An off-centred bulge or a satellite? Hydrodynamical $N$-body simulations of the disc galaxy NGC 5474

Abstract: We present dynamical models of the star-forming galaxy NGC 5474 based on $N$-body hydrodynamical numerical simulations. We investigate the possible origin of the compact round stellar structure, generally interpreted as the bulge of the galaxy, but unusually off-set by $\simeq1$ kpc in projection from the visual and the kinematic centres of both the star and the gas discs. We argue that it is very unlikely that the putative bulge is in a co-planar orbit in the disc plane, showing that such a configuration would be hardly compatible with its smooth and regular spatial distribution, and, in case its mass is above $10^8\,M_{\odot}$, also with the regular HI velocity field of NGC 5474. Instead, if the putative bulge is in fact an early-type satellite galaxy orbiting around NGC 5474, not only the off-set can be easily produced by projection effects, but our simulations suggest that the gravitational interaction between the two systems can explain also the warped HI distribution of NGC 5474 and the formation of its loose spiral arms. As a by-product of the simulations, we find that the peculiar over-density of old stars detected in the south-west region of NGC 5474 may be explained with the interaction between NGC 5474 and a smaller stellar system, unrelated to the putative bulge, accreted in the disc plane.