paper

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

arXiv:2004.09818 · doi:10.1093/mnras/staa1080

Abstract

Using new and archival data, we study the kinematic properties of the nearest field S0 galaxy, NGC 3115, out to half-light radii () from its stars (integrated starlight), globular clusters (GCs) and planetary nebulae (PNe). We find evidence of three kinematic regions with an inner transition at from a dispersion-dominated bulge () to a fast-rotating disk (), and then an additional transition from the disk to a slowly rotating spheroid at , as traced by the red GCs and PNe (and possibly by the blue GCs beyond ). 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.

arXiv comment22 pages (including 3 pages of Appendix material), 14 figures, published in MNRAS

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