Testing Photometric Techniques for Measuring the Rest-Frame UV Spectral Slope Against JWST PRISM Spectroscopy

TL;DR

This study evaluates various photometric methods for accurately measuring the UV spectral slope of high-redshift galaxies using JWST data, providing best practices for future research.

Contribution

It compares four photometric techniques against spectroscopic measurements to identify the most reliable method for estimating the UV slope in JWST observations.

Findings

Photometric power-law fitting best recovers spectroscopic slopes.

SED fitting performs moderately well, especially at low S/N.

Single-color estimates are less reliable and not recommended with multiple colors.

Abstract

We present a sample of 53 galaxy spectra at z_spec ~ 5-12 from the JWST CEERS and RUBIES surveys, combining NIRSpec PRISM spectroscopy with NIRCam photometry. We aim to use these data to establish best practices for measuring the UV spectral slope ($\beta$) in the era of JWST. We adopt power-law fits to the rest-frame UV continuum from the spectroscopic data as our fiducial, or `true', $\beta$ values, and compare them to photometric estimates derived through four methods: (1) photometric power-law fitting, (2) power-law fitting to an SED model fitted to the photometry, (3) single-color fitting near the Lyman break, and (4) single-color fitting at fixed rest-frame wavelengths. We find that photometric power-law fitting most closely recovers the spectroscopic slopes, with minimal bias and scatter. SED fitting performs moderately well, and can be preferable in cases of low signal-to-noise where photometric power-law fitting may become unreliable. Single-color estimates, while commonly used in past studies, show the most significant deviations and are not recommended when more than a single color is available. Our results highlight the limitations and strengths of each approach and provide practical guidance for measuring $\beta$ from photometry when spectra are unavailable or are of insufficient quality.