A new look at the infrared properties of z ∼ 5 galaxies

Recent Atacama Large Millimeter Array large surveys unveiled the presence of significant dust continuum emission in star-forming galaxies at z > 4. Unfortunately, such large programs – i.e. ALPINE (z ∼ 5) and REBELS (z ∼ 7) – only provide us with a single far-infrared (FIR) continuum data point for their individual targets. Therefore, high-z galaxies FIR spectral energy densities (SEDs) remain mostly unconstrained, hinging on an assumption for their dust temperature (T_d) in the SED fitting procedure. This introduces uncertainties in the inferred dust masses (M_d), infrared luminosities (L_(IR)), and obscured star formation rate (SFR) fraction at z > 4. In this work, we use a method that allows us to constrain T_d with a single-band measurement by combining the 158 µm continuum information with the overlying [C ɪɪ emission line. We analyse the 21 [C ɪɪ and FIR continuum-detected z ∼ 5 galaxies in ALPINE, finding a range of T_d = 25–60 K and M_d = 0.6–25.1 × 10⁷ M_⊙. Given the measured stellar masses of ALPINE galaxies, the inferred dust yields are around M_d/M_⋆ = (0.2–8) × 10⁻³, consistent with theoretical dust-production constraints. We find that eight out of the 21 ALPINE galaxies have L_(IR) ≥ 10¹² L_⊙, comparable to ultraluminous IR galaxies (ULIRGs). Relying on ultraviolet-to-optical SED fitting, the SFR was underestimated by up to two orders of magnitude in four of these eight ULIRGs-like galaxies. We conclude that these four peculiar sources should be characterized by a two-phase interstellar medium structure with 'spatially segregated' FIR and ultraviolet emitting regions.