The Fossil Record of Two-phase Galaxy Assembly: Kinematics and Metallicities in the Nearest S0 Galaxy

TL;DR

This study analyzes the kinematics and metallicity gradients of NGC 3115, revealing a two-phase galaxy assembly process involving an early violent phase and subsequent minor mergers, challenging the binary major merger scenario.

Contribution

It provides detailed kinematic and metallicity profiles of NGC 3115, supporting a two-phase assembly model over the binary major merger hypothesis.

Findings

Inner regions are rapidly rotating and metal-rich.

Outer regions show declining rotation and metallicity gradients.

Evidence supports a two-phase galaxy assembly process.

Abstract

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 > 1.5). At larger radii, the rotation declines dramatically to v/sigma ~ 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 +/- 0.04 and -0.38 +/- 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 ~ 15-20:1.