XXL-HSC: Link between AGN activity and star formation in the early Universe (z ⩾ 3.5)

¹ IAASARS, National Observatory of Athens, Ioannou Metaxa and Vasileos Pavlou, 15236 Athens, Greece
e-mail: epouliasis@noa.gr
² Instituto de Fisica de Cantabria (CSIC-Universidad de Cantabria), Avenida de los Castros, 39005 Santander, Spain
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁴ Astronomical Institute, Tohoku University, 6-3 Aramaki, Aoba-ku, Senda 980-8578, Japan
⁵ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
⁶ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
⁷ Research Center for Space and Cosmic Evolution, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan
⁸ Department of Astronomy, University of Geneva, ch. d’Écogia 16, 1290 Versoix, Switzerland
⁹ Academia Sinica Institute of Astronomy and Astrophysics, 11F of Astronomy-Mathematics Building, AS/NTU, No.1, Section 4, Roosevelt Road, Taipei 10617, Taiwan
¹⁰ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹¹ Università di Bologna, Dip. di Fisica e Astronomia “A. Righi”, Via P. Gobetti 93/2, 40129 Bologna, Italy

Received: 8 March 2022
Accepted: 27 June 2022

Abstract

In this work, our aim is to investigate the star formation rate (SFR) of the host galaxies of active galactic nuclei (AGNs) in the early Universe. To this end, we constructed a sample of 149 luminous (L_{2 − 10 keV} > 10⁴⁴ erg s⁻¹) X-ray AGNs at z ≥ 3.5 selected in three fields with different depths and observed areas from the Chandra COSMOS Legacy survey, XMM-XXL North, and eROSITA Final Equatorial-Depth Survey. We built their spectral energy distributions (SED) using the available multi-wavelength photometry from X-rays up to the far-IR. Then, we estimated the stellar mass, M_*, and the SFR of the AGNs using the X-CIGALE SED fitting algorithm. After applying several quality criteria, we ended up with 89 high-z sources. More than half (55%) of the X-ray sample have spectroscopic redshifts. Based on our analysis, our high-z X-ray AGNs live in galaxies with a median M_* = 5.6 × 10¹⁰ M_⊙ and SFR_* ≈ 240 M_⊙yr⁻¹. The majority of the high-z sources (∼89%) were found inside or above the main sequence (MS) of star-forming galaxies. Estimations of the normalised SFR, SFR_NORM, defined as the ratio of the SFR of AGNs, to the SFR of MS galaxies, show that the SFR of AGNs is enhanced by a factor of ∼1.8 compared to non-AGN star-forming systems. Combining our results with previous studies at lower redshifts, we confirmed that SFR_NORM does not evolve with redshift. Using the specific black hole accretion rate (i.e. L_X divided by M_*), λ_BHAR, which can be used as a tracer of the Eddington ratio, we found that the majority of AGNs that lie inside or above the MS have higher specific accretion rates compared to sources below the MS. Finally, we found indications that the SFR of the most massive AGN host galaxies (log (M_*/M_⊙) > 10^{11.5 − 12}) remains roughly constant as a function of M_*, in agreement with the SFR of MS star-forming galaxies.