高分数薄盘星系继续挑战ΛCDM宇宙学
The High Fraction of Thin Disk Galaxies Continues to Challenge ΛCDM Cosmology
摘要：
任何可行的宇宙学框架都必须与观测到的早型和晚型星系的比例相匹配。在这篇文章中，我们专注于宇宙学标准模型（λ冷暗物质，$Λ$ CDM）中星系形态类型的分布。利用最新的宇宙学$Λ$ CDM模拟被称为Illustris，IllustrisTNG和EAGLE，我们计算的内在和天空投影的深晕中的恒星质量$M_* > 10^{10}\,M_\odot$在红移$z=0$的纵横比分布。有一个显着的赤字本质薄盘星系，但包括大多数的本地观测到的星系人口。因此，这些$Λ$ CDM模拟产生的天空投影长宽比分布与星系和质量大会（GAMA）调查和斯隆数字巡天在$\geq 12.52σ$（TNG 50 -1）和$\geq 14.82σ$（EAGLE 50）置信度上不一致。本质上薄的星系的赤字可能是由于一个更少的层次合并驱动的积累所观察到的星系比$Λ$ CDM框架。这也可能是由于实施的子网格模型，或上述流体动力学模拟的有限分辨率。我们估计，$8^5$比TNG 50 -1更好的质量分辨率实现将使与GAMA的紧张关系降低到$5.58σ$水平。最后，我们表明，星系较少的主要合并有一个有点薄的长宽比分布。鉴于预计$Λ$清洁发展机制中小规模合并的频率很高，问题可能是小规模合并造成的。在这种情况下，角动量问题可以在米尔格罗米安动力学（MOND）中得到缓解，因为扩展的暗物质晕之间没有动力学摩擦，从而降低了合并频率。
英文摘要：
Any viable cosmological framework has to match the observed proportion of early- and late-type galaxies. In this contribution, we focus on the distribution of galaxy morphological types in the standard model of cosmology (Lambda cold dark matter, $Λ$CDM). Using the latest state-of-the-art cosmological $Λ$CDM simulations known as Illustris, IllustrisTNG, and EAGLE, we calculate the intrinsic and sky-projected aspect ratio distribution of the stars in subhalos with stellar mass $M_* > 10^{10}\,M_\odot$ at redshift $z=0$. There is a significant deficit of intrinsically thin disk galaxies, which however comprise most of the locally observed galaxy population. Consequently, the sky-projected aspect ratio distribution produced by these $Λ$CDM simulations disagrees with the Galaxy And Mass Assembly (GAMA) survey and Sloan Digital Sky Survey at $\geq 12.52σ$ (TNG50-1) and $\geq 14.82σ$ (EAGLE50) confidence. The deficit of intrinsically thin galaxies could be due to a much less hierarchical merger-driven build-up of observed galaxies than is given by the $Λ$CDM framework. It might also arise from the implemented sub-grid models, or from the limited resolution of the above-mentioned hydrodynamical simulations. We estimate that an $8^5$ times better mass resolution realization than TNG50-1 would reduce the tension with GAMA to the $5.58σ$ level. Finally, we show that galaxies with fewer major mergers have a somewhat thinner aspect ratio distribution. Given also the high expected frequency of minor mergers in $Λ$CDM, the problem may be due to minor mergers. In this case, the angular momentum problem could be alleviated in Milgromian dynamics (MOND) because of a reduced merger frequency arising from the absence of dynamical friction between extended dark matter halos.
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