We present a global analysis of kinematics and metallicity in the nearest S0 galaxy , NGC 3115 , along with implications for its assembly history .
The data include high-quality wide-field imaging from Suprime-Cam on the Subaru telescope , and multi-slit spectra of the field stars and globular clusters ( GCs ) obtained using Keck-DEIMOS/LRIS and Magellan-IMACS .
Within two effective radii , the bulge ( as traced by the stars and metal-rich GCs ) is flattened and rotates rapidly ( v / \sigma \gtrsim 1.5 ) .
At larger radii , the rotation declines dramatically to v / \sigma \sim 0.7 , but remains well-aligned with the inner regions .
The radial decrease in characteristic metallicity of both the metal-rich and metal-poor GC subpopulations produces strong gradients with power law slopes of -0.17 \pm 0.04 and -0.38 \pm 0.06 dex per dex , respectively .
We argue that this pattern is not naturally explained by a binary major merger , but instead by a two-phase assembly process where the inner regions have formed in an early violent , dissipative phase , followed by the protracted growth of the outer parts via minor mergers with typical mass ratios of \sim 15–20:1 .