PDRs4All

XIII. Empirical prescriptions for the interpretation of JWST imaging observations of star-forming regions

¹ Department of Physics & Astronomy, The University of Western Ontario, London ON N6A 3K7, Canada
² Institute for Earth and Space Exploration, The University of Western Ontario, London ON N6A 3K7, Canada
³ Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
⁴ I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
⁵ Carl Sagan Center, SETI Institute, 339 Bernardo Avenue, Suite 200, Mountain View, CA 94043, USA
⁶ Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁸ Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III – Paul Sabatier, CNRS, CNES, 9 Av. du colonel Roche, 31028 Toulouse Cedex 04, France
⁹ Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA
¹⁰ NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035-1000, USA
¹¹ Institut des Sciences Moléculaires d’Orsay, CNRS, Univ. Paris-Saclay, 91405 Orsay, France
¹² Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
¹³ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Bunkyo-ku, Tokyo 113-0033, Japan

^★ Corresponding author: rchown53@gmail.com

Received: 9 November 2024
Accepted: 9 April 2025

Abstract

Context. JWST continues to deliver incredibly detailed infrared (IR) images of star-forming regions in the Milky Way and beyond. IR emission from star-forming regions is very spectrally rich due to emission from gas-phase atoms, ions, and polycyclic aromatic hydrocarbons (PAHs). Physically interpreting IR images of these regions relies on assumptions about the underlying spectral energy distribution in the imaging bandpasses.

Aims. We aim to provide empirical prescriptions to derive line, PAH, and continuum intensities from JWST images. These prescriptions will facilitate the interpretation of images in a wide variety of astrophysical contexts. We also measure the level of agreement between JWST imaging and integral field spectroscopy.

Methods. We use JWST PDRs4All Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) imaging and Near-Infrared Spectrograph (NIRSpec) integral field unit (IFU) and MIRI Medium Resolution Spectrograph (MRS) spectroscopic observations of the Orion Bar, the prototypical photodissociation region (PDR), to directly compare and cross-calibrate imaging and IFU data at ~100 AU resolution over a region where the radiation field and ISM environment evolves from hot ionized gas to warm neutral gas followed by cold molecular gas. We study the relative contributions of line, PAH, and continuum emission to the NIRCam and MIRI filters as functions of local physical conditions, and investigate filter combinations that represent selected line and PAH emission.

Results. We provide empirical prescriptions based on NIRCam and MIRI images that may be used to derive intensities of strong emission lines and PAH features. Within the range of the environments probed in this study, these prescriptions accurately predict Pa α, Brα, and PAH 3.3 μm and 11.2 μm intensities, while those for [Fe II] 1.644 μm, H₂ 1–0 S(1) 2.12 μm and 0–0 S(9) 4.69 μm, and PAH 7.7 μm show more complicated environmental dependencies.

Conclusions. Linear combinations of JWST NIRCam and MIRI images provide effective tracers of ionized gas, H₂, and PAH emission in PDRs. We expect these recipes to be useful for both the Galactic and extragalactic communities. The flux calibration between imaging and spectroscopy is found to agree within 1–20% for NIRCam and NIRSpec, and 2–7% for MIRI Imager and MRS.