Detection of Deuterated Hydrocarbon Nanoparticles in the Whirlpool Galaxy, M51

Deuteration of hydrocarbon material, including polycyclic aromatic hydrocarbons (PAHs), has been proposed to account for the low gas-phase abundances of D in the interstellar medium.

JWST spectra of four star-forming regions in M51 show an emission feature, with central wavelength ∼4.647μm and FWHM 0.0265μm, corresponding to the C-D stretching mode in aliphatic hydrocarbons. The emitting aliphatic material is estimated to have (D/H)_aliph≈0.17±0.02 — a factor ∼10⁴ enrichment relative to the overall interstellar medium (ISM).

On ∼50pc scales, deuteration levels toward four HII regions in M51 are 2-3 times higher than in the Orion Bar photodissociation region (PDR), with implications for the processes responsible for the formation and evolution of hydrocarbon nanoparticles, including PAHs.

The deuteration of the aliphatic material is found to anticorrelate with helium ionization in the associated HII, suggesting that harsh FUV radiation may act to lower the deuteration of aliphatics in PDRs near massive stars. No evidence is found for deuteration of aromatic material, with (D/H)_arom≲0.016: deuteration of the aliphatic material exceeds that of the aromatic material by at least a factor 10.

The observed levels of deuteration may account for the depletion of D observed in the Galactic interstellar medium. If so, the 4.65μm feature may be detectable in absorption.

B. T. Draine, Karin Sandstrom, Daniel A. Dale, J.-D. T. Smith, Ryan Chown, Grant P. Donnelly, Sara E. Duval, Cory M. Whitcomb, Angela Adamo, L. Armus, Danielle A. Berg, Torsten Böker, Alberto D. Bolatto, Martha L. Boyer, Daniela Calzetti, B. G. Elmegreen, Brandt A. L. Gaches, Karl D. Gordon, L. K. Hunt, R. C. Kennicutt, Ralf S. Klessen, Thomas S.-Y. Lai, Adam K. Leroy, Sean T. Linden, Alex Pedrini, Noah S. J. Rogers, Julia C. Roman-Duval, Eva Schinnerer, Evan B. Skillman, Fabian Walter, Tony D. Weinbeck, Benjamin F. Williams

Comments: Revised. corresponding to published paper
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2504.02538 [astro-ph.GA] (or arXiv:2504.02538v2 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2504.02538
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Journal reference: Astrophys. J. Letters, 948:L42 (2025)
Related DOI:
https://doi.org/10.3847/2041-8213/adc991
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Submission history
From: Bruce T. Draine
[v1] Thu, 3 Apr 2025 12:39:47 UTC (2,423 KB)
[v2] Tue, 13 May 2025 18:30:29 UTC (2,423 KB)
https://arxiv.org/abs/2504.02538

Astrobiology, Astrochemistry,