The SRG/eROSITA all-sky survey: View of the Fornax galaxy cluster

Abstract

The Fornax cluster is one of the closest X-ray-bright galaxy clusters. Previous observations of the intracluster medium were limited to less than R500. We aim to significantly extend the X-ray coverage. We used data from 5 SRG/eROSITA all-sky surveys and performed a detailed 1- and 2-dimensional X-ray surface brightness analysis, tracing hot gas emission from kpc to Mpc scales with a single instrument. We compared the results to those from a recent numerical simulation of the local Universe (SLOW) and correlated the X-ray emission distribution with that of other tracers, including cluster member galaxies, ultra-compact dwarf galaxies, intracluster globular clusters, and HI-tail galaxies. We detect X-ray emission beyond the virial radius, R100=2.2 deg. In the inner regions within R500, we see previously known features, such as a large-scale spiral-shaped edge; however, we do not find obvious evidence of the bow shock several hundred kpc south of the cluster center predicted by previous numerical simulations of the Fornax cluster. Instead, we discover emission fingers beyond R500 to the west and southeast and excesses that stretch out far beyond the virial radius. They might be due to gas being pushed outward by the previous merger with NGC 1404 or due to warm-hot gas infall along large-scale filaments. Intriguingly, we find the distributions of the other tracers - galaxies and globular clusters - to be correlated with the X-ray-excess regions, favoring the infall scenario. Interestingly, we also discover an apparent bridge of low-surface-brightness emission beyond the virial radius connecting to the Fornax A galaxy group, which is also traced by the member galaxy and globular cluster distribution. The gas distribution in the SLOW simulation shows similar features as those we have discovered with eROSITA. With eROSITA, we witness the growth of a cluster along large-scale filaments.

12 pages plus 4 pages appendix, accepted by Astronomy & Astrophysics on 2025-08-08; updated figures and extended text, conclusions unchanged

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