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

Rachana Bhatawdekar, Christopher J. Conselice

Published 2020-05-29Version 1

We present new measurements of the UV spectral slope $\beta$ for galaxies at $z=6-9$ in the Frontier Field cluster MACSJ0416.1-2403 and its parallel field, to an unprecedented level of low stellar mass. To calculate $\beta$, we fit synthetic stellar population models to the observed spectral energy distribution and calculate its value by fitting a power law to the best-fit spectrum. This is the first derivation of rest-frame UV colours of galaxies extending out to $z=9$ for the Frontier Fields program that probes magnitudes as faint as $M\mathrm{_{UV}=-13.5}$. We find no correlation between $\beta$ and rest-frame UV magnitude $M_{1500}$ at all redshifts but we find a strong correlation between $\beta$ and stellar mass, with lower mass galaxies exhibiting bluer UV slopes. At $z=7$ we find that the bluest value of our sample is $\beta=-2.32\pm0.31$, which is redder than previously reported values at similar redshifts in the literature, whereas at $z\sim9$ we find that our bluest data point has a value of $\beta=-2.63\pm0.12$. Thus, we find no evidence for extreme stellar populations or evidence for Pop III stars in low-luminosity galaxies at $z>6$. Additionally, we find a strong correlation between $\beta$ and SFR such that galaxies with low SFRs exhibit bluer slopes, which appear to get bluer with increasing redshift at a given SFR. We also find a star formation main sequence up to $z = 9$ with a rising SFRs with increasing stellar mass. All of these relations show that $\beta$ values correlate with a process that drives both the overall star formation rate and stellar mass assembly. Furthermore, as we also observe no trend between $\beta$ and specific star formation rate (sSFR), this suggests that whatever is setting $\beta$ is not a local process but a global one driven by the scale of the galaxy.