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

¹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
e-mail: benjamin.magnelli@cea.fr
² Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ Aix-Marseille Univ, CNRS, CNES, LAM, Marseille, France
⁴ INAF – Osservatorio Astronomico di Brera 28, 20121, Milano, Italy and Via Bianchi 46, 23807 Merate, Italy
⁵ Waseda Research Institute for Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
⁶ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo, Japan
⁷ Center for Frontier Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
⁸ Department of Pure and Applied Physics, Graduate School of Advanced Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
⁹ The University of Texas at Austin, 2515 Speedway Blvd Stop C1400, Austin, TX 78712, USA
¹⁰ Istituto Nazionale di Astrofisica (INAF) – Osservatorio di Astrofisica e Scienza dello Spazio (OAS), Via Gobetti 101, 40129 Bologna, Italy
¹¹ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Antigua Carretera a Pátzcuaro #8701, Ex-Hda. San José de la Huerta, Morelia, Michoacán 58089, Mexico
¹² Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany
¹³ Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, PR China
¹⁴ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁵ Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128, København N 2200, Denmark
¹⁶ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁷ Dipartimento di Fisica e Astronomia (DIFA), Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy

Received: 22 March 2024
Accepted: 27 May 2024

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 (> 10^9.5 M_⊙) sample of SFGs at 4 < z < 5. Then, we measured their mean dust-attenuated properties (i.e., infrared luminosity, ⟨L_IR⟩; dust-attenuated star formation rate, ⟨SFR_IR⟩) by dividing our sample in three stellar mass (M_*) bins (i.e., 10^9.5 < M_*/M_⊙ < 10¹⁰, 10¹⁰ < M_*/M_⊙ < 10^10.5, and 10^10.5 < M_*/M_⊙ < 10^11.5) 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, ⟨L_UV⟩; UV spectral slope, ⟨β_UV⟩; and unattenuated SFR, ⟨SFR_UV⟩), and constrained the IRX (≡L_IR/L_UV)–β_UV, 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–β_UV 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 10^{9.5 − 10.5} M_⊙. Finally, the fraction of the cosmic SFRD that is attenuated by dust, SFRD_IR(> M_*)/SFRD(> M_*), is 90 ± 4% for M_* = 10¹⁰ M_⊙, 68 ± 10% for M_* = 10^8.9 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–β_UV 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.