The Grism Lens-amplified Survey from Space (GLASS). XII. Spatially Resolved Galaxy Star Formation Histories and True Evolutionary Paths at z > 1*

Abstract

Modern data empower observers to describe galaxies as the spatially and biographically complex objects they are. We illustrate this through case studies of four z  ∼ 1.3 systems based on deep, spatially resolved, 17-band + G102 + G141 Hubble Space Telescope grism spectrophotometry. Using full-spectrum rest-UV/-optical continuum fitting, we characterize these galaxies’ observed ∼kpc-scale structures and star formation rates (SFRs) and reconstruct their history over the age of the universe. The sample’s diversity—passive to vigorously star-forming; stellar masses log /  = 10.5 to 11.2—enables us to draw spatiotemporal inferences relevant to key areas of parameter space (Milky Way– to super-M31–mass progenitors). Specifically, we find signs that bulge mass fractions ( B / T ) and SF history shapes/spatial uniformity are linked, such that higher B / T s correlate with “inside-out growth” and central specific SFRs that peaked above the global average for all star-forming galaxies at that epoch. Conversely, the system with the lowest B / T had a flat, spatially uniform SFH with normal peak activity. Both findings are consistent with models positing a feedback-driven connection between bulge formation and the switch from rising to falling SFRs (“quenching”). While sample size forces this conclusion to remain tentative, this work provides a proof-of-concept for future efforts to refine or refute it: JWST , WFIRST , and the 30 m class telescopes will routinely produce data amenable to this and more sophisticated analyses. Such samples spanning representative mass, redshift, SFR, and environmental regimes will be ripe for converting into thousands of subgalactic-scale empirical windows on what individual systems actually looked like in the past, ushering in a new dialogue between observation and theory.