A3COSMOS: Measuring the cosmic dust-attenuated star formation rate density at 4 < z < 5
Authors: Magnelli Benjamin , Adscheid Sylvia , Wang Tsan-Ming , Ciesla Laure , Daddi Emanuele , Delvecchio Ivan , Elbaz David , Fudamoto Yoshinobu , Fukushima Shuma , Franco Maximilien , Gómez-Guijarro Carlos , Gruppioni Carlotta , Jiménez-Andrade Eric F. , Liu Daizhong , Oesch Pascal , Schinnerer Eva , Traina Alberto 
Published: May 2024

Context. In recent years, conflicting results have provided an uncertain view of the dust-attenuated star-forming properties of z ≳ 4 galaxies.
Aims. To solve this, we need to accurately measure the mean dust-attenuated properties of star-forming galaxies (SFGs) at 4 < z < 5 and therefore constrain the cosmic dust-attenuated star formation rate density (SFRD) of the Universe 1.3 Giga-years after the Big Bang.
Methods. We used the deepest optical-to-near-infrared data publicly available in the Cosmic Evolution Survey (COSMOS) field to build a mass-complete (> 109.5 M⊙) sample of SFGs at 4 < z < 5. Then, we measured their mean dust-attenuated properties (i.e., infrared luminosity, ⟨LIR⟩; dust-attenuated star formation rate, ⟨SFRIR⟩) by dividing our sample in three stellar mass (M*) bins (i.e., 109.5 < M*/M⊙ < 1010, 1010 < M*/M⊙ < 1010.5, and 1010.5 < M*/M⊙ < 1011.5) and by stacking in the uv domain all archival Atacama Large Millimeter/submillimeter Array (ALMA) band 6 and 7 observations available for these galaxies. Then, we combined this information with their mean rest-frame ultraviolet (UV) emission measured from the COSMOS2020 catalog (i.e., UV luminosity, ⟨LUV⟩; UV spectral slope, ⟨βUV⟩; and unattenuated SFR, ⟨SFRUV⟩), and constrained the IRX (≡LIR/LUV)–βUV, IRX–M*, and SFR–M* relations at z ∼ 4.5. Finally, using these relations and the stellar mass function of SFGs at z ∼ 4.5, we inferred the unattenuated and dust-attenuated SFRD at this epoch.
Results. SFGs follow an IRX–βUV relation that is consistent with that observed in local starbursts. Our measurements favors a steepening of the IRX–M* relation at z ∼ 4.5, compared to the redshift-independent IRX–M* relation observed at z ∼ 1 − 3. Our galaxies lie on a linear SFR–M* relation, whose normalization varies by 0.3 dex, when we exclude or include from our stacks the ALMA primary targets (i.e., sources within 3″ from the ALMA phase center). The cosmic SFRD( > M*) converges at M* ≲ 109 M⊙, with SFGs at 108 < M*/M⊙ < 109 contributing already less than 15% of the SFRD from all SFGs with M* > 108 M⊙. The cosmic SFRD at z ∼ 4.5 is dominated by SFGs with a stellar mass of 109.5 − 10.5 M⊙. Finally, the fraction of the cosmic SFRD that is attenuated by dust, SFRDIR(> M*)/SFRD(> M*), is 90 ± 4% for M* = 1010 M⊙, 68 ± 10% for M* = 108.9 M⊙ (i.e., 0.03 × M⋆; M⋆ being the characteristic stellar mass of SFGs at this epoch) and this value converges to 60 ± 10% for M* = 108 M⊙.
Conclusions. A non-evolving IRX–βUV relation suggests that the grain properties (e.g., size distribution, composition) of dust in SFGs at z ∼ 4.5 are similar to those in local starbursts. However, the mass and geometry of this dust result in lower attenuation in low-mass SFGs (≲1010 M⊙) at z ∼ 4.5 than at z ≲ 3. Nevertheless, the fraction of the cosmic SFRD that is attenuated by dust remains significant (∼68 ± 10%) even at such an early cosmic epoch.