Constraining the low-mass slope of the star formation sequence at 0.5 < z < 2.5

We constrain the slope of the star formation rate (SFR; log Ψ) to stellar mass (log M ∗) relation down to log (M ∗/M⊙) = 8.4 (log (M ∗/M⊙) = 9.2) at z = 0.5 (z = 2.5) with a mass-complete sample of 39,106 star-forming galaxies selected from the 3D-HST photometric catalogs, using deep photometry in the CANDELS fields. For the first time, we find that the slope is dependent on stellar mass, such that it is steeper at low masses (log Ψlog M ∗) than at high masses (log Ψ(0.3-0.6)log M ∗). These steeper low-mass slopes are found for three different star formation indicators: the combination of the ultraviolet (UV) and infrared (IR), calibrated from a stacking analysis of Spitzer/MIPS 24 μm imaging; β-corrected UV SFRs; and Hα SFRs. The normalization of the sequence evolves differently in distinct mass regimes as well: for galaxies less massive than log (M ∗/M⊙) < 10 the specific SFR (Ψ/M ∗) is observed to be roughly self-similar with Ψ/M ∗∝(1 + z)1.9, whereas more massive galaxies show a stronger evolution with Ψ/M ∗∝(1 + z)2.2-3.5 for log (M ∗/M⊙) = 10.2-11.2. The fact that we find a steep slope of the star formation sequence for the lower mass galaxies will help reconcile theoretical galaxy formation models with the observations.