Early Results from GLASS-JWST. XVII. Building the First Galaxies—Chapter 1. Star Formation Histories for 5 < z < 7 Galaxies

The JWST observations of high-redshift galaxies are used to measure their star formation histories—the buildup of stellar mass in the earliest galaxies. Here we use a novel analysis program, SEDz*, to compare near-IR spectral energy distributions for galaxies with redshifts 5 < z < 7 to combinations of stellar population templates evolved from z = 12. We exploit NIRCam imaging in seven wide bands covering 1–5 μ m taken in the context of the GLASS-JWST-ERS program and use SEDz* to solve for well-constrained star formation histories for 24 exemplary galaxies. In this first look, we find a variety of histories, from long, continuous star formation over 5 < z < 12 to short but intense starbursts, sometimes repeating, and, most commonly, contiguous mass buildup lasting ∼0.5 Myr, possibly the seeds of today’s typical M * galaxies.