“喷火”：z = 7.15 处“三大龙”中的热尘埃
Breathing Fire: Hot Dust in the Big Three Dragons at z = 7.15
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中文总结 AI 辅助
本研究利用ALMA多波段观测z=7.15的“三大龙”系统，发现其含热尘埃、高红外光度，两个莱曼断裂星系均高度尘埃遮挡，为高红移宇宙中存在大量被遮挡恒星形成提供了证据。
中文摘要 AI 辅助
我们利用阿塔卡马大型毫米波/亚毫米波阵列（ALMA）的9波段（观测波长λ_obs = 0.45mm）和4波段（观测波长λ_obs = 2.2mm）对“三大龙”开展观测，该系统是一对红移z=7.15的并合莱曼断裂星系（LBGs）。此前该系统已在ALMA的6、7、8波段（观测波长λ_obs = 0.73 - 1.32mm）被探测到，结合我们的新观测，可可靠约束其尘埃温度与被遮挡的恒星形成率（SFR）。未分辨的4波段观测获得3.5σ探测，分辨率0.4角秒的9波段观测分别以3.7σ和3.5σ探测到东侧与西侧的莱曼断裂星系。通过修正黑体拟合，我们推断该系统的整体尘埃温度T_d = 78_{-23}^{+35}K，对应红外光度log(L_IR/L_⊙) = 12.32_{-0.41}^{+0.43}，使其成为z>7已知的最亮红外系统之一，总紫外+红外恒星形成率SFR_UV+IR = 267_{-153}^{+418}M_⊙/年，几乎完全被遮挡（f_obs = 0.94_{-0.09}^{+0.04}）。利用ALMA 6、8、9波段的分辨观测，我们确认两个莱曼断裂星系均具有热尘埃温度（T_d ≈ 67 - 84K）及对应的高遮挡比例（f_obs ≈ 0.88 - 0.95）。我们发现西侧莱曼断裂星系比经典的IRX-β_UV关系高约1个数量级，表明存在斑块状尘埃遮挡；而致密的东侧成分在误差范围内与Calzetti型或SMC型尘埃屏一致。结合近期z=8.31星系MACS0416-Y1的热尘埃温度结果，我们的研究表明，即使在z≳7时，紫外光度函数亮端的恒星形成中也存在不可忽略的高度尘埃遮挡成分。
英文摘要
We present new Atacama Large Millimeter/submillimeter Array (ALMA) Band 9 ($λ_\mathrm{obs} = 0.45\,\mathrm{mm}$) and 4 ($λ_\mathrm{obs} = 2.2\,\mathrm{mm}$) observations towards the Big Three Dragons, a pair of merging Lyman-break galaxies (LBGs) at $z=7.15$. The system was previously detected in ALMA Bands 6, 7 and 8 ($λ_\mathrm{obs} = 0.73 - 1.32\,\mathrm{mm}$), which, combined with our new observations, allows us to robustly constrain its dust temperature and obscured SFR. The unresolved Band 4 observations yield a $3.5σ$ detection, and the $0.4''$ Band 9 observations detect the Eastern and Western LBGs at $3.7$ and $3.5σ$, respectively. Through modified blackbody fitting, we infer a global dust temperature of $T_d = 78_{-23}^{+35}\,\mathrm{K}$ for the system, which implies a high IR luminosity of $\log(L_\mathrm{IR}/L_\odot) = 12.32_{-0.41}^{+0.43}$. This makes the Big Three Dragons one of the most IR-luminous systems known at $z>7$, with a total $\mathrm{SFR}_\mathrm{UV+IR} = 267_{-153}^{+418}\,M_\odot\,\mathrm{yr}^{-1}$ that is almost completely obscured ($f_\mathrm{obs} = 0.94_{-0.09}^{+0.04}$). Using resolved ALMA observations in Bands 6, 8 and 9, we confirm both LBGs have hot dust temperatures ($T_d \approx 67 - 84\,\mathrm{K}$) and correspondingly high obscured fractions ($f_\mathrm{obs} \approx 0.88 -0.95$). We find the Western LBG to fall $\sim1\,\mathrm{dex}$ above the canonical IRX-$β_\mathrm{UV}$ relation, suggesting patchy dust obscuration. The compact Eastern component, on the other hand, is consistent with a Calzetti- or SMC-like dust screen within the uncertainties. Together with the similarly hot dust temperature recently reported for the $z=8.31$ galaxy MACS0416-Y1, our results suggest a non-negligible fraction of star formation at the bright end of the UV luminosity function is highly dust-obscured, even at $z\gtrsim7$.
