THE CHARACTERISTIC STAR FORMATION HISTORIES OF GALAXIES AT REDSHIFTSz∼ 2-7

A large sample of spectroscopically confirmed star-forming galaxies at redshifts 1. 4 ≤ z_ (spec) ≤ 3. 7, with imaging in the near- and mid-IR from the ground and from the Hubble Space Telescope and Space Telescope, is used to infer the average star formation histories (SFHs) of typical galaxies from z ∼ 2 7. For a subset of 302 galaxies at 1. 5 ≤ z_ (spec) 25. 5) galaxies—for _* ≳ 5 × 10⁸M_⊙ and SFRs ≳ 2M_⊙ yr^ (−1). We interpret this result in the context of several systematic biases that affect determinations of the SFR–M_* relation. The average specific SFRs at z ∼ 2–3 are remarkably similar a factor of two to those measured at z ≳ 4, implying that the average SFH is one where SFRs increase with. A consequence of these rising SFHs is that (1) a substantial fraction of UV-bright z ∼ 2–3 galaxies had faint -L* progenitors at z ≳ 4; and (2) gas masses must increase with time from z = 2 to 7, over which time the cold gas accretion rate—as inferred from the specific SFR and the Kennicutt–Schmidt relation—is ∼2–3 times than the SFR. However, if we evolve to higher redshift the SFHs and masses of the halos that are expected host L* galaxies at z ∼ 2, then we find that ≾10% of the baryons accreted onto typical halos at z ≳ 4 actually to star formation at those epochs. These results highlight the relative inefficiency of star formation even early cosmic times when galaxies were first assembling.