UV Spectral-Slopes at z=6-9 in the Hubble Frontier Fields: Lack of Evidence for Unusual or Pop III Stellar Populations
Abstract
We present new measurements of the UV spectral slope β for galaxies at z=6-9 in the Frontier Fields cluster MACSJ0416.1-2403 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 colours of galaxies for the Frontier Fields program extending out to z=9, probing magnitudes as faint as MUV=-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 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 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.
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