Rest-Frame UV Colors for Faint Galaxies at $z\sim 9-16$ with the \textit{JWST} NGDEEP Survey

Abstract

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, ${\beta }_{SED}=-2.3_{-0.1}^{+0.2}$ 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 ($M_{UV}=-18.1_{-0.9}^{+0.7}$) sample is low mass ($log[M_{\ast }/M_{\odot }]=7.7_{-0.5}^{+0.5}$), 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 $\log (\mathrm{SFR})=-0.3_{-0.4}^{+0.4}{M}_{\odot }/yr$. 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).

21 pages, 8 figures, 3 tables, submitted to ApjL