The eROSITA Final Equatorial-Depth Survey (eFEDS): An X-ray bright, extremely luminous infrared galaxy at z = 1.87

Abstract

In this study, we investigate the X-ray properties of WISE J090924.01+000211.1 (WISEJ0909+0002), an extremely luminous infrared (IR) galaxy (ELIRG) at $z_{\mathrm{spec}}$= 1.871 in the eROSITA final equatorial depth survey (eFEDS). WISEJ0909+0002 is a WISE 22 $\hat{I}¼$m source, located in the GAMA-09 field, detected by eROSITA during the performance and verification phase. The corresponding optical spectrum indicates that this object is a type-1 active galactic nucleus (AGN). Observations from eROSITA combined with Chandra and XMM-Newton archival data indicate a very luminous ($L$ (2--10 keV) = ($2.1\pm 0.2)\times 10^{45}$ erg s${}^{-1}$) unobscured AGN with a power-law photon index of $\hat{I}“$ = 1.73${}_{-0.15}^{+0.16}$, and an absorption hydrogen column density of $\log (N_{\mathrm{H}}/{\mathrm{cm}}^{-2})<21.0$. The IR luminosity was estimated to be $L_{\mathrm{IR}}$ = (1.79 $\pm$ 0.09) $\times 10^{14}{L}_{\odot }$ from spectral energy distribution modeling based on 22 photometric data (X-ray to far-IR) with X-CIGALE, which confirmed that WISEJ0909+0002 is an ELIRG. A remarkably high $L_{\mathrm{IR}}$ despite very low $N_{\mathrm{H}}$ would indicate that we are witnessing a short-lived phase in which hydrogen gas along the line of sight is blown outwards, whereas warm and hot dust heated by AGNs still exist. As a consequence of eROSITA all-sky survey, $6.8_{-5.6}^{+16}\times 10^2$ such X-ray bright ELIRGs are expected to be discovered in the entire extragalactic sky ($|b|>10^{\circ }$). This can potentially be the key population to constrain the bright-end of IR luminosity functions.

10 pages, 5 figures, and 3 tables, accepted for publication in A&A Letters (special Issue: First science highlights from SRG/eROSITA)