Starburst Energy Feedback Seen through HCO+/HOC+ Emission in NGC 253 from ALCHEMI
Starburst Energy Feedback Seen through HCO+/HOC+ Emission in NGC 253 from ALCHEMI
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DOI：
10.3847/1538-4357/ac26b8
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发表时间：
2021-09
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通讯作者：
N. Harada;S. Martín;J. Mangum;K. Sakamoto;S. Muller;Kunihiko Tanaka;K. Nakanishi;R. Herrero-Illana;Y. Yoshimura;S. Mühle;R. Aladro;L. Colzi;V. Rivilla;S. Aalto;E. Behrens;C. Henkel;J. Holdship;P. Humire;D. Meier;Y. Nishimura;P. P. van der Werf-P.;S. Viti
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作者：
N. Harada;S. Martín;J. Mangum;K. Sakamoto;S. Muller;Kunihiko Tanaka;K. Nakanishi;R. Herrero-Illana;Y. Yoshimura;S. Mühle;R. Aladro;L. Colzi;V. Rivilla;S. Aalto;E. Behrens;C. Henkel;J. Holdship;P. Humire;D. Meier;Y. Nishimura;P. P. van der Werf-P.;S. Viti
分子丰度对紫外光子通量和宇宙射线电离率敏感。在星暴环境中，高能光子和粒子的影响预计会更强。我们研究这些天体化学特征，通过多个过渡的HCO+和亚稳异构体HOC+在星爆星系NGC 253的中心，使用阿塔卡马大型毫米/亚毫米阵列大型计划阿尔马综合高分辨率河外分子清单的数据。HOC+（1−0）积分强度的分布显示它与“超级气泡”（superbubbles）有关，即由超新星或膨胀的H ii区域产生的空洞。观测到的HCO+/HOC+丰度比为10-150，HOC+相对于H2的丰度分数为10 - 1.5 × 10−11-6 × 10−10，这意味着NGC 253中心的HOC+丰度明显高于银河系和银河系中心云的静止旋臂暗云。与化学模型的比较意味着，如果最大可见光消光为105，星际辐射场为G 0 <$103，或者宇宙射线电离率为100 <$10 −14 s−1（比银河旋臂中的云高3-4个数量级），以重现观测结果。根据HOC+形成途径的不同，我们提出HOC+的低激发线追踪宇宙线主导区，而高激发线追踪光解离区。我们的研究结果表明，NGC 253中心的星际介质受到来自紫外光子和宇宙射线的能量输入的显着影响，这些能量反馈源。
Molecular abundances are sensitive to the UV photon flux and cosmic-ray ionization rate. In starburst environments, the effects of high-energy photons and particles are expected to be stronger. We examine these astrochemical signatures through multiple transitions of HCO+ and its metastable isomer HOC+ in the center of the starburst galaxy NGC 253 using data from the Atacama Large Millimeter/submillimeter Array large program ALMA Comprehensive High-resolution Extragalactic Molecular inventory. The distribution of the HOC+(1−0) integrated intensity shows its association with “superbubbles,” cavities created either by supernovae or expanding H ii regions. The observed HCO+/HOC+ abundance ratios are ∼10–150, and the fractional abundance of HOC+ relative to H2 is ∼1.5 × 10−11–6 × 10−10, which implies that the HOC+ abundance in the center of NGC 253 is significantly higher than in quiescent spiral arm dark clouds in the Galaxy and the Galactic center clouds. Comparison with chemical models implies either an interstellar radiation field of G 0 ≳ 103 if the maximum visual extinction is ≳5, or a cosmic-ray ionization rate of ζ ≳ 10−14 s−1 (3–4 orders of magnitude higher than that within clouds in the Galactic spiral arms) to reproduce the observed results. From the difference in formation routes of HOC+, we propose that a low-excitation line of HOC+ traces cosmic-ray dominated regions, while high-excitation lines trace photodissociation regions. Our results suggest that the interstellar medium in the center of NGC 253 is significantly affected by energy input from UV photons and cosmic rays, sources of energy feedback.