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, beta, for a sample of 36 faint star-forming galaxies at z similar to 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 similar to-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 beta from photometric power-law fitting of beta PL = - 2.7( - 0.5) (+ 0.5) and from SED fitting, beta 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 SED , 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-circle star ] = 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 + 0.4 M(circle star)yr (- 1) . We find no strong evidence for ultrablue UV spectral slopes (beta similar to-3) within our sample, as would be expected for exotically metal-poor (Z/Z(circle star) < 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 similar to 9-16 should have only modestly low metallicities (Z/Z(circle star)similar to 0.1-0.2).