Chemical analysis of the Milky Way's Nuclear Star Cluster: Evidence for a metallicity gradient

Abstract

The Milky Way nuclear star cluster (MWNSC) is located together with its surrounding nuclear stellar disc (MWNSD) in the Galactic centre and they dominate the gravitational potential within the inner 300\,pc. However, the formation and evolution of both systems and their possible connections are still under debate. We reanalyse the low-resolution KMOS spectra in the MWNSC with the aim to improve the stellar parameters (${\mathrm{T}}_{\mathrm{eff}}$, $\log \mathrm{g}$, and $[M/H])$ for the MWNSC. We use an improved line-list, especially dedicated for cool M giants allowing to improve the stellar parameters and to obtain in addition global $\alpha$-elements. A comparison with high-resolution IR spectra (IGRINS) gives very satisfactory results pinning down the uncertainties to ${\mathrm{T}}_{\mathrm{eff}}\simeq 150\mathrm{K}$, $\log \mathrm{g}\simeq 0.4\mathrm{dex}$, and $[M/H]\simeq 0.2\mathrm{dex}$. Our $\alpha$-elements agree within 0.1\,dex compared to the IGRINS spectra. We obtain a high-quality sample of 1140 M giant stars where we see an important contribution of a metal-poor population ($\sim 20\%$) centered at $[M/H]\simeq -0.7\mathrm{dex}$ while the most dominant part comes from the metal-rich population with $[M/H]\simeq 0.26\mathrm{dex}$. We construct a metallicity map and find a metallicity gradient of $\sim -0.1\pm 0.02dex/pc$ favouring the inside-out formation scenario for the MWNSC.

Accepted for publication in Astronomy&Astrophysics

References in corpus (23)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• The old nuclear star cluster in the Milky Way: dynamics, mass, statistical parallax, and black hole mass
• Line lists for the A2PI-X2Sigma+ (red) and {B2Sigma+-X2Sigma} (violet) Systems of CN, 13C14N, and 12C15N, and Application to Astronomical Spectra
• Clues to Nuclear Star Cluster Formation from Edge-on Spirals
• Large scale kinematics and dynamical modelling of the Milky Way nuclear star cluster
• The GALAH Survey: Non-LTE departure coefficients for large spectroscopic surveys
• The Milky Way's nuclear star cluster: Old, metal-rich, and cuspy
• The nuclear cluster of the Milky Way: Our primary testbed for the interaction of a dense star cluster with a massive black hole
• KMOS view of the Galactic Centre - II. Metallicity distribution of late-type stars
• The solar silicon abundance based on 3D non-LTE calculations
• The Structure of Nuclear Star Clusters in Nearby Late-type Spiral Galaxies from Hubble Space Telescope Wide Field Camera 3 Imaging
• New constraints on the structure of the nuclear stellar cluster of the Milky Way from star counts and MIR imaging
• Dissipative transformation of non-nucleated dwarf galaxies into nucleated systems
• Formation and evolution of nuclear star clusters with in-situ star-formation: Nuclear cores and age segregation
• 3D non-LTE iron abundances in FG-type dwarfs
• On the origin of a rotating metal-poor stellar population in the Milky Way Nuclear Cluster
• Revealing the Formation of the Milky Way Nuclear Star Cluster via Chemo-Dynamical Modeling
• Formation of central massive objects via tidal compression
• Ca line formation in late-type stellar atmospheres: I. The model atom
• The nuclear star cluster and nuclear stellar disc of the Milky Way: Different stellar population and star formation history
• Kinematic data rebuild the Nuclear star cluster as the most metal rich region of the Galaxy
• Effects of Magnetic Fields on Gas Dynamics and Star Formation in Nuclear Rings
• A Wide Metallicity Range for Gyr-old Stars in the Nuclear Star Cluster