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

Abstract

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 ${\beta }_{\mathrm{PL}}=-{2.7}_{-0.5}^{+0.5}$ and from SED fitting, ${\beta }_{\mathrm{SED}}=-{2.3}_{-0.1}^{+0.2}$ 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 ${\beta }_{\mathrm{SED},\mathrm{corr}}=-{2.5}_{-0.2}^{+0.2}$ . We measure physical properties for our galaxies with Bagpipes and find that our faint ( ${M}_{\mathrm{UV}}=-{18.1}_{-0.9}^{+0.7}$ ) sample is low in mass ( $\mathrm{log}[{M}_{* }/{M}_{\odot }]={7.7}_{-0.5}^{+0.5}$ ), fairly dust-poor ( ${A}_{{\rm{v}}}={0.1}_{-0.1}^{+0.2}$ mag), and modestly young ( $\mathrm{log}[\mathrm{age}]={7.8}_{-0.8}^{+0.2}$ yr) with a median star formation rate of $\mathrm{log}(\mathrm{SFR})=-{0.3}_{-0.4}^{+0.4}{M}_{\odot }\,{\mathrm{yr}}^{-1}$ . 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).

Keywords

• Ultraviolet color
• High-redshift galaxies
• Early universe
• Galaxy evolution
• Galaxy formation