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

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 similar to 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 mu m 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 greater than or similar to 2. The SFRs of z similar to 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-* greater than or similar to 5 x 10(8) M-circle dot and SFRs greater than or similar to 2M(circle dot) 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 similar to 2-3 are remarkably similar within a factor of two to those measured at z greater than or similar to 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 similar to 2-3 galaxies had faint sub-L* progenitors at z similar to 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 similar to 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 similar to 2, then we find that less than or similar to 10% of the baryons accreted onto typical halos at z greater than or similar to 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.