We investigate the infrared (IR) emission of high-redshift (z ∼ 6), highly star-forming (${{\rm SFR}\gt 100\,{\rm M}_{\odot }\, {\rm yr}^{-1}}$) galaxies, with/without active galactic nuclei (AGN), using a suite of cosmological simulations featuring dust radiative transfer. Synthetic spectral energy distributions (SEDs) are used to quantify the relative contribution of stars/AGN to dust heating. In dusty (Md ≳ 3 × 107 M⊙) galaxies, ≳50–90 per cent of the ultraviolet (UV) radiation is obscured by dust inhomogeneities on scales ≳100 pc. In runs with AGN, a clumpy, warm (≈250 K) dust component coexists with a colder (≈60 K) and more diffuse one, heated by stars. Warm dust provides up to ${50 {{\ \rm per\ cent}}}$ of the total infrared (IR) luminosity, but only ${\lesssim}0.1 {{\ \rm per\ cent}}$ of the total mass content. The AGN boosts the MIR flux by 10–100 times with respect to star-forming galaxies, without significantly affecting the far-IR. Our simulations successfully reproduce the observed SED of bright (MUV ∼ −26) z ∼ 6 quasars, and show that these objects are part of complex, dust-rich merging systems, containing multiple sources (accreting black holes and/or star-forming galaxies) in agreement with recent HST and ALMA observations. Our results show that the proposed ORIGINS missions will be able to investigate the mid-IR (MIR) properties of dusty star-forming galaxies and to obtain good-quality spectra of bright quasars at z ∼ 6. Finally, the MIR-to-FIR flux ratio of faint (MUV ∼ −24) AGN is >10 times higher than for normal star-forming galaxies. This implies that combined JWST/ORIGINS/ALMA observations will be crucial to identify faint and/or dust-obscured AGN in the distant Universe.

中文翻译：

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z ∼ 6 星系的红外发射：AGN 印记

我们研究了高红移 (z ∼ 6)、高恒星形成 (${{\rm SFR}\gt 100\,{\rm M}_{\odot }\, {\rm yr}^{-1}}$) 个星系，有/没有活动星系核（AGN），使用一套以尘埃辐射转移为特征的宇宙学模拟。合成光谱能量分布 (SED) 用于量化恒星/AGN 对尘埃加热的相对贡献。在尘埃 (Md ≳ 3 × 107 M⊙) 星系中，≳50-90% 的紫外线 (UV) 辐射被≳100 pc. 在 AGN 的运行中，一种块状、温暖 (≈250 K) 的尘埃成分与更冷 (≈60 K) 和更分散的尘埃成分共存，由恒星加热。暖尘埃提供高达 ${50 {{\ \rm per\ cent}}}$ 的总红外 (IR) 光度，但只有 ${\lesssim}0.1 {{\ \rm per\ cent}}$总质量含量。AGN 将 MIR 通量相对于恒星形成星系提高了 10-100 倍，而不会显着影响远红外。我们的模拟成功地再现了观测到的明亮 (MUV ∼ -26) z ∼ 6 类星体的 SED，并表明这些物体是复杂的、富含尘埃的合并系统的一部分，包含多个来源（吸积黑洞和/或恒星形成星系) 与最近的 HST 和 ALMA 观察结果一致。我们的结果表明，提议的 ORIGINS 任务将能够研究尘埃恒星形成星系的中红外（MIR）特性，并在 z ∼ 6 处获得明亮类星体的高质量光谱。最后，MIR-to-FIR微弱 (MUV ∼ -24) AGN 的通量比比正常的恒星形成星系高 10 倍以上。