A new look at the infrared properties of z $\sim$ 5 galaxies

Abstract

Recent ALMA large surveys unveiled the presence of significant dust continuum emission in star-forming galaxies at $z>4$. Unfortunately, such large programs -- i.e. ALPINE ($z\sim 5$) and REBELS ($z\sim 7$) -- 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 ($T_{\mathrm{d}}$) in the SED fitting procedure. This introduces uncertainties in the inferred dust masses ($M_{\mathrm{d}}$), infrared luminosities ($L_{\mathrm{IR}}$), and obscured Star Formation Rate (SFR) fraction at $z>4$. In this work we use a method that allows us to constrain $T_{\mathrm{d}}$ with a single band measurement by combining the $158\hat{\mathrm{I}}¼\mathrm{m}$ continuum information with the overlying [CII] emission line. We analyse the $21$ [CII] and FIR continuum detected $z\sim 5$ galaxies in ALPINE, finding a range of $T_{\mathrm{d}}=25-60\mathrm{K}$ and $M_{\mathrm{d}}=0.6-25.1\times 10^7{\mathrm{M}}_{\odot }$. Given the measured stellar masses of ALPINE galaxies, the inferred dust yields are around $M_{\mathrm{d}}/M_{\star }=(0.2-8)\times 10^{-3}$, consistent with theoretical dust-production constraints. We find that $8$ out of $21$ ALPINE galaxies have $L_{\mathrm{IR}}\ge 10^{12}{\mathrm{L}}_{\odot }$, comparable to UltraLuminous IR Galaxies (ULIRGs). Relying on ultraviolet-to-optical SED fitting, the SFR was underestimated by up to $2$ orders of magnitude in $4$ of these $8$ ULIRGs-like galaxies. We conclude that these $4$ peculiar sources should be characterised by a two-phase interstellar medium structure with "spatially-segregated" FIR and ultraviolet emitting regions.

Accepted for publication in MNRAS. 13 pages, 4 figures