Abstrakt
The majority of galaxies with current star formation rates (SFRs), SFRo >= 10(-3) M-circle dot yr(-1), in the Local Cosmological Volume, where observations should be reliable, have the property that their observed SFRo is larger than their average SFR. This is in tension with the evolution of galaxies described by delayed-t models, according to which the opposite would be expected.
The tension is apparent in that local galaxies imply the star formation time-scale tau approximate to 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 proportional to(t - t(1))(eta) for t(1) = 1.8 Gyr (with no stars forming prior to t(1)) 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.