable, have the property that their observed SFRo is larger than theiraverage star formation rate. This is in tension with the evolution of galaxiesdescribed by delayed-tau models, according to which the opposite would beexpected. The tension is apparent in that local galaxies imply the starformation timescale tau approx 6.7 Gyr, much longer than the 3.5-4.5 Gyrobtained 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 raisesnon-trivial problems for the evolution and formation time of galaxies, but isbroadly consistent with the observed decreasing main sequence with increasingage of the Universe.

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