2025A&A...696A.138K

Context. The MeerKAT Fornax Survey (MFS) is a large survey project mapping the H I in the Fornax cluster. Most of the cluster members detected in H I show significant signs of interaction with the intracluster medium or other galaxies. The galaxy ESO 358-60, however, stands out as its large H I disk appears regular and undisturbed. Combined with the fact that the galaxy's systemic velocity is at the edge of the velocity distribution of Fornax, a possible explanation for this undisturbed disk is that the galaxy is not in Fornax.
Aims. Our goal is to understand the detailed morphology and kinematics of the H I disk of ESO 358-60 and, by doing so, establish whether the galaxy is a member of Fornax.
Methods. We analyzed the H I distribution within and around ESO 358-60 based on the MFS observations in a 2 deg² field around the galaxy. We visually inspected the low resolution data in order to study the H I disk from the center to its outskirts and look for low column density gas that could reveal recent interactions. We then constructed a detailed parameterization of the H I disk by fitting a tilted ring model to the high resolution data cube. Using a bootstrap method, we established accurate errors on our best-fit models. We used the fitted rotational velocity to place the galaxy on the baryonic Tully-Fisher relation. By equating the galaxy's H I and 3.6 μm fluxes to the thus retrieved baryonic mass, we obtained a redshift-independent distance.
Results. We confirm that the immediate surroundings of ESO 358-60 are quiescent relative to other MFS detections and find no obvious companion interacting with the galaxy. Our modeling confirms the regularity of the H I disk in ESO 358-60 but also shows that the galaxy's H I distribution contains a significant line-of-sight warp and that radial motions, on the order of 10 km s^–1, cover the extent of the optical disk. From the modeling we obtain a velocity V_flat = 48.1 ± 1.4 km s^–1 for the best-fit rotation curve. This leads to a distance from the baryonic Tully-Fisher relation of 9.4 ± 2.5 Mpc which is ∼10 Mpc less than the distance to the Fornax cluster. This distance fits better not only with V_flat but also with the overall rotation curves and H I content of low mass galaxies and the fact that the galaxy appears undisturbed and reasonably symmetric. It is also consistent with the distance calculated in the Cosmicflows project. At 9.4 Mpc, ESO 358-60 cannot be a member of the Fornax cluster; it is instead a foreground field galaxy.