The inner two degrees of the Milky Way. Evidence of a chemical difference between the Galactic Center and the surrounding inner bulge stellar populations

Abstract

Although there have been numerous studies of chemical abundances in the Galactic bulge, the central two degrees have been relatively unexplored due to the heavy and variable interstellar extinction, extreme stellar crowding, and the presence of complex foreground disk stellar populations. In this paper we discuss the metallicity distribution function, vertical and radial gradients and chemical abundances of $\alpha$-elements in the inner two degrees of the Milky Way, as obtained by recent IR spectroscopic surveys. We use a compilation of recent measurements of metallicities and $\alpha$-element abundances derived from medium-high resolution spectroscopy. We compare these metallicities with low-resolution studies. Defining "metal-rich" as stars with $[Fe/H]>0$, and "metal-poor" as stars with $[Fe/H]<0$, we find compelling evidence for a higher fraction ($\sim 80\%$) of metal-rich stars in the Galactic Center (GC) compared to the values (50-60\%) measured in the low latitude fields within the innermost 600 pc. The high fraction of metal-rich stars in the GC region implies a very high mean metallicity of +0.2 dex, while in the inner 600 pc of the bulge the mean metallicity is rather homogenous around the solar value. A vertical metallicity gradient of -0.27 dex/kpc in the inner 600 pc is only measured if the GC is included, otherwise the distribution is about flat and consistent with no vertical gradient. In addition to its high stellar density, the Galactic center/nuclear star cluster is also extreme in hosting high stellar abundances, compared to the surrounding inner bulge stellar populations; this has implications for formation scenarios and strengthens the case for the NSC being a distinct stellar system.

arXiv comment9 pages, 8 figures, accepted for Astronomy&Astrophysics

References in corpus (65)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• The Apache Point Observatory Galactic Evolution Experiment (APOGEE)
• The Galactic Center Massive Black Hole and Nuclear Star Cluster
• The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy's Central Black Hole: A Paradox of Youth
• The Nuclear Bulge of the Galaxy. III. Large-Scale Physical Characteristics of Stars and Interstellar Matter
• The Metal Content of Bulge Field Stars from FLAMES-GIRAFFE Spectra. I. Stellar parameters and Iron Abundances
• Line derived infrared extinction towards the Galactic Center
• On the metallicity of open clusters. III. Homogenised sample
• Enhanced star formation rates in AGN hosts with respect to inactive galaxies from PEP-Herschel observations
• Chemodynamical history of the Galactic Bulge
• Surface Brightness Profile of the Milky Way's Nuclear Star Cluster
• On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk
• Co-Evolution of Galactic Nuclei and Globular Cluster Systems
• A remarkably flat relationship between the average star formation rate and AGN luminosity for distant X-ray AGN
• Chemical Cartography with APOGEE: Large-scale Mean Metallicity Maps of the Milky Way
• The GIRAFFE Inner Bulge Survey (GIBS) III. Metallicity distributions and kinematics of 26 Galactic bulge fields
• Origin and growth of nuclear star clusters around massive black holes
• On the Origin of Nuclear Star Clusters in Late Type Spiral Galaxies
• Chemical Abundances of Luminous Cool Stars in the Galactic Center from High-Resolution Infrared Spectroscopy
• Red giants observed by CoRoT and APOGEE: The evolution of the Milky Way's radial metallicity gradient
• The Evolution of the Globular Cluster System in a Triaxial Galaxy: Can a Galactic Nucleus Form by Globular Cluster Capture?
• Large scale kinematics and dynamical modelling of the Milky Way nuclear star cluster
• The First Stellar Abundance Measurements in the Galactic Center: the M Supergiant IRS 7
• The First Detailed Abundances for M giants in Baade's Window from Infrared Spectroscopy
• The Galactic Centre Chimney
• The Gaia-ESO Survey: Exploring the complex nature and origins of the Galactic bulge populations
• Reddening and metallicity maps of the Milky Way bulge from VVV and 2MASS III. The first global photometric metallicity map of the Galactic bulge
• Chemical Evolution of the Galactic Bulge as Derived from High-Resolution Infrared Spectroscopy of K and M Red Giants
• Stellar Iron Abundances at the Galactic Center
• The First Detailed Abundances for M giants in the inner Bulge from Infrared Spectroscopy
• Mapping the Milky Way bulge at high resolution: the 3D dust extinction, CO, and X factor maps
• The Nuclear Cluster of the Milky Way: Total Mass and Luminosity (long version)
• KMOS view of the Galactic Centre - II. Metallicity distribution of late-type stars
• APOGEE Chemical Abundances of Globular Cluster Giants in the Inner Galaxy
• Discovery of low-metallicity stars in the central parsec of the Milky Way
• GALACTICNUCLEUS: A high angular resolution $JH{K}_s$ imaging survey of the Galactic Centre. I. Methodology, performance, and near-infrared extinction towards the Galactic Centre
• Recurrent gas accretion by massive star clusters, multiple stellar populations and mass thresholds for spherical stellar systems
• The disc origin of the Milky Way bulge: Dissecting the chemo-morphological relations using N-body simulations and APOGEE
• The Terzan 5 puzzle: discovery of a third, metal-poor component
• Detailed abundances for M giants in two inner bulge fields from Infrared Spectroscopy
• Merging of globular clusters within inner galactic regions. II. The Nuclear Star Cluster formation
• Metallicity of Red Giants in the Galactic Bulge from Near-Infrared Spectroscopy
• APOGEE DR14/DR15 Abundances in the Inner Milky Way
• On the rotation of nuclear star clusters formed by cluster-inspirals
• Revised Geometric Estimates of the North Galactic Pole and the Sun's Height Above the Galactic Midplane
• Mapping a stellar disk into a boxy bulge: The outside-in part of the Milky Way bulge formation
• Metallicity gradients through disk Instability: A simple model for the Milky Way's boxy bulge
• The origin of stellar populations in the Galactic bulge from chemical abundances
• Detailed Abundances for the Old Population near the Galactic Center: I. Metallicity distribution of the Nuclear Star Cluster
• Metallicity and kinematics of the bar in-situ
• The Bulge Metallicity Distribution from the APOGEE Survey
• Chemical evolution of the inner 2 degrees of the Milky Way bulge: [alpha/Fe] trends and metallicity gradients
• Star formation history and metallicity in the Galactic inner bulge revealed by the red giant branch bump
• Chemical Abundances of M giants in the Galactic Center: a Single Metal-Rich Population with Low [alpha/Fe]
• Interstellar Extinction and Long-Period Variables in the Galactic Center
• Evidence for a metal-poor population in the inner Galactic Bulge
• Near-Infrared Variability Study of the Central 2.3 arcmin x 2.3 arcmin of the Galactic Centre II. Identification of RR Lyrae Stars in the Milky Way Nuclear Star Cluster
• Evidence against anomalous compositions for giants in the Galactic Nuclear Star Cluster
• Weighing the two stellar components of the Galactic Bulge
• Detailed abundance analysis of a metal-poor giant in the Galactic Center
• Effects of the selection function on metallicity trends in spectroscopic surveys of the Milky Way
• Chemical evolution of the Galactic Center
• Chemical Characterization of the Inner Galactic bulge:North-South Symmetry
• SiO Masers in the Galactic Bulge and Disk: Kinematics from the BAaDE Survey
• Systematic trend of water vapour absorption in red giant atmospheres revealed by high resolution TEXES 12 micron spectra

