Issue |
A&A
Volume 689, September 2024
|
|
---|---|---|
Article Number | L12 | |
Number of page(s) | 14 | |
Section | Letters to the Editor | |
DOI | https://doi.org/10.1051/0004-6361/202451741 | |
Published online | 26 September 2024 |
Letter to the Editor
H3+ absorption and emission in local (U)LIRGs with JWST/NIRSpec: Evidence for high H2 ionization rates
1
Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
2
Universidad de Alcalá, Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
3
Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, E-28692 Villanueva de la Cañada, Madrid, Spain
4
Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
5
Department of Astronomy, University of Florida, P.O. Box 112055, Gainesville, FL 32611, USA
6
Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto F.no, (Firenze), Italy
7
INAF – Osservatorio Astrofisco di Arcetri, largo E. Fermi 5, 50127 Firenze, Italy
8
Centro de Astrobiología (CAB), CSIC-INTA, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
9
School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus
Received:
31
July
2024
Accepted:
25
August
2024
We study the 3.4 − 4.4 μm fundamental rovibrational band of H3+, a key tracer of the ionization of the molecular interstellar medium (ISM), in a sample of 12 local (d < 400 Mpc) (ultra)luminous infrared galaxies ((U)LIRGs) observed with JWST/NIRSpec. The P, Q, and R branches of the band are detected in 13 out of 20 analyzed regions within these (U)LIRGs, which increases the number of extragalactic H3+ detections by a factor of 6. For the first time in the ISM, the H3+ band is observed in emission; we detect this emission in three regions. In the remaining ten regions, the band is seen in absorption. The absorptions are produced toward the 3.4 − 4.4 μm hot dust continuum rather than toward the stellar continuum, indicating that they likely originate in clouds associated with the dust continuum source. The H3+ band is undetected in Seyfert-like (U)LIRGs where the mildly obscured X-ray radiation from the active galactic nuclei might limit the abundance of this molecule. For the detections, the H3+ abundances, N(H3+)/NH = (0.5 − 5.5)×10−7, imply relatively high ionization rates, ζH2, of between 3 × 10−16 and > 4 × 10−15 s−1, which are likely associated with high-energy cosmic rays. In half of the targets, the absorptions are blueshifted by 50–180 km s−1, which is lower than the molecular outflow velocities measured using other tracers such as OH 119 μm or rotational CO lines. This suggests that H3+ traces gas close to the outflow-launching sites before it has been fully accelerated. We used nonlocal thermodynamic equilibrium models to investigate the physical conditions of these clouds. In seven out of ten objects, the H3+ excitation is consistent with inelastic collisions with H2 in warm translucent molecular clouds (Tkin ∼ 250–500 K and n(H2) ∼102 − 3 cm−3). In three objects, dominant infrared pumping excitation is required to explain the absorptions from the (3,0) and (2,1) levels of H3+ detected for the first time in the ISM.
Key words: cosmic rays / ISM: molecules / galaxies: active / galaxies: starburst / infrared: ISM
© The Authors 2024
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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