PDRs4All

VII. The 3.3 μm aromatic infrared band as a tracer of physical properties of the interstellar medium in galaxies^★

¹ Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III – Paul Sabatier CNRS, CNES, 9 Av. du colonel Roche, 31028 Toulouse Cedex 04, France
e-mail: ilane.schroetter@gmail.com
² Department of Physics & Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada
³ Institute for Earth and Space Exploration, The University of Western Ontario, London ON N6A 3K7, Canada
⁴ Carl Sagan Center, SETI Institute, 339 Bernardo Avenue, Suite 200, Mountain View, CA 94043, USA
⁵ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, The Netherlands
⁶ Department of Chemistry, GITAM School of Science, GITAM Deemed to be University, Bangalore, India
⁷ Institut de Physique de Rennes, UMR CNRS 6251, Université de Rennes 1, Campus de Beauties, 35042 Rennes Cedex, France
⁸ Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
⁹ IPAC, California Institute of Technology, Pasadena, CA, USA
¹⁰ Molecular Photonics, Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹¹ Space Telescope Science Institute, Baltimore, MD 21218, USA
¹² Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA

Received: 15 December 2023
Accepted: 16 February 2024

Abstract

Aromatic infrared bands (AIBs) are a set of broad emission bands at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 μm, seen in the infrared spectra of most galaxies. With the James Webb Space Telescope (JWST), the 3.3 μm AIB can in principle be detected up to a redshift of ~7. Relating the evolution of the 3.3 μm AIB to local physical properties of the interstellar medium (ISM) is thus of paramount importance. By applying a dedicated machine learning algorithm to JWST NIRSpec observations of the Orion Bar photodissociation region obtained as part of the PDRs4All Early Release Science (ERS) program, we extracted two template spectra capturing the evolution of the AIB-related emission in the 3.2–3.6 μm range, which includes the AIB at 3.3 μm and its main satellite band at 3.4 μm. In the Orion Bar, we analyzed the spatial distribution of the templates and their relationship with the ro-vibrational H₂ line at 2.12 μm, the pure rotational line of H₂ at 4.69 μm and the Pfund δ line at 3.29 μm. We find that one template (AIB_Irrad) traces regions of neutral atomic gas with strong far-UV fields, while the other template (AIB_Shielded) corresponds to shielded regions with lower FUV fields and a higher molecular gas fraction. We then show that these two templates can be used to fit the NIRSpec AIB-related spectra of nearby galaxies. The relative weight of the two templates (AIB_Irrad/_Shielded) is a tracer of the radiative feedback from massive stars on the ISM. We derive an estimate of AIB_{Irrad/Shielded} in a z = 4.22 lensed galaxy and find that it has a lower value than for local galaxies. This pilot study illustrates how a detailed analysis of AIB emission in nearby regions can be used to probe the physical conditions of the extragalactic ISM.