UV Spectral Slopes at z = 6?9 in the Hubble Frontier Fields: Lack of Evidence for Unusual or Population III Stellar Populations

We present new measurements of the UV spectral slope $\beta$ for galaxies at $z=6-9$ in Frontier Fields cluster MACSJ0416.1-2403 and its parallel field, to an unprecedented level low stellar mass. fit synthetic population models observed energy distribution calculate by fitting a power law best-fit spectrum. With this method, we report derivation rest-frame colours program extending out $z=9$, probing magnitudes as faint $M\mathrm{_{UV}=-13.5}$ $z=6$. find no significant correlation between magnitude $M_{1500}$ all redshifts, but do strong mass with lower exhibiting bluer slopes. At $z=7$ bluest median value our sample is redder than previously reported values literature, whereas $z=9$ data point has $\beta=-2.63_{-0.43}^{+0.52}$. Thus, evidence extreme populations $z>6$. also observe SFR, such that SFRs exhibit Additionally, there exists star formation main sequence up $z = 9$ correlating All these relations show correlate process drives both overall SFR assembly. Furthermore, trend specific suggesting getting set global driven scale galaxy.