Rest-frame UV Colors for Faint Galaxies at z ∼ 9–16 with the JWST NGDEEP Survey

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 β PL = − 2.7 − 0.5 + 0.5 and from SED fitting, β 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 β SED , corr = − 2.5 − 0.2 + 0.2 . We measure physical properties for our galaxies with Bagpipes and find that our faint ( M UV = − 18.1 − 0.9 + 0.7 ) sample is low in mass ( log [ M * / M ⊙ ] = 7.7 − 0.5 + 0.5 ), fairly dust-poor ( A v = 0.1 − 0.1 + 0.2 mag), and modestly young ( log [ age ] = 7.8 − 0.8 + 0.2 yr) with a median star formation rate of log ( SFR ) = − 0.3 − 0.4 + 0.4 M ⊙ 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).