Kinematics of the lens host S0 galaxy NGC 1553: role of secular processes

Abstract: We present an investigation of the central structure of the S0 galaxy NGC 1553, to understand its origin and the underlying dynamical processes that shape it. The high-resolution integral field spectroscopic data from the Multi Unit Spectroscopic Explorer (MUSE) reveal a well-ordered rotation pattern, consisting of a rapidly rotating nuclear disc that is somewhat decoupled from the main disc, together with an inner lens; we collectively refer to these structures as the "disc-lens". The central peak in the velocity dispersion indicates the presence of a classical bulge. The nuclear disc is dynamically colder than the surrounding disc, while the lens is dynamically hotter. The higher-order Gauss-Hermite moments, $h_{3}$ and $h_{4}$, further characterise the stellar kinematics. An anti-correlation between the line-of-sight velocity and skewness ($h_{3}$) is consistent with regular rotation. In contrast, the ring-like enhancement in kurtosis ($h_{4}$) confirms the presence of the nuclear disc component. Unsharp masking of HST images (Erwin et al. 2015) reveals a nuclear bar and faint spiral structures within the central 10 arcsec, supporting the role of secular evolution. The mass-weighted stellar age map shows an old stellar population in the central regions, with high metallicity that suggests the in-situ formation of the disc-lens from disc material. We discuss possible formation scenarios for the disc-lens, including both minor mergers and secular processes, and examine the influence of the Dorado group environment on NGC 1553. Our findings suggest that the disc-lens in NGC 1553 formed during the early stages of the galaxy's evolution. However, its subsequent development has been shaped by internal and external processes. These results provide new insights into the origin and evolution of kinematically distinct substructures in S0 galaxies.