Steep-spectrum AGN in eROSITA Final Equatorial-Depth Survey (eFEDS): Their host galaxies and multi-wavelength properties

¹ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès, 1, 08028 Barcelona, Spain
e-mail: kazushi.iwasawa@icc.ub.edu
² ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching bei München, Germany
⁴ Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
⁵ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
⁶ Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
⁷ Department of Economics, Management and Information Science, Onomichi City University, Onomichi, Hiroshima 722-8506, Japan
⁸ Research Center for Space and Cosmic Evolution, Ehime University, Matsuyama, Ehime 790-8577, Japan
⁹ National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
¹⁰ Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 10617, Taiwan
¹¹ Kavli Institute for the Physics and Mathematics of the Universe, WPI, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
¹² MIT Kavli Institute for Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139, USA
¹³ Universität Erlangen/Nürnberg, Dr.-Remeis-Sternwarte, Sternwartstraße 7, 96049 Bamberg, Germany
¹⁴ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁵ Dr. Karl Remeis-Sternwarte & Erlangen Centre for Astroparticle Physics, Sternwartstraße 7, 96049 Bamberg, Germany

Received: 18 October 2023
Accepted: 3 February 2024

Abstract

We selected sources with a steep soft-X-ray-band spectrum with a photon index of Γ > 2.5 – measured by eROSITA on board the Spectrum-Röntgen-Gamma (SRG) – from the eFEDS AGN catalogue as candidates of highly accreting supermassive black holes, and investigated their multi-wavelength properties. Among 601 bright AGN with 0.2−5 keV counts of greater than 100, 83 sources (≈14%) are classified as steep-spectrum sources. These sources have typical 0.5−2 keV luminosities of L_SX ≈ 10⁴⁴ erg s⁻¹ and the majority of them are found at redshifts below z = 1. In comparison with sources with flatter spectra, these sources have, on average, a UV (or optical) to 2 keV luminosity ratio that is larger by ∼0.3 dex and bluer optical-to-UV continuum emission. They also appear to be radio quiet based on the detection rate in the FIRST and VLASS surveys. Their host galaxies – at least in the redshift range of z = 0.2 − 0.8, where the AGN–galaxy decomposition results from the Subaru Hyper Suprime-Cam imaging are available – tend to be late-type and have smaller stellar masses (M_⋆ ∼ 10^10.5 M_⊙) than those of sources with flatter spectra. These properties are similar to those found in nearby narrow-line Seyfert 1 galaxies, in agreement with the picture that they are AGN with elevated accretion rates and are in the early growth phase of black hole and galaxy co-evolution. However, the steep-spectrum sources are not exclusively narrow-line Seyfert 1 galaxies; indeed many are broad-line Seyfert 1 galaxies, as found by a catalogue search. This suggests that these steep-spectrum sources may be black holes generally with high accretion rates but of a wide mass range, including a few objects emitting at L_SX ≥ 10⁴⁵ erg s⁻¹, of which black hole masses can be close to 10⁹ M_⊙.