2021ApJ...909..144B

We present new measurements of the UV spectral slope β for galaxies at z = 6-9 in the Frontier Field cluster MACS J0416.1-2403 and its parallel field to an unprecedented level of low stellar mass. We fit synthetic stellar population models to the observed spectral energy distribution and calculate β by fitting a power law to the best-fit spectrum. With this method, we report the derivation of rest-frame UV colors of galaxies for the Frontier Fields program extending out to z = 9, probing magnitudes as faint as M_UV = -13.5 at z = 6. We find no significant correlation between β and rest-frame UV magnitude M₁₅₀₀ all redshifts, but we do find a strong correlation between β and stellar mass, with lower-mass galaxies exhibiting bluer UV slopes. At z = 7, the bluest median value of our sample is redder than the previously reported values in the literature, whereas at z = 9, our bluest data point has a median value of β=-2.63_–0.43^+0.52. Thus, we find no evidence for extreme stellar populations at z > 6. We also observe a strong correlation between β and star formation rate (SFR), such that galaxies with low SFRs exhibit bluer slopes. Additionally, there exists a star formation main sequence up to z = 9 with SFRs correlating with stellar mass. All of these relations show that β values correlate with a process that drives both the overall SFR and stellar mass assembly. Furthermore, we observe no trend between β and specific SFR, suggesting that β is getting set by a global process driven by the scale of the galaxy.