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