不同路径，相同外观：一个红移z~0.13的中年红色致密星系
Different paths, same appearance: an intermediate-age red nugget at z~0.13
AI总结：
研究红移z~0.13的大质量超致密星系G79071，结合多种光谱拟合方法和不同恒星族群模型，分析其恒星运动学等性质，发现其有显著旋转，由中年恒星主导，给出暗物质占比，为大质量星系形成途径多样性提供新约束。
AI中文摘要：
大质量超致密星系通常被视为红移z~2-3的致密静止星系群在附近的遗迹，迄今为止详细研究仅聚焦于致密环境中的古老星系系统。本文基于甚大望远镜/ X - 射手长缝光谱，对一个处于低质量星系群环境、红移z~0.13、超致密（R_e<2kpc）、大质量（M*~10^{11}M☉）的椭圆星系G79071进行空间分辨分析。通过结合全光谱拟合、全指数拟合和指数拟合，使用不同恒星族群模型，提取约4R_e范围内的恒星运动学信息，约束恒星族群性质和恒星初始质量函数的低质量端。G79071有显著旋转，由年龄约3 - 4Gyr的中年恒星族群主导，中心金属度超太阳且随半径减小，多数丰度比呈平坦径向趋势。分析表明恒星初始质量函数在约2R_e内仍为重底型，暗物质占比f_DM(<R_e)=0.18 ± 0.07。这些结果为大质量星系形成途径的多样性提供了新约束。
英文摘要：
Massive ultra-compact galaxies are commonly regarded as nearby relics of the compact quiescent population at z~2-3, and detailed studies have focused so far only on old systems in dense environments. Here we present a spatially resolved analysis of G79071, an ultra-compact (R_e<2kpc), massive (M*~10^{11}MSun) ETG at z~0.13 residing in a low-mass group environment, based on deep VLT/X-Shooter long-slit spectroscopy. We extract stellar kinematics out to ~4R_e and constrain stellar population properties and the low-mass end of the stellar IMF, by combining full spectral fitting, full-index fitting, and index fitting, using different stellar population models. G79071 shows significant rotation, consistent with a fast-rotator-like kinematic structure, and is dominated by an intermediate-age stellar population (~3-4Gyr) with a flat age profile and no evidence for a significantly old (>5Gyr) component. The central metallicity is supersolar and decreases with radius, while most abundance ratios show flat radial trends and are consistent with typical massive, low-redshift ETGs at similar velocity dispersion. Moreover, metallicity and [Na/Fe] abundance are significantly enhanced, reflecting a very efficient chemical enrichment process.Our analysis suggests that the IMF remains bottom-heavy out to ~2R_e, implying a mass-excess factor alpha>~2. Jeans anisotropic models with an NFW halo are consistent with the IMF-based stellar mass normalization and with compactness-corrected (non-homologous) dynamical mass estimators, yielding a modest projected dark-matter fraction within one effective radius, f_DM(<R_e)=0.18 \pm 0.07. These results show that at least some massive compact galaxies with a bottom-heavy IMF can form and persist at lower redshift and outside high-density environments, providing new constraints on the diversity of formation pathways for massive galaxies.
