Infrared emission of z ∼ 6 galaxies: AGN imprints

Abstract

We investigate the infrared (IR) emission of high-redshift (z ∼ 6), highly star-forming (SFR > 100 M⊙ 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% of the UV radiation is obscured by dust inhomogeneities on scales ≳ 100 pc. In runs with AGN, a clumpy, warm (≈250 K) dust component co-exists with a colder (≈60 K) and more diffuse one, heated by stars. Warm dust provides up to ${50 \%}$ of the total IR luminosity, but only $\lesssim 0.1 \%$ of the total mass content. The AGN boosts the MIR flux by 10 − 100 × with respect to star forming galaxies, without significantly affecting the FIR. 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 BHs 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 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 × 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.

Super

“…Although we are limited by photometric coverage at long wavelengths in most of the SEDs, which makes the exact percentage of the AGN contribution uncertain, all our analyses indicate that the contribution from dust heated by the AGN to the 260 µm emission, and hence to our maps produced from the ALMA data, is negligible. We note that some studies based on radiative transfer models have highlighted the possibility that AGN could contribute significantly to the heating of diffuse warm dust on host-galaxy scales (Schneider et al 2015;Duras et al 2017;Viaene et al 2020;Di Mascia et al 2021;McKinney et al 2021). This emission is not related to the torus emission and therefore it is not taken into account in our SED fitting decomposition.…”

“…However, another interpretation is that the AGN are contributing significantly to the heating of the diffuse dust in the central region of the galaxy, resulting in more concentrated FIR emission in AGN host galaxies (e.g. Schneider et al 2015;Viaene et al 2020;Di Mascia et al 2021;McKinney et al 2021). On the other hand, Ni et al (2021) recently reported a relation between the black hole accretion rate and the compactness of the host galaxy using optical/NIR imaging.…”

Dust-enshrouded AGNs Can Dominate Host-galaxy-scale Cold Dust Emission

“…The origin of the FIR emission in galaxies hosting luminous AGNs has been debated over the past few decades (e.g., Downes & Solomon 1998;Page et al 2001;Franceschini et al 2003;Ruiz et al 2007;Kirkpatrick et al 2012Kirkpatrick et al , 2015. Over the years, a growing body of literature has been making the case for AGN-powered FIR emission at wavelengths 100 μm in QSOs (e.g., Sanders et al 1989;Yun & Scoville 1998;Nandra & Iwasawa 2007;Petric et al 2015;Schneider et al 2015;Symeonidis et al 2016;Symeonidis 2017;Symeonidis & Page 2018); however, other works find conflicting results (e.g., Stanley et al 2017;Shangguan et al 2020;DiMascia et al 2021). Spatially resolved observations of heavily obscured nearby systems find that a significant fraction of the galaxyintegrated FIR/submillimeter emission can originate from very small regions (e.g., 15 pc for Arp 220; Scoville et al 2017), suggestive that deeply dust-obscured AGNs power this emission (as it is unlikely that a star-forming region forming stars at a rate 100 M e yr −1 would be this compact).…”

Dust-Enshrouded AGN can Dominate Host-Galaxy-Scale Cold-Dust Emission

“…The origin of the far-IR emission in galaxies hosting luminous AGN has been debated over the past few decades (e.g., Downes & Solomon 1998;Page et al 2001;Franceschini et al 2003;Ruiz et al 2007;Kirkpatrick et al 2012Kirkpatrick et al , 2015. Over the years, a growing body of literature has been making the case for AGN-powered FIR emission in QSOs (e.g., Sanders et al 1989;Yun & Scoville 1998;Nandra & Iwasawa 2007;Petric et al 2015;Schneider et al 2015;Symeonidis et al 2016;Symeonidis 2017;Symeonidis & Page 2018); however, other works find conflicting results (e.g., Stanley et al 2017;Shangguan et al 2020;DiMascia et al 2021).…”