Testing Photometric Techniques for Measuring the Rest-frame UV Spectral Slope against JWST PRISM Spectroscopy
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 ( β ) 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,” β values, and compare them to photometric estimates derived through three methods: (1) photometric power-law fitting, (2) power-law fitting to a spectral energy distribution (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 and SED fitting are comparable and most closely recover the spectroscopic slopes, with minimal bias and scatter. 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 β from photometry when spectra are unavailable or are of insufficient quality.
Authors
- Ray A. Lucas (ORCID: https://orcid.org/0000-0003-1581-7825)
- Mauro Giavalisco (ORCID: https://orcid.org/0000-0002-7831-8751)
- Alexa M. Morales (ORCID: https://orcid.org/0000-0003-4965-0402)
- Stephen M. Wilkins (ORCID: https://orcid.org/0000-0003-3903-6935)
- Alexander de la Vega (ORCID: https://orcid.org/0000-0002-6219-5558)
- Óscar A. Chávez Ortiz (ORCID: https://orcid.org/0000-0003-2332-5505)
- Fabio Pacucci (ORCID: https://orcid.org/0000-0001-9879-7780)
- Nimish P. Hathi (ORCID: https://orcid.org/0000-0001-6145-5090)
- Pablo Arrabal Haro (ORCID: https://orcid.org/0000-0002-7959-8783)
- Casey Papovich (ORCID: https://orcid.org/0000-0001-7503-8482)
- Anton M. Koekemoer (ORCID: https://orcid.org/0000-0002-6610-2048)
- Anthony J. Taylor (ORCID: https://orcid.org/0000-0003-1282-7454)
- Antonello Calabrò (ORCID: https://orcid.org/0000-0003-2536-1614)
- L. Y. Aaron Yung (ORCID: https://orcid.org/0000-0003-3466-035X)
- Kelcey Davis (ORCID: https://orcid.org/0000-0001-8047-8351)
- Nor Pirzkal (ORCID: https://orcid.org/0000-0003-3382-5941)
- Steven L. Finkelstein (ORCID: https://orcid.org/0000-0001-8519-1130)
- Mark Dickinson (ORCID: https://orcid.org/0000-0001-5414-5131)
- Micaela B. Bagley (ORCID: https://orcid.org/0000-0002-9921-9218)
- Eric Gawiser (ORCID: https://orcid.org/0000-0003-1530-8713)
- Arianna S. Long (ORCID: https://orcid.org/0000-0002-7530-8857)
- Jeyhan S. Kartaltepe (ORCID: https://orcid.org/0000-0001-9187-3605)
- Michaela Hirschmann (ORCID: https://orcid.org/0000-0002-3301-3321)
- Borja Pautasso
Institutions
- Rutgers, The State University of New Jersey (US)
- University of California, Riverside (US)
- Space Telescope Science Institute (US)
- Goddard Space Flight Center (US)
- Woodwell Climate Research Center (US)
- Harvard University (US)
- University of Connecticut (US)
- Rochester Institute of Technology (US)
- University of Sussex (GB)
- University of Malta (MT)
- University of Washington (US)
- University of Massachusetts Amherst (US)
- Astronomical Observatory of Rome (IT)
- Mitchell Institute (US)
- Université Bourgogne Franche-Comté (FR)
- Center for Astrophysics Harvard & Smithsonian (US)
- NSF NOIRLab (US)
- Deleted Institution
- École Polytechnique Fédérale de Lausanne (CH)
- Seattle University (US)
- The University of Texas at Austin (US)
- National Institute for Astrophysics (IT)
- Texas A&M University (US)
Publication Details
- Journal
- The Astrophysical Journal
- Published
- 2025-11-26
- DOI
- https://doi.org/10.3847/1538-4357/ae145c
- Primary Topic
- Galaxies: Formation, Evolution, Phenomena
- Type
- article
- Field-Weighted Citation Impact
- 0.00