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

Modern data empower observers to describe galaxies as the spatially and biographically complex objects they are. We illustrate this through case studies of four, $z\sim 1.3$ 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 $\sim$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 starforming; stellar masses $\log M_*/M_\odot=10.5$to$11.2$—enables us to draw spatio-temporal inferences relevant to key areas of parameter space (Milky Way- to super-Andromeda-mass progenitors). Specifically, we find signs that bulge mass-fractions ($B\mathrm{/}T$) and SF history shapes/spatial uniformity are linked, such that higher $B\mathrm{/}T$s correlate with “inside-out growth” and central specific SFRs that peaked above the global average for all starforming galaxies at that epoch. Conversely, the system with the lowest $B\mathrm{/}T$ 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. These samples—spanning representative mass, redshift, SFR, and environmental regimes—will be ripe for converting into thousands of sub-galactic-scale empirical windows on what individual systems actually looked like in the past, ushering in a new dialog between observation and theory.