The eROSITA Final Equatorial Depth Survey (eFEDS)

X-ray emission around star-forming and quiescent galaxies at 0.05 < z < 0.3

¹ Max-Planck-Institut für extraterrestrische Physik (MPE), Gießenbachstraße 1, 85748 Garching bei München, Germany
e-mail: comparat@mpe.mpg.de
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
⁴ International Centre for Radio Astronomy Research (ICRAR), University of Western Australia, Perth, Western Australia, Australia
⁵ International Space Centre (ISC), University of Western Australia, Perth, Western Australia, Australia
⁶ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁷ Institute for Astronomy, Royal Observatory, University of Edinburgh, Edinburgh EH9 3HJ, UK
⁸ School of Physics & Astronomy, University of Leicester, University Road, Leicester LE1 7RJ, UK
⁹ Institute for Astronomy and Astrophysics, National Observatory of Athens, V. Paulou & I. Metaxa 11532, Greece
¹⁰ European Southern Observatory, 85748 Garching bei München, Germany

Received: 12 January 2022
Accepted: 6 July 2022

Abstract

Aims. The circumgalactic medium (CGM) plays an important role in galaxy evolution as the main interface between the star-forming body of galaxies and the surrounding cosmic network of in- and out-flowing matter. In this work, we aim to characterize the hot phase of the CGM in a large sample of galaxies using recent soft-X-ray observations made by SRG/eROSITA.

Methods. We stack X-ray events from the ‘eROSITA Final Equatorial Depth Survey’ (eFEDS) around central galaxies in the 9hr field of the ‘GAlaxy and Mass Assembly’ (GAMA) survey to construct radially projected X-ray luminosity profiles in the 0.5–2 keV rest frame energy band as a function of their stellar mass and specific star formation rate. We consider samples of quiescent (star-forming) galaxies in the stellar mass range 2 × 10¹⁰–10¹² M_⊙ (3 × 10⁹–6 × 10¹¹ M_⊙).

Results. For quiescent galaxies, the X-ray profiles are clearly extended throughout the available mass range; however, the measured profile is likely biased high because of projection effects, as these galaxies tend to live in dense and hot environments. For the most massive star-forming samples (≥10¹¹ M_⊙), there is a hint of detection of extended emission. On the other hand, for star-forming galaxies with < 10¹¹ M_⊙ the X-ray stacked profiles are compatible with unresolved sources and are consistent with the expected emission from faint active galactic nuclei (AGN) and X-ray binaries. We measure for the first time the mean relation between average X-ray luminosity and stellar mass separately for quiescent and star-forming galaxies. We find that the relation is different for the two galaxy populations: high-mass (≥10¹¹ M_⊙) star-forming or quiescent galaxies follow the expected scaling of virialized hot haloes, while lower mass star-forming galaxies show a less prominent luminosity and a weaker dependence on stellar mass consistent with empirical models of the population of weak AGN. When comparing our results with state-of-the-art numerical simulations (IllustrisTNG and EAGLE), we find overall consistency on the average emission on large (> 80 kpc) scales at masses ≥10¹¹ M_⊙, but disagreement on the small scales, where brighter-than-observed compact cores are predicted. The simulations also do not predict the clear differentiation that we observe between quiescent and star-forming galaxies in our samples.

Conclusions. This is a stepping stone towards a more profound understanding of the hot phase of the CGM, which holds a key role in the regulation of star formation. Future analysis using eROSITA all-sky survey data, combined with future generation galaxy evolution surveys, shall provide much enhanced quantitative measurements and mapping of the CGM and its hot phase(s).