EPOCHS III: Unbiased UV continuum slopes at 6.5<z<13 from combined PEARLS GTO and public JWST NIRCam imaging

Abstract

We present an analysis of rest-frame UV continuum slopes, ${\hat{I}}^2$, using a sample of 1011 galaxies at $6.5<z<13$ from the EPOCHS photometric sample collated from the GTO PEARLS and public ERS/GTO/GO (JADES, CEERS, NGDEEP, GLASS) JWST NIRCam imaging across $178.9{\mathrm{arcmin}}^2$ of unmasked blank sky. We correct our UV slopes for the photometric error coupling bias using $200,000$ power law SEDs for each ${\hat{I}}^2=\{-1,-1.5,-2,-2.5,-3\}$ in each field, finding biases as large as $\hat{I}”{\hat{I}}^2\simeq -0.55$ for the lowest SNR galaxies in our sample. Additionally, we simulate the impact of rest-UV line emission (including Ly$\hat{I}\pm$) and damped Ly$\hat{I}\pm$ systems on our measured ${\hat{I}}^2$, finding biases as large as $0.5-0.6$ for the most extreme systems. We find a decreasing trend with redshift of ${\hat{I}}^2=-1.51\pm 0.08-(0.097\pm 0.010)\times z$, with potential evidence for Pop.~III stars or top-heavy initial mass functions (IMFs) in a subsample of 68 ${\hat{I}}^2+\ddot{I}{ƒ}_{\hat{I}}^2<-2.8$ galaxies. At $z\simeq 11.5$, we measure an extremely blue ${\hat{I}}^2(M_{\mathrm{UV}}=-19)=-2.73\pm 0.06$, deviating from simulations, indicative of low-metallicity galaxies with non-zero Lyman continuum escape fractions $f_{\mathrm{esc},\mathrm{LyC}}\gtrsim 0$ and minimal dust content. The observed steepening of $\mathrm{d}{\hat{I}}^2/\mathrm{d}{\log }_{10}(M_{\star }/{\mathrm{M}}_{\odot })$ from $0.22\pm 0.02$ at $z=7$ to $0.81\pm 0.13$ at $z=11.5$ implies that dust produced in core-collapse supernovae (SNe) at early times may be ejected via outflows from low mass galaxies. We also observe a flatter $\mathrm{d}{\hat{I}}^2/\mathrm{d}{M}_{\mathrm{UV}}=0.03\pm 0.02$ at $z=7$ and a shallower $\mathrm{d}{\hat{I}}^2/\mathrm{d}{\log }_{10}(M_{\star }/{\mathrm{M}}_{\odot })$ at $z<11$ than seen by HST, unveiling a new population of low mass, faint, galaxies reddened by dust produced in the stellar winds of asymptotic giant branch (AGB) stars or carbon-rich Wolf-Rayet binaries.

37 pages, 18 figures, 8 tables. Data available via the Github page at https://github.com/duncanaustin98/EPOCHS-III-data. Submitted to ApJ on 16/04/2024