The BPT Diagram in Cosmological Galaxy Formation Simulations: Understanding the Physics Driving Offsets at High Redshift
The BPT Diagram in Cosmological Galaxy Formation Simulations: Understanding the Physics Driving Offsets at High Redshift
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DOI：
10.3847/1538-4357/ac43b8
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发表时间：
2022-01
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通讯作者：
P. Garg;D. Narayanan;N. Byler;R. Sanders;A. Shapley;A. Strom;Romeel Dav'e;M. Hirschmann;C. Lovell;Justin A. Otter;G. Popping;G. Privon
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作者：
P. Garg;D. Narayanan;N. Byler;R. Sanders;A. Shapley;A. Strom;Romeel Dav'e;M. Hirschmann;C. Lovell;Justin A. Otter;G. Popping;G. Privon
Baldwin，Philips，& Terlevich的[O iii]/Hβ对[N ii]/Hα图（以下简称N2-BPT）长期以来一直被用作基于电离辐射的主要来源对星系进行分类的工具。最近的观测表明，z = 1.2的星系与N2-BPT空间中的本星系存在偏移。在本文中，我们进行了一系列的控制数值实验，以了解驱动这种偏移的潜在物理过程。我们在一个大样本的星系模型星云线发射，从辛巴宇宙学流体动力学星系形成模拟，使用多云光电离代码来计算星云线亮度从H II区域。我们发现，在高红移下观测到的[O iii]/Hβ和[N ii]/Hα值向更高方向的偏移是由于样本的选择：当我们只考虑质量最大的星系M *<$1010 -11 M <$时，由于它们的高金属丰度，偏移自然出现。我们预测，探测低质量星系的更深入的观测将揭示位于与z = 0观测相当的轨迹上的星系。即使考虑到样本选择效应，我们也发现模拟和观测之间存在微妙的不匹配。为了解决这一矛盾，我们调查的影响，不同的电离参数，H II区域密度，气相丰度模式，并增加辐射场硬度N2-BPT图。我们发现，在O/H不变的情况下，无论是减小星系的电离参数还是增大星系的N/O比，都能使星系在N2-BPT空间中沿沿着自相似弧运动，而该弧被高红移星系占据.
The Baldwin, Philips, & Terlevich diagram of [O iii]/Hβ versus [N ii]/Hα (hereafter N2-BPT) has long been used as a tool for classifying galaxies based on the dominant source of ionizing radiation. Recent observations have demonstrated that galaxies at z ∼ 2 reside offset from local galaxies in the N2-BPT space. In this paper, we conduct a series of controlled numerical experiments to understand the potential physical processes driving this offset. We model nebular line emission in a large sample of galaxies, taken from the simba cosmological hydrodynamic galaxy formation simulation, using the cloudy photoionization code to compute the nebular line luminosities from H ii regions. We find that the observed shift toward higher [O iii]/Hβ and [N ii]/Hα values at high redshift arises from sample selection: when we consider only the most massive galaxies M * ∼ 1010–11 M ⊙, the offset naturally appears, due to their high metallicities. We predict that deeper observations that probe lower-mass galaxies will reveal galaxies that lie on a locus comparable to z ∼ 0 observations. Even when accounting for samples-selection effects, we find that there is a subtle mismatch between simulations and observations. To resolve this discrepancy, we investigate the impact of varying ionization parameters, H ii region densities, gas-phase abundance patterns, and increasing radiation field hardness on N2-BPT diagrams. We find that either decreasing the ionization parameter or increasing the N/O ratio of galaxies at fixed O/H can move galaxies along a self-similar arc in N2-BPT space that is occupied by high-redshift galaxies.