UV Spectral-Slopes at $z=6-9$ in the Hubble Frontier Fields: Lack of
  Evidence for Unusual or Pop III Stellar Populations
  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. 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 $\beta=-2.63_{-0.43}^{+0.52}$. Thus, we
find no evidence for extreme stellar populations at $z>6$. We also observe a
strong correlation between $\beta$ 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 $\beta$ values correlate with a process that drives both the overall
SFR and stellar mass assembly. Furthermore, we observe no trend between $\beta$
and specific SFR, suggesting that $\beta$ is getting set by a global process
driven by the scale of the galaxy.
