摘要
We present measurements of the rest-frame UV spectral slope,$\beta$ , 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 (NGDEEP) Survey. We use robust photometric measurements for UV-faint galaxies (down to$M_{UV}$~ -16), originally published in Leung+23, and measure values of the UV spectral slope via photometric power-law fitting to both the observed photometry and to 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,$\beta_{SED} = -2.3^{+0.2}_{-0.1}$for the full sample. We show that when only 2-3 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 ( $M_{UV} = -18.1^{+0.7}_{-0.9}$ ) sample is low mass ( ${log}[M_{\ast}/M_\odot] = 7.7^{+0.5}_{-0.5}$ ), fairly dust-poor ( $A_{v} = 0.1^{+0.2}_{-0.1}$mag), and modestly young ( ${log[age]} = 7.8^{+0.2}_{-0.8}$yr) with a median star formation rate of$\mathrm{log(SFR)} = -0.3^{+0.4}_{-0.4} M_\odot{/yr}$ . We find no strong evidence for ultra-blue UV spectral slopes ( $\beta$~ -3) within our sample, as would be expected for exotically metal-poor ( $Z/Z_{\odot}$< 10 $^{-3}$ ) stellar populations with very high LyC 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 ( $Z/Z_{\odot}$~ 0.1--0.2).