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 $\sim 6.5$ 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 $\sim 0.2R_{\mathrm{e}}$ 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 $\sim 2-2.5R_{\mathrm{e}}$, as traced by the red GCs and PNe (and possibly by the blue GCs beyond $\sim 5R_{\mathrm{e}}$). 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.

22 pages (including 3 pages of Appendix material), 14 figures, published in MNRAS

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