[论文解读] New constraints on dust emission and UV attenuation of z= 6.5 7.5 galaxies from IRAM and ALMA observations ⋆

D. Schaerer, F. Boone+5 more|arXiv (Cornell University)|Jul 22, 2014

Galaxies: Formation, Evolution, Phenomena被引用 1

一句话总结

本研究利用深空IRAM PdBI与ALMA观测，对红移z ≈ 6.8和z = 7.5的两个高红移Lyman break星系的尘埃发射和紫外消光进行约束。尽管未探测到[CII]线和尘埃连续谱，但数据集给出了红外 luminosity（LIR < 8.1×10¹⁰ L⊙）、尘埃质量（Md < 1.6×10⁷ M⊙）和恒星形成速率的严格上限，结果与Calzetti的IRX–β关系一致，且其恒星质量对应的紫外消光低于预期值。

ABSTRACT

Context. Determining the dust properties and UV attenuation of distant star-forming galaxies is of great interest for our unders tanding of galaxy formation and cosmic star formation in the early Universe. However, few direct measurements exist so far. Aims. To shed new light on these questions we have targeted two recently discovered Lyman break galaxies (LBGs) at z≈ 6.8 and z = 7.508 to search for dust continuum and [Cii]λ158µm line emission. Methods. The strongly lensed z≈ 6.8 LBG A1703-zD1 behind the galaxy cluster Abell 1703, and the spectroscopically confirmed z = 7.508 LBG z8-GND-5296 in the GOODS-N field have been observed with the Plateau de Bure interferometric (PdBI) observations at 1.2mm. These observations have been combined with those of three z> 6.5 Lyα emitters (named HCM6A, Himiko, and IOK-1), for which deep measurements were recently obtained with the PdBI and ALMA. Results. [Cii] is undetected in both galaxies, providing a deep upper limit of L[CII] < 2.8× 10 7 L⊙ for A1703-zD1, comparable to the non-detections of Himiko and IOK-1 with ALMA. Dust continuum emission from A1703-zD1 and z8-GND-5296 is not detected with an rms of 0.12 and 0.16 mJy/beam. From these non-detections and earlier multi-wavelength observations we derive upper limits on their IR luminosity and star formation rate, dust mass, and UV attenuation. Thanks to strong gravitational lensing the achieved limit for A1703-zD1 is comparable to those achieved with ALMA, probing the sub-LIRG regime (LIR< 8.1× 10 10 L⊙) and very low dust masses (Md< 1.6× 10 7 M⊙). We find that all five galaxies are compatible with the Calzet ti IRX‐β relation, their UV attenuation is compatible with several indirect estimates from other methods (the UV slope, extrapolation of the attenuation measured from the IR/UV ratio at lower redshift, and SED fits), and the dust-to-ste llar mass ratio is not incompatible with that of galaxies fro m z = 0 to 3. For their stellar mass the high-z galaxies studied here have an attenuation below the one expected from the mean relation of low redshift (z< 1.5) galaxies. Conclusions. More and deeper (sub)-mm data are clearly needed to directly determine the UV attenuation and dust content of the dominant population of high-z distant star-forming galaxies and to establish more firmly t heir dependence on stellar mass, redshift, and other properties.

研究动机与目标

• 直接测量红移z ≈ 6.5–7.5的高红移星系中尘埃性质与紫外消光，此类数据在该红移区间极为稀缺。
• 评估早期宇宙中间接紫外消光估计方法（如紫外谱指数或IR/UV比值）的可靠性。
• 评估高红移星系是否与本地及中等红移星系遵循相同的尘埃-恒星质量关系。
• 判断观测到的[CII]线与尘埃连续谱非探测结果，是否与早期星系的理论预期一致。

提出的方法

• 利用帕莱索-布里干涉仪（Plateau de Bure Interferometer, PdBI）对两个强引力透镜效应的z ≈ 6.8和z = 7.508 Lyman break星系进行1.2 mm连续谱与[CII]λ158µm谱线观测。
• 将PdBI数据与三颗额外z > 6.5的Lyα发射体（HCM6A、Himiko、IOK-1）的深空ALMA与PdBI观测数据结合，以提升灵敏度与统计效力。
• 利用[CII]线与尘埃连续谱的非探测结果，通过多波段SED建模推导出红外 luminosity、尘埃质量与恒星形成速率的上限。
• 应用Calzetti的IRX–β关系，将观测到的紫外消光与理论预期进行比较，评估不同红移下的行为一致性。
• 借助A1703-zD1中的强引力透镜效应，实现亚LIRG量级的灵敏度（LIR < 8.1×10¹⁰ L⊙）与低尘埃质量上限（Md < 1.6×10⁷ M⊙）。
• 评估尘埃-恒星质量比，并与z = 0至3星系的比值进行比较，以评估演化趋势。

实验结果

研究问题

• RQ1 基于1.2 mm波段的非探测结果，z ≈ 6.8与z = 7.508 Lyman break星系的尘埃质量与红外 luminosity上限为何？
• RQ2 这些高红移星系的紫外消光特性与Calzetti IRX–β关系及基于紫外谱指数或IR/UV比值的间接估计相比如何？
• RQ3 这些高红移星系的尘埃-恒星质量比是否与z = 0至3星系观测结果一致？
• RQ4 A1703-zD1与z8-GND-5296中[CII]线的非探测结果，与ALMA对类似星系的观测预期一致到何种程度？
• RQ5 与低红移（z < 1.5）星系相比，这些星系的紫外消光是否低于其恒星质量所预期的水平？

主要发现

• 在A1703-zD1与z8-GND-5296中均未探测到[CII]线，A1703-zD1的深上限为L[CII] < 2.8×10⁷ L⊙。
• 两星系均未探测到尘埃连续谱发射，1σ噪声分别为0.12 mJy/beam（A1703-zD1）与0.16 mJy/beam（z8-GND-5296）。
• 红外 luminosity上限为LIR < 8.1×10¹⁰ L⊙，表明星系处于亚LIRG状态，尘埃质量被约束在Md < 1.6×10⁷ M⊙以内。
• 包括HCM6A、Himiko、IOK-1在内的五颗观测星系均与Calzetti IRX–β关系一致，表明其紫外消光行为合理。
• 高红移星系的紫外消光低于低红移（z < 1.5）星系中基于恒星质量的平均关系所预期的水平。
• 这些高红移星系的尘埃-恒星质量比与z = 0至3星系的观测结果不矛盾，表明尘埃含量趋势具有连续性。