PDRs4All

XVI. Tracing aromatic infrared band characteristics in photodissociation region spectra with PAHFIT in the JWST era

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 Carl Sagan Center, SETI Institute, 339 Bernardo Avenue, Suite 200, Mountain View, CA 94043, USA
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, B-9000 Gent, Belgium
⁶ Ritter Astrophysical Research Center, University of Toledo, Toledo, OH 43606, USA
⁷ IPAC, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
⁸ NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035-1000, USA
⁹ Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III – Paul Sabatier, CNRS, CNES, 9 Av. du colonel Roche, 31028 Toulouse Cedex 04, France
¹⁰ Institut des Sciences Moléculaires d’Orsay, Université Paris-Saclay, CNRS, Bâtiment 520, 91405 Orsay Cedex, France
¹¹ Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
¹² Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Bunkyo-ku, Tokyo 113-0033, Japan
¹³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁴ Astronomy Department, University of Maryland, College Park, MD 20742, USA

^★ Corresponding author.

Received: 1 April 2025
Accepted: 26 June 2025

Abstract

Context. Photodissociation regions (PDRs) exhibit strong emission bands between 3–20 μm known as the aromatic infrared bands (AIBs), and they originate from small carbonaceous species such as polycyclic aromatic hydrocarbons (PAHs) excited by UV radiation. The AIB spectra observed in Galactic PDRs are considered a local analog for those seen in extragalactic star-forming regions. Recently, the PDRs4All JWST program observed the Orion Bar PDR, revealing the subcomponents and profile variations of the AIBs in very high detail.

Aims. We present the Python version of PAHFIT, a spectral decomposition tool that separates the contributions by AIB subcomponents, thermal dust emission, gas lines, stellar light, and dust extinction. We aim to provide a configuration that enables highly detailed decompositions of JWST spectra of PDRs (3.1–26 μm) and to test if the same configuration is suitable to characterize AIB emission in extragalactic star forming regions.

Methods. We determined the central wavelength and FWHM of the AIB subcomponents by fitting selected segments of the Orion Bar spectra and compiled them into a “PDR pack” for PAHFIT. We tested the PDR pack by applying PAHFIT to the full 3.1–26 μm PDRs4All templates. We applied PAHFIT with this PDR pack and the default continuum model to seven spectra extracted from the central star forming ring of the galaxy NGC7469.

Results. We introduce an alternate dust continuum model to fit the Orion Bar spectra, as the default PAHFIT continuum model mismatches the intensity at 15–26 μm. Using the PDR pack and the alternate continuum model, PAHFIT reproduces the Orion Bar template spectra with residuals of a few percent. A similar performance is achieved when applying the PDR pack to the NGC7469 spectra. We provide PAHFIT-based diagnostics that trace the profile variations of the 3.3, 3.4, 5.7, 6.2, and 7.7 μm AIBs and thus the photochemical evolution of the AIB carriers. The 5.7 μm AIB emission originates from at least two subpopulations, one more prominent in highly irradiated environments and one preferring more shielded environments. Smaller PAHs as well as very small grains or PAH clusters both thrive in the more shielded environments of the molecular zone in the Orion Bar. Based on these new diagnostics, we show and quantify the strong similarity of the AIB profiles observed in NGC7469 to the Orion Bar template spectra.