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

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We present new measurements of the UV spectral slope $\beta$ 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 $\beta$ 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 $M\mathrm{_{UV}=-13.5}$ at $z=6$.
• We find no significant correlation between $\beta$ and rest-frame UV magnitude $M_{1500}$ all redshifts, but we do find a strong correlation between $\beta$ and stellar mass with lower mass galaxies exhibiting bluer UV slopes.