2024ApJ...964L..24M

We present measurements of the rest-frame UV spectral slope, β, for a sample of 36 faint star-forming galaxies at z ∼ 9-16 discovered in one of the deepest JWST NIRCam surveys to date, the Next Generation Deep Extragalactic Exploratory Public Survey. We use robust photometric measurements for UV-faint galaxies (down to M_UV ∼ -16), originally published in Leung et al., and measure values of the UV spectral slope via photometric power-law fitting to both the observed photometry and stellar population models obtained through spectral energy distribution (SED) fitting with BAGPIPES. We obtain a median and 68% confidence interval for β from photometric power-law fitting of ${\beta }_{\mathrm{PL}}=-{2.7}_{-0.5}^{+0.5}$ and from SED fitting, ${\beta }_{\mathrm{SED}}=-{2.3}_{-0.1}^{+0.2}$ for the full sample. We show that when only two to three photometric detections are available, SED fitting has a lower scatter and reduced biases than photometric power-law fitting. We quantify this bias and find that after correction the median ${\beta }_{\mathrm{SED},\mathrm{corr}}=-{2.5}_{-0.2}^{+0.2}$. We measure physical properties for our galaxies with BAGPIPES and find that our faint (${M}_{\mathrm{UV}}=-{18.1}_{-0.9}^{+0.7}$) sample is low in mass ($\mathrm{log}[{M}_{* }/{M}_{\odot }]={7.7}_{-0.5}^{+0.5}$), fairly dust-poor (${A}_{{\rm{v}}}={0.1}_{-0.1}^{+0.2}$ mag), and modestly young ($\mathrm{log}[\mathrm{age}]={7.8}_{-0.8}^{+0.2}$ yr) with a median star formation rate of $\mathrm{log}(\mathrm{SFR})=-{0.3}_{-0.4}^{+0.4}{M}_{\odot }\,{\mathrm{yr}}^{-1}$. We find no strong evidence for ultrablue UV spectral slopes (β ∼ -3) within our sample, as would be expected for exotically metal-poor (Z/Z_⊙ < 10^–3) stellar populations with very high Lyman continuum escape fractions. Our observations are consistent with model predictions that galaxies of these stellar masses at z ∼ 9-16 should have only modestly low metallicities (Z/Z_⊙ ∼ 0.1-0.2).