The Variability of Star Formation Rate in Galaxies. II. Power Spectrum Distribution on the Main Sequence

Abstract

We constrain the temporal power spectrum of the specific star formation rate of star-forming galaxies, using a well-defined sample of main sequence galaxies from MaNGA and our earlier measurements of the ratio of the star formation rate averaged within the last 5 Myr to that averaged over the last 800 Myr. We explore the assumptions of stationarity and ergodicity that are implicit in this approach. We assume a single power-law form of the power spectrum distribution (PSD) but introduce an additional free parameter, the “intrinsic scatter”, to try to account for any non-ergodicity introduced from various sources. We analyze both an “integrated” sample consisting of global measurements of all of the galaxies, as well as 25 subsamples obtained by considering five radial regions and five bins of integrated stellar mass. Assuming that any intrinsic scatter is not the dominant contribution to the main sequence dispersion of galaxies, we find that the PSDs have slopes between 1.0 and 2.0, indicating that the power (per log interval of frequency) is mostly contributed by longer-timescale variations. We find a correlation between the returned PSDs and the inferred gas depletion times ( ) obtained from application of the extended Schmidt Law, such that regions with shorter gas depletion times show larger integrated power and flatter PSD. Intriguingly, it is found that shifting the PSDs by the inferred causes all 25 PSDs to closely overlap, at least in the region where the PSD is best constrained and least affected by uncertainties about any intrinsic scatter. A possible explanation for these results is the dynamical response of the gas-regulator system of Lilly et al. to a uniform time-varying inflow, as previously proposed in Wang et al.