Testing Intermittency of the Galactic Star Formation History along with the Infall Model

Abstract

We analyze the star formation history (SFH) of the Galactic disk by using an infall model. Based on the observed SFH of the Galactic disk, we first determine the timescales of the gas infall into the Galactic disk ($t_{\rm in}$) and that of the gas consumption to form stars ($t_{\rm sf}$). Since each of the two timescales does not prove to be determined independently from the SFH, we first fix $t_{\rm sf}$. Then, $t_{\rm in}$ is determined so that we minimize $\chi^2$. Consequently, we choose three parameter sets: ($t_{\rm sf}$ [Gyr], $t_{\rm in}$ [Gyr]) = (6.0, 23), (11, 12), and (15, 9.0), where we set the Galactic age as 15 Gyr. All of the three cases predict almost identical star formation history. Next, we test the intermittency (or variability) of the star formation rate (SFR) along with the smooth SFH suggested from the infall model. The large value of the $\chi^2$ statistic supports the violent time-variation of the SFH. If we interpret the observed SFH with smooth and variable components, the amplitude of the variable component is comparable to the smooth component. Thus, intermittent SFH of the Galactic disk is strongly suggested. We also examined the metallicity distribution of G-dwarfs. We found that the true parameter set lies between ($t_{\rm sf}$ [Gyr], $t_{\rm in}$ [Gyr]) = (6, 23) and (11, 12), though we should need a more sophisticated model including the process of metal enrichment within the Galactic halo.