The characteristic star formation histories of galaxies at redshifts z ∼ 2-7

Abstract

A large sample of spectroscopically confirmed star-forming galaxies at redshifts 1.4 ≤ z spec ≤ 3.7, with complementary imaging in the near- and mid-IR from the ground and from the Hubble Space Telescope and Spitzer Space Telescope, is used to infer the average star formation histories (SFHs) of typical galaxies from z ∼ 2 to 7. For a subset of 302 galaxies at 1.5 ≤ z spec < 2.6, we perform a detailed comparison of star formation rates (SFRs) determined from spectral energy distribution (SED) modeling (SFRs[SED]) and those calculated from deep Keck UV and Spitzer/MIPS 24 μm imaging (SFRs[IR+UV]). Exponentially declining SFHs yield SFRs[SED] that are 5-10 times lower on average than SFRs[IR+UV], indicating that declining SFHs may not be accurate for typical galaxies at z ≳ 2. The SFRs of z ∼ 2-3 galaxies are directly proportional to their stellar masses (M *), with unity slope - a result that is confirmed with Spitzer/IRAC stacks of 1179 UV-faint (R ≥ 25.5) galaxies - for M * ≳ 5 × 108 M⊙ and SFRs ≳ 2 M⊙yr-1. We interpret this result in the context of several systematic biases that can affect determinations of the SFR-M * relation. The average specific SFRs at z ∼ 2-3 are remarkably similar within a factor of two to those measured at z ≳ 4, implying that the average SFH is one where SFRs increase with time. A consequence of these rising SFHs is that (1) a substantial fraction of UV-bright z ∼ 2-3 galaxies had faint sub-L* progenitors at z ≳ 4; and (2) gas masses must increase with time from z = 2 to 7, over which time the net cold gas accretion rate - as inferred from the specific SFR and the Kennicutt-Schmidt relation - is ∼2-3 times larger than the SFR. However, if we evolve to higher redshift the SFHs and masses of the halos that are expected to host L* galaxies at z ∼ 2, then we find that ≲ 10% of the baryons accreted onto typical halos at z ≳ 4 actually contribute to star formation at those epochs. These results highlight the relative inefficiency of star formation even at early cosmic times when galaxies were first assembling. © 2012. The American Astronomical Society. All rights reserved.