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

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 LSX ≈ 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⋆ ∼ 1010.5 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 LSX ≥ 1045 erg s−1, of which black hole masses can be close to 109 M⊙.