Cited by in corpus (32)

• Nuclear Star Clusters
• Chemodynamics of barred galaxies in cosmological simulations: On the Milky Way's quiescent merger history and in-situ bulge
• The Milky Way bar and bulge revealed by APOGEE DR16 and Gaia EDR3
• The Milky Way's nuclear star cluster: Old, metal-rich, and cuspy
• How Many Components? Quantifying the Complexity of the Metallicity Distribution in the Milky Way Bulge with APOGEE
• The Milky Way's nuclear stellar disc: A dynamically cool and metal-rich component formed from the Central Molecular Zone?
• GALACTICNUCLEUS: A high-angular-resolution $JH{K}_s$ imaging survey of the Galactic centre. IV. Extinction maps and de-reddened photometry
• GALACTICNUCLEUS: A high-angular-resolution JHKs imaging survey of the Galactic centre III. Evidence for wavelength dependence of the extinction curve in the near-infrared
• Magnesium and silicon in interstellar dust: an X-ray overview
• Blanco DECam Bulge Survey (BDBS) II: Project Performance, Data Analysis, and Early Science Results
• Variability of the near-infrared extinction curve towards the Galactic centre
• The Pristine Inner Galaxy Survey (PIGS) V: a chemo-dynamical investigation of the early assembly of the Milky Way with the most metal-poor stars in the bulge
• Understanding the evolution and dust formation of carbon stars in the LMC with a look at the JWST
• The nuclear star cluster and nuclear stellar disc of the Milky Way: Different stellar population and star formation history
• A KMOS survey of the nuclear disk of the Milky Way I: Survey design and metallicities
• Cool stars in the Galactic Center as seen by APOGEE: M giants, AGB stars and supergiant stars/candidates
• The Blanco DECam Bulge Survey. I. The Survey Description and Early Results
• The extinction law in the inner $3\times 3$ deg${}^2$ of the Milky Way and the red clump absolute magnitude in the inner bar-bulge
• Evidence of an age gradient along the line of sight in the nuclear stellar disc of the Milky Way
• A wind-blown bubble in the Central Molecular Zone cloud G0.253+0.016
• A spectroscopic map of the Galactic centre -- Observations and resolved stars
• The Detection of Deuterated Water in the Large Magellanic Cloud with ALMA
• The S stars' zone of avoidance in the Galactic center
• Double progenitor origin of the S-star cluster
• The multi-zone chemical evolution of the Galactic bulge: predicting abundances for different radial zones
• Cosmic rays, gas and dust in the Central Molecular Zone I -- $X_{CO}$ factors, cosmic-ray densities and dust opacities
• The Near-Infrared Spectrum of the Nuclear Star Cluster: Looking Below the Tip of the Iceberg and Comparisons with Extragalactic Nuclei
• Searching for stars ejected from the Galactic Centre in DESI
• A Near-Infrared Extinction and Reddening Map Towards the Galactic Bulge Using UKIRT
• Photometric properties of nuclear star clusters and their host galaxies in the Fornax cluster
• Blanco DECam Bulge Survey (BDBS) IV: Metallicity Distributions and Bulge Structure from 2.6 Million Red Clump Stars
• Self-consistent modelling of the Milky Way's Nuclear Stellar Disc