We introduce SERRA, a suite of zoom-in high-resolution (1.2 × 104 M⊙, ≃ 25 pc at z = 7.7) cosmological simulations including non-equilibrium chemistry and on-the-fly radiative transfer. The outputs are post-processed to derive galaxy UV+FIR continuum and emission line properties. Results are compared with available multi-wavelength data to constrain the physical properties (e.g. star formation rates, stellar/gas/dust mass, metallicity) of high-redshift 6 ≲ z ≲ 15 galaxies. This flagship paper focuses on the z = 7.7 sub-sample, including 202 galaxies with stellar mass 107 M⊙ ≲ M⋆ ≲ 5 × 1010 M⊙, and specific star formation ranging from sSFR ∼ 100 Gyr−1 in young, low-mass galaxies to ∼10 Gyr−1 for older, massive ones. At this redshift, serra galaxies are typically bursty, i.e. they are located above the Schmidt-Kennicutt relation by a factor $\kappa _s = 3.03^{+4.9}_{-1.8}$, consistent with recent findings for [O iii] and [C ii] emitters at high-z. They also show relatively large IRX =LFIR/LUV values as a result of their compact/clumpy morphology effectively blocking the stellar UV luminosity. Note that this conclusion might be affected by insufficient spatial resolution at the molecular cloud level. We confirm that early galaxies lie on the standard [C ii]$-\rm SFR$ relation; their observed L[OIII]/L[CII] ≃ 1 − 10 ratios can be reproduced by a part of the SERRA galaxies without the need of a top-heavy IMF and/or anomalous C/O abundances. [O i] line intensities are similar to local ones, making ALMA high-z detections challenging but feasible ($\sim 6\, \rm hr$ for an SFR of 50  M⊙ yr−1).

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.

We investigate the attenuation law in z ∼ 6 quasars by combining cosmological zoom-in hydrodynamical simulations of quasar host galaxies, with multi-frequency radiative transfer calculations. We consider several dust models differing in terms of grain size distributions, dust mass and chemical composition, and compare the resulting synthetic Spectral Energy Distributions (SEDs) with data from bright, early quasars. We show that only dust models with grain size distributions in which small grains ($a\lesssim 0.1~\mu {\rm m}$, corresponding to $\approx 60{{\ \rm per\ cent}}$ of the total dust mass) are selectively removed from the dusty medium provide a good fit to the data. Removal can occur if small grains are efficiently destroyed in quasar environments and/or early dust production preferentially results in large grains. Attenuation curves for these models are close to flat, and consistent with recent data; they correspond to an effective dust-to-metal ratio fd ≃ 0.38, i.e. close to the Milky Way value.

We present ALMA Band 9 continuum observation of the ultraluminous quasi-stellar object (QSO) SDSS J0100+2802 providing a ∼10σ detection at ∼670 GHz. SDSS J0100+2802 is the brightest QSO with the most massive supermassive black hole (SMBH) known at z > 6, and we study its dust spectral energy distribution in order to determine the dust properties and the star formation rate (SFR) of its host galaxy. We obtain the most accurate estimate so far of the temperature, mass, and emissivity index of the dust, which are T dust = 48.4 ± 2.3 K, M dust = (2.29 ± 0.83) × 107 M ⊙, and β = 2.63 ± 0.23, respectively. This allows us to measure the SFR with the smallest statistical error for this QSO, SFR = 265 ± 32 M ⊙yr−1. Our results enable us to evaluate the relative growth of the SMBH and host galaxy of J0100+2802. We find that the SMBH is dominating the process of black-hole galaxy growth in this QSO at z = 6.327, when the universe was 865 Myr old. Such unprecedented constraints on the host-galaxy SFR and dust temperature can only be obtained through high-frequency observations and highlight the importance of ALMA Band 9 to obtain a robust overview of the buildup of the first quasars’ host galaxies at z > 6.

The large total infrared (TIR) luminosities (LTIR ≳ 1012 L⊙) observed in z ∼ 6 quasars are generally converted into high star formation rates (SFR ≳ 102 M⊙ yr−1) of their host galaxies. However, these estimates rely on the assumption that dust heating is dominated by stellar radiation, neglecting the contribution from the central Active Galactic Nuclei (AGN). We test the validity of this assumption by combining cosmological hydrodynamic simulations with radiative transfer calculations. We find that, when AGN radiation is included in the simulations, the mass (luminosity)-weighted dust temperature in the host galaxies increases from T ≈ 50 K (T ≈ 70 K) to T ≈ 80 K (T ≈ 200 K), suggesting that AGN effectively heat the bulk of dust in the host galaxy. We compute the AGN-host galaxy SFR from the synthetic spectral energy distribution by using standard SFR − LTIR relations, and compare the results with the “true” values in the simulations. We find that the SFR is overestimated by a factor of ≈3 (≳ 10) for AGN bolometric luminosities of Lbol ≈ 1012 L⊙ (≳ 1013 L⊙), implying that the star formation rates of z ∼ 6 quasars can be overestimated by over an order of magnitude.