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arXiv 2603.26353astro-ph.GAastro-ph.COastro-ph.SR
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X射线背景下的第三族恒星形成:V. 环境依赖性与晕占据概率

Population III star formation in an X-ray background: V. Environmental dependence and halo occupation probability

Yonsei University(延世大学);University of Maryland(马里兰大学)

Jongwon Park, Massimo Ricotti

AI总结:

利用宇宙学zoom-in模拟,研究软X射线背景如何通过降低宿主晕质量并提高晕占据分数来促进Pop III恒星形成,发现低密度区域增强最显著,并提供了晕占据概率的拟合函数。

摘要:

早期宇宙中的X射线背景增强了分子氢的形成,分子氢是原始气体的主要冷却剂,从而降低了Pop III恒星形成的阈值。延续我们关于X射线对Pop III恒星形成影响的系列研究,我们利用十个覆盖不同晕数密度的宇宙体积的宇宙学zoom-in模拟,研究了软X射线背景如何促进Pop III恒星形成。每个体积都受到Lyman-Warner (LW) H$_2$解离背景以及由Pop III恒星产生的对不稳定超新星(PISNe)产生的弱(J$_{21} \sim 10^{-5}$)、软($E \sim 0.2-2.0$ keV) X射线背景的照射,该X射线背景根据配套论文中描述的方法自洽计算。我们还比较了有无X射线的相同模拟,以隔离X射线效应。该背景通过两种方式促进Pop III恒星形成:(1) 将平均宿主晕质量降低约2-3倍,(2) 使原本无法形成恒星的晕中形成Pop III恒星,从而提高晕占据分数。由此产生的Pop III数密度增益在低密度区域最大(平均约3倍,最高可达7倍)。在最极端的情况下,Pop II恒星仅在X射线存在时形成,气相金属丰度增加了一个数量级,表明低密度区域的矮星系可能受到早期X射线背景的显著影响。我们还为X射线和仅LW模拟提供了Pop III恒星晕占据概率作为红移函数的拟合函数,可作为半解析模型的输入。

英文摘要:

An X-ray background in the early Universe enhances molecular hydrogen formation, the main coolant of primordial gas, thereby lowering the threshold for Pop III star formation. Continuing our series on X-ray impacts on Pop III star formation, we investigate how a soft X-ray background promotes Pop III star formation using cosmological zoom-in simulations of ten cosmic volumes spanning a range of halo number densities. Each volume is irradiated by the Lyman-Warner (LW) H$_{2}$ dissociating background and a weak (J$_{21} \sim 10^{-5}$), soft ($E \sim 0.2-2.0$ keV) X-ray background produced by pair-instability SNe (PISNe) from Pop III stars and calculated self-consistently as described in a companion paper. We also compare the same simulations with and without X-rays to isolate the X-ray effect. The background promotes Pop III star formation in two ways: (1) by reducing the mean host halo mass by a factor of $\sim 2-3$, and (2) by enabling Pop III star formation in haloes that would otherwise remain sterile, thereby increasing the halo occupation fraction. The resulting gain in Pop III number density is largest in underdense regions (a factor of $\approx 3$ on average, reaching up to 7). In the most extreme case, Pop II stars form only in the presence of X-rays and the gas-phase metallicity rises by an order of magnitude, suggesting that dwarf galaxies in underdense regions may be significantly influenced by an early X-ray background. We also provide fitting functions for the halo occupation probability of Pop III stars as a function of redshift for both X-ray and LW-only simulations, which can serve as inputs for semi-analytic models.

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