Top-heavy stellar mass distribution in galactic nuclei inferred from the universally high abundance ratio of [Fe/Mg]
Top-heavy stellar mass distribution in galactic nuclei inferred from the universally high abundance ratio of [Fe/Mg]
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DOI：
10.1093/mnras/stac640
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发表时间：
2021-12
影响因子：
4.8
通讯作者：
Daisuke Toyouchi;K. Inayoshi;M. Ishigaki;N. Tominaga
中科院分区：
文献类型：
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作者：
Daisuke Toyouchi;K. Inayoshi;M. Ishigaki;N. Tominaga
最近对活动星系核(AGN)的观测表明，在它们的宽线区域有很高的Fe II/Mg II线通量比，几乎与红移无关，直到z≳6。高通量比要求星系核中迅速产生铁，以达到与本宇宙成熟星系中观测到的[Fe/mg]≳0.2的丰度比。我们提出了一种可能的解释，解释了活动星系核中通过大质量恒星的形成来快速富铁的原因，该解释遵循一个顶部重的初始质量函数，其幂定律指数Γ大于Salpeter IMF的正则值Γ=−2.35。考虑到不同类型SIME的金属生产通道，我们发现，[Fe/mg]≳0.2值的高要求为Γ≳−1(Γ≳0)和在Mmax≃100M-150M⊙(Mmax≳250M⊙)处的高质量截止。在此条件下，具有M*≳70M⊙的核崩塌和成对不稳定的近场E对铁的富集度有较大贡献。这种顶重的恒星IMF是恒星质量增长的自然结果，这些恒星形成在致密的活动星系核圆盘中，受到M*≳10M⊙反馈调节的邦迪式气体吸积。在活动星系核圆盘中形成的大量恒星群也留下了恒星质量的黑洞残留物，其与引力波发射有关的合并至多占从LIGO/室女座观测推断的合并率的10%，以同时解释金属抛射的高[Fe/Mg]比。
Recent observations of active galactic nuclei (AGNs) have shown a high Fe ii/Mg ii line-flux ratio in their broad-line regions, nearly independent of redshift up to z ≳ 6. The high flux ratio requires rapid production of iron in galactic nuclei to reach an abundance ratio of [Fe/Mg] ≳ 0.2 as high as those observed in matured galaxies in the local universe. We propose a possible explanation of rapid iron enrichment in AGNs by massive star formation that follows a top-heavy initial mass function (IMF) with a power-law index of Γ larger than the canonical value of Γ = −2.35 for a Salpeter IMF. Taking into account metal production channels from different types of SNe, we find that the high value of [Fe/Mg] ≳ 0.2 requires the IMF to be characterized with Γ ≳ −1 (Γ ≳ 0) and a high-mass cutoff at Mmax ≃ 100–150 M⊙ (Mmax ≳ 250 M⊙). Given the conditions, core-collapse SNe with M* ≳ 70 M⊙ and pair-instability SNe give a major contribution for iron enrichment. Such top-heavy stellar IMFs would be a natural consequence from mass growth of stars formed in dense AGN disks under Bondi-like gas accretion that is regulated by feedback at M* ≳ 10 M⊙. The massive stellar population formed in AGN disks also leave stellar-mass black hole remnants, whose mergers associated with gravitational-wave emission account for at most 10 per cent of the merger rate inferred from LIGO/Virgo observations to simultaneously explain the high [Fe/Mg] ratio with metal ejection.