CEERS中一个红移z=1.85的星系群：演化的、无尘埃的大质量晕内光以及正在形成中的最亮群星系
A z=1.85 galaxy group in CEERS: evolved, dustless, massive intra-halo light and a brightest group galaxy in the making
AI总结：
本研究利用JWST/NIRCam观测红移z=1.85的大质量星系群，首次构建该红移下晕内光的静止帧紫外-近红外光谱能量分布，发现晕内光无尘埃、演化程度高、占总恒星质量约10%，挑战了现有晕内光形成模型。
摘要：
我们展示了CEERS项目利用JWST/NIRCam对一个红移z=1.85的大质量星系群的成像观测，旨在借助JWST的早期观测视角探索遥远宇宙中大质量星系群的形成过程。该星系群包含超过16个成员星系（其中6个经光谱确认），恒星质量下限为log10(Mstar/Msun)=8.5，其中包括一个在此红移下正处于活跃组装过程中的最亮群星系（BGG）。该最亮群星系由多个并合成分组成，在天球上延伸约3.6角秒（对应30千秒差距）。最亮群星系贡献了星系群总恒星质量的69%，其中一个并合成分占最亮群星系总质量的76%，且恒星形成率（SFR）大于1810倍太阳质量每年。\n最重要的是，我们在多个HST和JWST/NIRCam波段中探测到了晕内光（IHL），这使我们首次在该高红移下构建了最先进的晕内光静止帧紫外-近红外光谱能量分布。结合赫歇尔（Herschel）数据，我们得以对晕内光和成员星系的恒星族进行表征，同时分析星系群星系的形态分布与其恒星形成活动的关系。我们制作了晕内光的叠加图像，使我们对延伸辐射的探测灵敏度在静止帧1微米处达到28.5星等每平方角秒。我们发现晕内光极度缺乏尘埃（消光量Av约为0），包含一个演化的恒星族，其log10(t50/yr)=8.8，对应50%恒星物质的形成时期比红移z=1.85早0.63吉年。没有证据表明晕内光中存在正在进行的恒星形成。\n这个红移z=1.85的星系群中的晕内光贡献了总恒星质量的约10%，与在近邻星系团中观测到的结果相当。这表明，晕内光比例随红移的演化比部分模型预测的更具自相似性，对我们关于高红移星系团组装过程中晕内光形成的理解提出了挑战。JWST正在揭示该红移下星系群形成的新侧面，这些星系群将演化为近邻宇宙中类似室女座星系团的结构。
英文摘要：
We present CEERS JWST/NIRCam imaging of a massive galaxy group at z=1.85, to explore the early JWST view on massive group formation in the distant Universe. The group contains >16 members (including 6 spectros. confirmations) down to log10(Mstar/Msun)=8.5, including the brightest group galaxy (BGG) in the process of actively assembling at this redshift. The BGG is comprised of multiple merging components extending ~3.6" (30kpc) across the sky. The BGG contributes 69% of the group's total galactic stellar mass, with one of the merging components containing 76% of the total mass of the BGG and a SFR>1810Msun/yr. Most importantly, we detect intra-halo light (IHL) in several HST and JWST/NIRCam bands, allowing us to construct a state-of-the-art rest-frame UV-NIR Spectral Energy Distribution of the IHL for the first time at this high redshift. This allows stellar population characterisation of both the IHL and member galaxies, as well as the morphology distribution of group galaxies vs. their star-formation activity when coupled with Herschel data. We create a stacked image of the IHL, giving us a sensitivity to extended emission of 28.5 mag/arcsec2 at rest-frame 1um. We find that the IHL is extremely dust poor (Av~0), containing an evolved stellar population of log10(t50/yr)=8.8, corresponding to a formation epoch for 50% of the stellar material 0.63Gyr before z=1.85. There is no evidence of ongoing star-formation in the IHL. The IHL in this group at z=1.85 contributes ~10% of the total stellar mass, comparable with what is observed in local clusters. This suggests that the evolution of the IHL fraction is more self-similar with redshift than predicted by some models, challenging our understanding of IHL formation during the assembly of high-redshift clusters. JWST is unveiling a new side of group formation at this redshift, which will evolve into Virgo-like structures in the local Universe.
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