[Paper Review] EPOCHS III: Unbiased UV continuum slopes at 6.5

Duncan Austin, Christopher J. Conselice+35 more|arXiv (Cornell University)|Apr 16, 2024

Atmospheric and Environmental Gas Dynamics9 citations

TL;DR

This paper measures rest-frame UV continuum slopes (β) for 1011 galaxies in the redshift range 6.5–13 using combined PEARLS GTO and public JWST NIRCam imaging, correcting for photometric biases and examining implications for dust, metallicity, and possible exotic stellar populations.

ABSTRACT

We present an analysis of rest-frame UV continuum slopes, $β$, using a sample of 1011 galaxies at $6.5

Motivation & Objective

• Quantify rest-frame UV continuum slopes (β) for a large, unbiased sample of galaxies at 6.5<z<13.
• Correct β measurements for photometric error coupling bias and line-emission contamination.
• Investigate the evolution of β with redshift, mass, and UV magnitude to infer dust content and stellar population properties.

Proposed method

• Compile a photometric galaxy sample from JWST/NIRCam imaging in PEARLS GTO and public ERS/GO fields across 178.9 arcmin^2 of unmasked sky.
• Compute β via two methods: (i) power-law fit to rest-frame 1250–3000 Å photometry and (ii) fitting 10 Calzetti-like (C94) templates to Bayesian SED posteriors.
• Correct β for photometric error coupling bias using 200,000 power-law SEDs per β value to quantify and remove biases up to Δβ≈−0.55 for low-SNR sources.
• Assess the impact of rest-UV line emission and damped Lyα systems on β, finding biases up to 0.5–0.6 for extreme cases.
• Select high-z candidates with stringent criteria to minimize contamination from brown dwarfs and Balmer-break interlopers, and limit analysis to 6.5<z<13 (excluding 6.5<z<7.5 in fields lacking blue HST data).
• Report β trends with redshift and stellar mass, and discuss implications for dust production and potential exotic stellar populations.

Figure 1: Contamination from lower redshift SFGs in the El Gordo (upper panels) and JADES-Deep-GS (lower panels) fields for the entire redshift range used in this work ( $7.5<z<13.0$ for NIRCam only fields and $6.5<z<13.0$ otherwise) as a function of $\Theta=(M_{\mathrm{UV}},\beta)$ (left panels) an

Experimental results

Research questions

• RQ1 What are the rest-frame UV continuum slopes (β) of galaxies in the 6.5<z<13 range observed by JWST NIRCam?
• RQ2 How do photometric errors and rest-UV line emission affect β measurements, and how can these biases be corrected?
• RQ3 What do the β–redshift, β–mass, and β–UV magnitude relations imply about dust content and stellar populations in early galaxies?
• RQ4 Is there evidence for exotic stellar populations (e.g., Pop III or top-heavy IMFs) in the highest-β blue outliers?

Key findings

• β shows a decreasing trend with redshift: β = −1.51 ± 0.08 − (0.097 ± 0.010) × z.
• A subsample of 68 galaxies with β + σβ < −2.8 hints at possible Pop. III stars or top-heavy IMFs.
• At z ≈ 11.5, β(MUV = −19) = −2.73 ± 0.06, indicating very blue continua consistent with low metallicity and nonzero LyC escape fractions (fesc,LyC ≳ 0) and minimal dust.
• The slope of β with stellar mass steepens from 0.22 ± 0.02 at z ≈ 7 to 0.81 ± 0.13 at z ≈ 11.5, suggesting dust ejected in outflows from low-mass galaxies.
• A flatter β–MUV relation (dβ/dMUV = 0.03 ± 0.02 at z ≈ 7) and a shallower dβ/dlog(M★) at z<11 reveal a population of low-mass, faint galaxies reddened by dust from AGB winds or carbon-rich Wolf-Rayet binaries.
• Results imply that early dust production mechanisms (e.g., core-collapse SNe) and dust processing influence observed UV slopes, with evidence for low dust content at very high redshift and potential deviations from some simulations.

Figure 2: Top: Rest-frame UV coverage of the JWST/NIRCam filters used in this work as a function of both redshift and $\lambda_{\mathrm{rest}}$ . The right-hand side shows the rest-wavelength coverage of the 10 C94 filters in shaded lime green which are used to avoid prominent rest frame UV nebular