一个测光红移的警示故事：COSBO-7 在 z=2.625 的红移光谱证认
A photo-z cautionary tale: Redshift confirmation of COSBO-7 at z=2.625
AI总结：
本文通过 ALMA 多谱线观测将强透镜尘埃恒星形成星系 COSBO-7 的光谱红移证认为 z=2.625，纠正了此前基于 JWST 多波段测光和单谱线探测得到的 z≳7.0 错误解，并指出中红外测光数据对提高测光红移可靠性的关键作用。
摘要：
测光红移广泛用于尘埃恒星形成星系（DSFGs）的研究，但灾难性的 photo-z 失败可能破坏所有依赖红移的结果。本文报告了 COSBO-7 的光谱红移证认，COSBO-7 是 COSMOS-PRIMER 天区中一个强透镜化的尘埃恒星形成星系。最近，基于詹姆斯·韦布空间望远镜（JWST）NIRCam 和 MIRI 成像数据的十个波段，有人报告了 COSBO-7 的测光红移解为 z≳7.0。这一 z 值得到四个独立光谱能量分布（SED）拟合程序的支持，该结果提供了一个迄今已知最遥远大质量尘埃恒星形成星系的有吸引力的候选体。这一 photo-z 解还得到阿塔卡马大型毫米波阵列（ALMA）Band 3 中一条单谱线探测的支持，该谱线与 z=7.46 处的 CO(7-6) 一致。然而，我们新的 ALMA 观测在 Band 6 中稳健地探测到两条谱线，证认为 z_spec=2.625 处的 CO(7-6) 和 [CI](2-1)，因此 Band 3 中的谱线应为 CO(3-2)。这三条稳健的谱线探测明确地将 COSBO-7 定位于 z=2.625，否定了该 photo-z 解。我们通过拟合近红外（NIR）至毫米波（mm）测光数据和透镜模型推导物理参数，揭示 COSBO-7 是一个主序星系。我们考察了这一 photo-z 失败的可能原因，并将其归因于：（1）0.9 μm 和 1.15 μm 处测光不确定度可能被低估；（2）缺乏波长超过 20 μm 的测光数据。值得注意的是，通过纳入现有的 MIPS 24 μm 测光数据，我们恢复出一个真实的 z_phot∼2.3，证明了中红外（MIR）数据在增强 photo-z 测量方面的关键重要性。这项工作凸显了在 DSFGs 的 SED 建模中普遍存在的挑战，并就测光红移以及基于单谱线探测的伪光谱红移的可靠性提供了一个警示故事。
英文摘要：
Photometric redshifts are widely used in studies of dusty star-forming galaxies (DSFGs), but catastrophic photo-$z$ failure can undermine all redshift-dependent results. Here we report the spectroscopic redshift confirmation of COSBO-7, a strongly lensed DSFG in the COSMOS-PRIMER field. Recently, a photometric redshift solution of $z\gtrsim7.0$ was reported for COSBO-7 based on ten bands of {\it James Webb} Space Telescope (JWST) NIRCam and MIRI imaging data. This $z$ value was favored by four independent spectral energy distribution (SED) fitting codes, and the result provided an appealing candidate for the most distant massive DSFG known to date. This photo-$z$ solution was also supported by a single line detection in Atacama Large Millimeter Array (ALMA) Band 3 consistent with CO(7-6) at $z=7.46$. However, our new ALMA observations robustly detect two lines in Band 6 identified as CO(7-6) and [CI](2-1) at $z_{\rm spec}=2.625$, and thus the Band 3 line as CO(3-2). These three robust line detections decidedly place COSBO-7 at $z=2.625$, refuting the photo-$z$ solution. We derive physical parameters by fitting near-infrared(NIR)-to-millimeter(mm) photometry and lens modeling, revealing that COSBO-7 is a main sequence galaxy. We examine possible reasons for this photo-$z$ failure and attribute it to (1) the likely underestimation of photometric uncertainties at 0.9\,$μ$m and 1.15 \,$μ$m; and (2) the lack of photometry at wavelengths beyond 20\,$μ$m. Notably, we recover a bona fide $z_{\rm phot}\sim 2.3$ by including the existing MIPS $24\,μ$m photometry, demonstrating the critical importance of mid-infrared (MIR) data in bolstering photo-$z$ measurements. This work highlights a common challenge in modeling the SEDs of DSFGs, and provides a cautionary tale regarding the reliability of photometric redshifts as well as pseudo-spectroscopic redshifts based on single line detection.
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