ALCHEMI Finds a “Shocking” Carbon Footprint in the Starburst Galaxy NGC 253
ALCHEMI Finds a “Shocking” Carbon Footprint in the Starburst Galaxy NGC 253
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DOI：
10.3847/1538-4357/ac8dfc
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发表时间：
2022-08
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通讯作者：
N. Harada;S. Martín;J. Mangum;K. Sakamoto;S. Muller;V. Rivilla;Christian Henkel;D. Meier;L. Colzi;M. Yamagishi;Kunihiko Tanaka;K. Nakanishi;R. Herrero-Illana;Y. Yoshimura;P. Humire;R. Aladro;P. Werf;K. Emig
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作者：
N. Harada;S. Martín;J. Mangum;K. Sakamoto;S. Muller;V. Rivilla;Christian Henkel;D. Meier;L. Colzi;M. Yamagishi;Kunihiko Tanaka;K. Nakanishi;R. Herrero-Illana;Y. Yoshimura;P. Humire;R. Aladro;P. Werf;K. Emig
星暴星系的中心可能具有特殊的气体和冰化学特征，这是由于它们的气体动力学和各种冰解吸机制的存在。这可能会导致一个特殊的可观察到的组成。我们分析了CO2的丰度，一种可靠的冰化学示踪剂，从阿塔卡马大型毫米/亚毫米阵列大型计划ALCHEMI收集的数据中，对星爆星系NGC 253进行了宽频率光谱扫描，角分辨率为1.“6。我们约束的CO2丰度在气相中使用其质子化形式HOCO+。HOCO+的分布与甲醇的分布相似，这表明HOCO+确实是由冰升华的CO2的质子化产生的。HOCO+丰度在中心分子带（CMZ）的外部为（1-2）× 10−9，而在运动中心附近较低（10 × 10−10）。外CMZ的峰值丰度与银河系CMZ相当，并且比银河系盘中的恒星形成区域高出几个数量级。根据化学模型所建议的HOCO+/CO2比值范围，在CMZ外部的气相CO2丰度估计为（1-20）× 10−7，在中心附近的数量级更低。根据文献，我们估计CMZ外的CO2冰丰度为（2-5）× 10−6。冰和气体CO2丰度之间的比较表明一个有效的升华机制。这种升华是由于在酒吧和CMZ的轨道交叉点的大规模冲击。
The centers of starburst galaxies may be characterized by a specific gas and ice chemistry due to their gas dynamics and the presence of various ice desorption mechanisms. This may result in a peculiar observable composition. We analyse the abundances of CO2, a reliable tracer of ice chemistry, from data collected as part of the Atacama Large Millimeter/submillimeter Array large program ALCHEMI, a wide-frequency spectral scan toward the starburst galaxy NGC 253 with an angular resolution of 1.″6. We constrain the CO2 abundances in the gas phase using its protonated form HOCO+. The distribution of HOCO+ is similar to that of methanol, which suggests that HOCO+ is indeed produced from the protonation of CO2 sublimated from ice. The HOCO+ fractional abundances are found to be (1–2) × 10−9 at the outer part of the central molecular zone (CMZ), while they are lower (∼10−10) near the kinematic center. This peak fractional abundance at the outer CMZ is comparable to that in the Milky Way CMZ, and orders of magnitude higher than that in Galactic disk, star-forming regions. From the range of HOCO+/CO2 ratios suggested from chemical models, the gas-phase CO2 fractional abundance is estimated to be (1–20) × 10−7 at the outer CMZ, and orders of magnitude lower near the center. We estimate the CO2 ice fractional abundances at the outer CMZ to be (2–5) × 10−6 from the literature. A comparison between the ice and gas CO2 abundances suggests an efficient sublimation mechanism. This sublimation is attributed to large-scale shocks at the orbital intersections of the bar and CMZ.