H2 emission in the Orion Bar, NGC7023 and the Horsehead Nebula observed with the James Webb Space Telescope
Abstract
Molecular hydrogen (H2), the most abundant molecule in the interstellar medium, is a handy tool to study Photodissociation Regions. The JWST, with its high spatial resolution, sensitivity, and wavelength coverage, provides unique access to the detection and spatial morphology of the H2 rotational and rovibrational lines. Our goal is to analyze H2 line emission detected with JWST in several PDRs (the Orion Bar, NGC7023, and the Horsehead Nebula) to constrain the physical structure of dissociation front, and compare the impact of the ultraviolet field and the gas density across different regions. The analysis of H2 spatial morphology reveals similar filamentary structures across all regions. Spatial shifts between H2 lines (0.5") are observed and linked to differences in excitation mechanisms (thermalized vs FUV-pumped lines) and the temperature gradient. Despite differences in incident irradiation conditions, analysis of H2 rotational excitation yields high gas temperatures that are similar across the three PDRs. This is expected for a certain regime of excited PDRs (such as Orion and NGC7023), where the G0 at the DFs and the density are similar. However, it is surprising for less excited PDRs like the Horsehead Nebula. OPRs in the rotational and rovibrational levels differ across the three PDRs. We also used H2 rovibrational lines and the 0-0 S(3) line to estimate the visual extinction across the field of view, yielding markedly different results that constrain both the geometry and the grain composition. Due to the complex 3D geometry, H2 emission is observed throughout the FOV and is dominated by the thin UV illuminated layer, making H2 an unreliable tracer of gas temperature in deeper and coolers molecular layers of the PDR. Therefore, H2 emission is not sufficient to trace the possible temperature and density gradient across PDRs and must therefore be associated with other tracers.
Related
- Newly Identified Planetary Nebula Candidates in M31 using Deep-Learning Search of Hubble Space Telescope Images
- Photoionization of the Composite Nebula Surrounding NGC 5408 X-1: Implications for Beamed Emission
- Dynamics and geometry of the inner sub-parsec-scale jet in 3C 279 observed with the Event Horizon Telescope
- Photoionization of the Composite Nebula Surrounding NGC 5408 X-1: Implications for Beamed Emission
- The Silk Nebula: A Newly Discovered Giant [O iii ] Emission Filament
BibTeX
@misc{zannese2026emission,
title = {{H2 emission in the Orion Bar, NGC7023 and the Horsehead Nebula observed with the James Webb Space Telescope}},
author = {M. Zannese and A. Sidhu and A. Guerras and M. Pound and M. Wolfire and A. G. G. M. Tielens and E. Peeters and E. Habart and A. Abergel and M. Baes and O. Berné and C. Boersma and E. Bron and J. Cami and R. Chown and E. Dartois and P. Dell'Ova and J. R. Goicoechea and K. D. Gordon and P. Guillard and O. Kannavou and K. Misselt and N. Monnier and A. Noriega-Crespo and T. Onaka and D. Van de Putte and L. Verstraete and A. N. Witt},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.31287},
eprint = {2609.31287},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.31287},
note = {CC BY-NC-ND 4.0},
}
RIS
TY - UNPB
TI - H2 emission in the Orion Bar, NGC7023 and the Horsehead Nebula observed with the James Webb Space Telescope
AU - M. Zannese
AU - A. Sidhu
AU - A. Guerras
AU - M. Pound
AU - M. Wolfire
AU - A. G. G. M. Tielens
AU - E. Peeters
AU - E. Habart
AU - A. Abergel
AU - M. Baes
AU - O. Berné
AU - C. Boersma
AU - E. Bron
AU - J. Cami
AU - R. Chown
AU - E. Dartois
AU - P. Dell'Ova
AU - J. R. Goicoechea
AU - K. D. Gordon
AU - P. Guillard
AU - O. Kannavou
AU - K. Misselt
AU - N. Monnier
AU - A. Noriega-Crespo
AU - T. Onaka
AU - D. Van de Putte
AU - L. Verstraete
AU - A. N. Witt
PY - 2026
DA - 2026/09/25
PB - arXiv
DO - 10.48550/arxiv.2609.31287
UR - https://arxiv.org/abs/2609.31287
AB - Molecular hydrogen (H2), the most abundant molecule in the interstellar medium, is a handy tool to study Photodissociation Regions. The JWST, with its high spatial resolution, sensitivity, and wavelength coverage, provides unique access to the detection and spatial morphology of the H2 rotational and rovibrational lines. Our goal is to analyze H2 line emission detected with JWST in several PDRs (the Orion Bar, NGC7023, and the Horsehead Nebula) to constrain the physical structure of dissociation front, and compare the impact of the ultraviolet field and the gas density across different regions. The analysis of H2 spatial morphology reveals similar filamentary structures across all regions. Spatial shifts between H2 lines (0.5") are observed and linked to differences in excitation mechanisms (thermalized vs FUV-pumped lines) and the temperature gradient. Despite differences in incident irradiation conditions, analysis of H2 rotational excitation yields high gas temperatures that are similar across the three PDRs. This is expected for a certain regime of excited PDRs (such as Orion and NGC7023), where the G0 at the DFs and the density are similar. However, it is surprising for less excited PDRs like the Horsehead Nebula. OPRs in the rotational and rovibrational levels differ across the three PDRs. We also used H2 rovibrational lines and the 0-0 S(3) line to estimate the visual extinction across the field of view, yielding markedly different results that constrain both the geometry and the grain composition. Due to the complex 3D geometry, H2 emission is observed throughout the FOV and is dominated by the thin UV illuminated layer, making H2 an unreliable tracer of gas temperature in deeper and coolers molecular layers of the PDR. Therefore, H2 emission is not sufficient to trace the possible temperature and density gradient across PDRs and must therefore be associated with other tracers.
ER -