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 β PL = − 2.7 − 0.5 + 0.5 and from SED fitting, β 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 β 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 in mass ( log [ M * / M ⊙ ] = 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 ( SFR ) = − 0.3 − 0.4 + 0.4 M ⊙ 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).

The Complete CEERS Early Universe Galaxy Sample: A Surprisingly Slow Evolution of the Space Density of Bright Galaxies at z ∼ 8.5–14.5

“…To simulate the fluxes in all observed HST and JWST/ NIRCam filters, we use BC03 (Bruzual & Charlot 2003) stellar population models with a distribution of stellar population age, dust attenuation, and metallicity tuned to reproduce the expected (and now observed; e.g., Cullen et al 2023;Morales et al 2024) blue colors of very high redshift galaxies (see Finkelstein et al 2015 for details on these models). The result is a lognormal distribution of rest-UV colors, which peaks at F200W-F277W = −0.05, with a 68% spread from F200W-F277W = −0.25 to +0.3, comparable to the measured colors of our observed objects (median of −0.1, 68% spread from −0.3 to 0.3).…”

Haro 11: The Spatially Resolved Lyman Continuum Sources

“…On the other hand, while dwarf starbursts seem to be promising candidates for reionization agents (Bouwens et al 2012;Sharma et al 2017;Yeh et al 2023), they may not have sufficiently high escape fractions (e.g., Fontanot et al 2014). JWST is now revealing the blue UV slopes of the earliest, z = 8-16, galaxies, pointing to young and dust-poor stellar populations (Cullen et al 2023(Cullen et al , 2024Morales et al 2024;Topping et al 2024), in further support of galaxy-driven reionization. JWST is moreover uncovering an abundance of high-redshift starbursts with high ionizing photon production efficiencies, implying that galaxies could have reionized the Universe with somewhat lower escape fractions than previously assumed (Matthee et al 2023;Atek et al 2024).…”