Late Bloomer Galaxies: Hiding in Plain Sight

Dr. Alan Dressler (Carnegie)
June 7, 2018 - 4:00pm

Abstract

Measuring the star formation histories (SFHs) and stellar mass growth of individual galaxies is challenging because stellar populations older than 2 Gyr have largely degenerate spectrophotometric properties.  For this reason, the SFHs of  present-epoch galaxies are largely obscured: the ubiquitous "tau-model" of explonential decline of  the the star formation rate is a result of that insensitivity.  In contast, the study of galaxies between 5 and 7 Gyr in lookback time allows a reliable measure of individual SFHs from z~1 to z ~ 0.5 and a secure comparison to their star formation before z~1.  From this better vantage point, we have found a wide diversity of SFHs in our study of ~8000 CSI-Survey galaxies log M = 10-11 Msun.  In particular,  our group has identified a ~20% population of "late bloomers"--Milky Way mass galaxies that formed most of their stellar mass between z~1 and z~0.5.  This discovery could have important implications for popular models where quasi-universal SFHs derived by abundance matching or integrating SFR scaling laws are "quenched" later in life, and also for the utility of linking stellar mass growth to that of the dark matter halo as a tool in studies of galaxy evolution.