Neutral gas heating by X-rays in primitive galaxies: Infrared observations of the blue compact dwarf I Zw 18 with Herschel
Neutral gas heating by X-rays in primitive galaxies: Infrared observations of the blue compact dwarf I Zw 18 with Herschel
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原始星系中 X 射线加热中性气体：赫歇尔对蓝致密矮星 I Zw 18 的红外观测
DOI：
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发表时间：
2017
期刊：
影响因子：
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通讯作者：
F. Polles
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文献类型：
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作者：
V. Lebouteiller;D. Péquignot;D. Cormier;D. Cormier;S. Madden;M. Pakull;D. Kunth;F. Galliano;M. Chevance;M. Chevance;S. Heap;Min;F. Polles
语境。星系的中性星际介质充当恒星形成的燃料库。在金属极度贫乏的恒星形成星系中，这一阶段的主要加热和冷却机制是不确定的。此类物体中的低尘气质量比和低多环芳烃丰度表明传统的光电效应加热可能无效。目标。我们的目标是确定这样一个星系 I Zw 18 (1/30Z⊙) 中的主要热机制，评估精细结构冷却线的诊断价值，并估计分子气体含量。尽管分子气体是恒星形成的重要催化剂和示踪剂，但在金属最贫乏的星系中，对分子气体质量的限制仍然难以捉摸。方法。基于先前描述多扇形拓扑中 I Zw 18-NW 的巨大 H ii 区域的光电离模型，我们特别使用 [C ii] 157 μm 和 [O i] 63 μm 线以及最近用赫歇尔太空望远镜测量的尘埃质量提供了额外的约束。结果。 Hi 区域的加热似乎主要是由于来自明亮的 X 射线二元源的辐射产生的光电离，而光电效应可以忽略不计。不排除显着的宇宙射线加热。由于 X 射线加热在 H i 气体中占主导地位，因此在过去的 104 年中，红外精细结构线提供了 4 × 1040 erg s-1 数量级的平均 X 射线光度。 [Ne v] 线的上限对双星产生的软 X 射线通量提供了严格的限制。预计 H2 的质量可以忽略不计。尽管如此，高达 ~107 M⊙ 的 H2 可能隐藏在一些足够密集的、大小为 ≲5 pc (≲0.05″) 的云中。尽管存在大量的 H2 气体，[C ii] 和 [O i] 并不追踪所谓的“CO 暗气体”，但它们追踪几乎纯粹的原子介质。尽管 I Zw 18 中的 [C ii]+[O i] 与总红外比率与更富含金属的源中的值 (~1%) 相似，但它不能安全地用作光电加热效率代理。由于 X 射线光度与恒星形成率之间的相关性，这个比率似乎保持稳定。结论。 X 射线加热对于金属极其贫乏的源来说可能是一个重要的过程。由于低尘气比而导致的光电加热的缺乏往往会被低金属丰度星系中 X 射线双星的较大出现和功率所补偿。我们推测 X 射线加热可能会抑制恒星的形成。关键词：X射线：双星/红外线：ISM/光子主导区（PDR）/星系：ISM/星系：个体：I Zw 18 /星系：恒星形成⋆ 赫歇尔是欧空局太空天文台，其科学仪器由欧洲领导的首席研究员联盟提供，并得到了美国宇航局的重要参与。
Context. The neutral interstellar medium of galaxies acts as a reservoir to fuel star formation. The dominant heating and cooling mechanisms in this phase are uncertain in extremely metal-poor star-forming galaxies. The low dust-to-gas mass ratio and low polycyclic aromatic hydrocarbon abundance in such objects suggest that the traditional photoelectric effect heating may not be effective. Aims. Our objective is to identify the dominant thermal mechanisms in one such galaxy, I Zw 18 (1/30Z⊙), assess the diagnostic value of fine-structure cooling lines, and estimate the molecular gas content. Even though molecular gas is an important catalyst and tracer of star formation, constraints on the molecular gas mass remain elusive in the most metal-poor galaxies. Methods. Building on a previous photoionization model describing the giant H ii region of I Zw 18-NW within a multi-sector topology, we provide additional constraints using, in particular, the [C ii] 157 μm and [O i] 63 μm lines and the dust mass recently measured with the Herschel Space Telescope. Results. The heating of the H i region appears to be mainly due to photoionization by radiation from a bright X-ray binary source, while the photoelectric effect is negligible. Significant cosmic ray heating is not excluded. Inasmuch as X-ray heating dominates in the H i gas, the infrared fine-structure lines provide an average X-ray luminosity of order 4 × 1040 erg s-1 over the last few 104 yr in the galaxy. The upper limits to the [Ne v] lines provide strong constraints on the soft X-ray flux arising from the binary. A negligible mass of H2 is predicted. Nonetheless, up to ~107 M⊙ of H2 may be hidden in a few sufficiently dense clouds of order ≲5 pc (≲0.05′′) in size. Regardless of the presence of significant amounts of H2 gas, [C ii] and [O i] do not trace the so-called “CO-dark gas”, but they trace the almost purely atomic medium. Although the [C ii]+[O i] to total infrared ratio in I Zw 18 is similar to values in more metal-rich sources (~1%), it cannot be safely used as a photoelectric heating efficiency proxy. This ratio seems to be kept stable owing to a correlation between the X-ray luminosity and the star formation rate. Conclusions. X-ray heating could be an important process in extremely metal-poor sources. The lack of photoelectric heating due to the low dust-to-gas ratio tends to be compensated for by the larger occurrence and power of X-ray binaries in low-metallicity galaxies. We speculate that X-ray heating may quench star formation. Key words: X-rays: binaries / infrared: ISM / photon-dominated region (PDR) / galaxies: ISM / galaxies: individual: I Zw 18 / galaxies: star formation⋆ Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
DOI：
10.3847/2041-8205/831/2/l14
发表时间：
2016-09
期刊：
The Astrophysical Journal Letters
影响因子：
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作者：
F. Fuerst;D. Walton;F. Harrison;D. Stern;D. Barret;M. Brightman;A. Fabian;B. Grefenstette;
通讯作者：
F. Fuerst;D. Walton;F. Harrison;D. Stern;D. Barret;M. Brightman;A. Fabian;B. Grefenstette;