Tracing the Hidden Molecular Gas in the Small Magellanic Cloud with SOFIA and APEX
Abstract
In low-metallicity environments, reduced dust abundance weakens molecular cloud shielding against far-ultraviolet radiation, enhancing CO photodissociation and producing extended CO-faint or CO-dark molecular gas. The [CII] 158 μm line is a key tracer of these CO-faint phases. We characterize the multiphase origin of [CII] emission and quantify the CO-faint molecular gas fraction across several regions of the Small Magellanic Cloud (SMC), the nearest low-metallicity (∼ 0.2 Z⊙) galaxy. We combine high spectral resolution SOFIA/GREAT and upGREAT [CII], APEX CO(2-1), and ASKAP+Parkes H I 21 cm data to decompose [CII] into its atomic and molecular contributions. On average, (78± 1)% of [CII] intensity originates from molecular gas and (18± 1)% from atomic gas. The [CII]-traced H2 accounts for (77±4)% of the total molecular gas column density, indicating that the SMC molecular reservoir is dominated by CO-faint gas. We derive a H2-to-CO conversion factor XCO≈ 8.9×1020 cm−2 (K km s−1)−1 on 4 pc scales in CO-emitting regions, 4.5 times the canonical Galactic value. We find no significant correlation between the CO-dark fraction and either total column density or ΣSFR, suggesting that the CO-faint fraction is primarily governed by dust and gas shielding rather than current star formation activity. Our results demonstrate that [CII] is the dominant tracer of molecular gas in the SMC and confirm the prevalence of an extended, stable CO-faint molecular phase in low-metallicity environments.
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BibTeX
@misc{rodriguezambler2026tracing,
title = {{Tracing the Hidden Molecular Gas in the Small Magellanic Cloud with SOFIA and APEX}},
author = {K. Rodriguez-Ambler and R. Herrera-Camus and M. Rúbio and A. D. Bolatto and D. Riquelme-Vásquez and K. Jameson and C. Muñoz-López and E. J. Tarantino and J. Stutzki},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.05680},
eprint = {2609.05680},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.05680},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Tracing the Hidden Molecular Gas in the Small Magellanic Cloud with SOFIA and APEX
AU - K. Rodriguez-Ambler
AU - R. Herrera-Camus
AU - M. Rúbio
AU - A. D. Bolatto
AU - D. Riquelme-Vásquez
AU - K. Jameson
AU - C. Muñoz-López
AU - E. J. Tarantino
AU - J. Stutzki
PY - 2026
DA - 2026/09/04
PB - arXiv
DO - 10.48550/arxiv.2609.05680
UR - https://arxiv.org/abs/2609.05680
AB - In low-metallicity environments, reduced dust abundance weakens molecular cloud shielding against far-ultraviolet radiation, enhancing CO photodissociation and producing extended CO-faint or CO-dark molecular gas. The [CII] 158 μm line is a key tracer of these CO-faint phases. We characterize the multiphase origin of [CII] emission and quantify the CO-faint molecular gas fraction across several regions of the Small Magellanic Cloud (SMC), the nearest low-metallicity (∼ 0.2 Z_⊙) galaxy. We combine high spectral resolution SOFIA/GREAT and upGREAT [CII], APEX CO(2-1), and ASKAP+Parkes H I 21 cm data to decompose [CII] into its atomic and molecular contributions. On average, (78± 1)% of [CII] intensity originates from molecular gas and (18± 1)% from atomic gas. The [CII]-traced H_2 accounts for (77±4)% of the total molecular gas column density, indicating that the SMC molecular reservoir is dominated by CO-faint gas. We derive a H_2-to-CO conversion factor X_CO≈ 8.9×10^20 cm^−2 (K km s^−1)^−1 on 4 pc scales in CO-emitting regions, 4.5 times the canonical Galactic value. We find no significant correlation between the CO-dark fraction and either total column density or Σ_SFR, suggesting that the CO-faint fraction is primarily governed by dust and gas shielding rather than current star formation activity. Our results demonstrate that [CII] is the dominant tracer of molecular gas in the SMC and confirm the prevalence of an extended, stable CO-faint molecular phase in low-metallicity environments.
ER -