Late Bloomer Galaxies: Growing Up in Cosmic Autumn

Late bloomers are massive ($M_* >10^{10}\,M_\odot$) galaxies at $z<1$ that formed the majority of their stars within $\sim$2 Gyr of the epoch of observation. Our improved methodology for deriving star formation histories (SFHs) of galaxies at redshifts $0.45 < z < 0.75$ from the Carnegie-Spitzer-IMACS Survey includes confidence intervals that robustly distinguish late bloomers from "old" galaxies. We use simulated SFHs to test for "false positives" and contamination from old galaxies to demonstrate that the late bloomer population is not an artifact of our template modeling technique. We show that late bloomers account for $\sim$20% of $z\sim0.6$ galaxies with masses of the modern Milky Way, with a moderate dependence on mass. We take advantage of a 1% overlap of our sample with HST (CANDELS) imaging to construct a "gold standard" catalog of 74 galaxies with high-confidence SFHs, SEDs, basic data, and HST images to facilitate comparison with future studies by others. This small subset suggests that galaxies with both old and young SFHs cover the full range of morphology and environment (excluding rich groups or clusters), albeit with a mild but suggestive correlation with local environment. We begin the investigation of whether late bloomers of sufficient mass and frequency are produced in current-generation $\Lambda$CDM-based semi-analytic models of galaxy formation. In terms of halo growth, we find a late-assembling halo fraction within a factor-of-two of our late bloomer fraction. However, sufficiently delaying star formation in such halos may be a challenge for the baryon component of such models.