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

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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 βPL=−2.7−0.5+0.5 and from SED fitting, βSED=−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 βSED,corr=−2.5−0.2+0.2 . We measure physical properties for our galaxies with Bagpipes and find that our faint ( MUV=−18.1−0.9+0.7 ) sample is low in mass ( log[M*/M⊙]=7.7−0.5+0.5 ), fairly dust-poor ( Av=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+0.4M⊙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).