The assembly history of the nearest S0 galaxy NGC 3115 from its kinematics out to six half-light radii

Abstract

Using new and archival data, we study the kinematic properties of the nearest field S0 galaxy, NGC 3115, out to \(\sim6.5\) half-light radii (\(R_\mathrm{e}\)) from its stars (integrated starlight), globular clusters (GCs) and planetary nebulae (PNe). We find evidence of three kinematic regions with an inner transition at \(\sim0.2\ R_\mathrm{e}\) from a dispersion-dominated bulge (\(V_\mathrm{rot}/\sigma <1\)) to a fast-rotating disk (\(V_\mathrm{rot}/\sigma >1\)), and then an additional transition from the disk to a slowly rotating spheroid at \(\sim2-2.5\, R_\mathrm{e}\), as traced by the red GCs and PNe (and possibly by the blue GCs beyond \(\sim5\, R_\mathrm{e}\)). From comparison with simulations, we propose an assembly history in which the original progenitor spiral galaxy undergoes a gas-rich minor merger that results in the embedded kinematically cold disk that we see today in NGC 3115. At a later stage, dwarf galaxies, in mini mergers (mass-ratio \(<\) 1:10), were accreted building-up the outer slowly rotating spheroid, with the central disk kinematics largely unaltered. Additionally, we report new spectroscopic observations of a sample of ultra-compact dwarfs (UCDs) around NGC 3115 with the Keck/KCWI instrument. We find that five UCDs are inconsistent with the general rotation field of the GCs, suggesting an \textit{ex-situ} origin for these objects, i.e. perhaps the remnants of tidally stripped dwarfs. A further seven UCDs follow the GC rotation pattern, suggesting an \textit{in-situ} origin and, possibly a GC-like nature.