The Next Generation X-ray Galaxy Survey with eROSITA

TL;DR

The paper predicts that the eROSITA all-sky X-ray survey will detect over 15,000 normal galaxies within 50-200 Mpc, vastly increasing the known X-ray galaxy population and providing new insights into galaxy and AGN properties.

Contribution

It introduces a comprehensive prediction of eROSITA's capabilities to detect normal galaxies and assesses their impact on X-ray luminosity functions using simulations and scaling relations.

Findings

eROSITA will detect over 15,000 galaxies at 50-200 Mpc.

Normal galaxies will outnumber detected AGN by a factor of 100.

The study provides detailed predictions of X-ray emission from galaxies based on multi-wavelength data.

Abstract

eROSITA, launched on 13 July 2019, will be completing the first all-sky survey in the soft and medium X-ray band in nearly three decades. This 4-year survey, finishing in late 2023, will present a rich legacy for the entire astrophysics community and complement upcoming multi-wavelength surveys (with, e.g. the Large Synoptic Survey Telescope and the Dark Energy Survey). Besides the major scientific aim to study active galactic nuclei (AGN) and galaxy clusters, eROSITA will contribute significantly to X-ray studies of normal (i.e., not AGN) galaxies. Starting from multi-wavelength catalogues, we measure star formation rates and stellar masses for 60 212 galaxies constrained to distances of 50-200 Mpc. We chose this distance range to focus on the relatively unexplored volume outside the local Universe, where galaxies will be largely spatially unresolved and probe a range of X-ray luminosities that overlap with the low luminosity and/or highly obscured AGN population. We use the most recent X-ray scaling relations as well as the on-orbit eROSITA instrument performance to predict the X-ray emission from XRBs and diffuse hot gas and to perform both an analytic prediction and an end-to-end simulation using the mission simulation software, SIXTE. We consider potential contributions from hidden AGN and comment on the impact of normal galaxies on the measurement of the faint end of the AGN luminosity function. We predict that the eROSITA 4-year survey, will detect $\gtrsim$ 15 000 galaxies (3 $\sigma$ significance) at 50-200 Mpc, which is ~100X more normal galaxies than detected in any X-ray survey to date.