A3COSMOS: Measuring the cosmic dust-attenuated star formation rate density at 4 < z < 5

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Abstract

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, hLIRi; dust-attenuated star formation rate, hSFRIRi) by dividing our sample in three stellar mass (M∗) bins (i.e., 109.5 < 1010, 1010 < 1010.5, and 1010.5 < 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, hLUVi; UV spectral slope, hβUVi; and unattenuated SFR, hSFRUVi), 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 300 from the ALMA phase center). The cosmic SFRD(>M∗) converges at M∗ . 109 M , with SFGs at 108 < 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.

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Magnelli, B., Adscheid, S., Wang, T. M., Ciesla, L., Daddi, E., Delvecchio, I., … Traina, A. (2024). A3COSMOS: Measuring the cosmic dust-attenuated star formation rate density at 4 < z < 5. Astronomy and Astrophysics, 688. https://doi.org/10.1051/0004-6361/202450081

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