The Metal-Poor Metallicity Distribution of the Ancient Milky Way

Abstract

We present a low metallicity map of the Milky Way consisting of $\sim$111,000 giants with $-3.5\lesssim$ [Fe/H] $\lesssim -$0.75, based on public photometry from the second data release of the SkyMapper survey. These stars extend out to $\sim$7kpc from the solar neighborhood and cover the main Galactic stellar populations, including the thick disk and the inner halo. Notably, this map can reliably differentiate metallicities down to [Fe/H] $\sim -3.0$, and thus provides an unprecedented view into the ancient, metal-poor Milky Way. Among the more metal-rich stars in our sample ([Fe/H] $>-2.0$), we recover a clear spatial dependence of decreasing mean metallicity as a function of scale height that maps onto the thick disk component of the Milky Way. When only considering the very metal-poor stars in our sample ([Fe/H] $<-$2), we recover no such spatial dependence in their mean metallicity out to a scale height of $|Z|\sim 7$ kpc. We find that the metallicity distribution function (MDF) of the most metal-poor stars in our sample ($-3.0<$ [Fe/H] $<-2.3$) is well fit with an exponential profile with a slope of $\Delta \log (N)/\Delta$[Fe/H] = 1.52$\pm$0.05, and shifts to $\Delta \log (N)/\Delta$[Fe/H] = 1.53$\pm$0.10 after accounting for target selection effects. For [Fe/H] $<-2.3$, the MDF is largely insensitive to scale height $|Z|$ out to $\sim 5$kpc, showing that very and extremely metal-poor stars are in every galactic component.

arXiv comment9 pages, 5 figures; accepted for publication in ApJL. Minor corrections after acceptance addressing referee report for Chiti et al. ApJS submitted

References in corpus (46)

• Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds
• Astropy: A Community Python Package for Astronomy
• emcee: The MCMC Hammer
• The Gaia mission
• SDSS-III: Massive Spectroscopic Surveys of the Distant Universe, the Milky Way Galaxy, and Extra-Solar Planetary Systems
• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• The Apache Point Observatory Galactic Evolution Experiment (APOGEE)
• SEGUE: A Spectroscopic Survey of 240,000 stars with g=14-20
• Tracing Galaxy Formation with Stellar Halos I: Methods
• Co-formation of the Galactic disc and the stellar halo
• Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole
• The GALAH Survey: Scientific Motivation
• Optimal Techniques in Two-dimensional Spectroscopy: Background Subtraction for the 21st Century
• Two Stellar Components in the Halo of the Milky Way
• The Milky Way Tomography with SDSS: II. Stellar Metallicity
• SkyMapper and the Southern Sky Survey
• Near-Field Cosmology with Metal-Poor Stars
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• The Hamburg/ESO R-process Enhanced Star survey (HERES) II. Spectroscopic analysis of the survey sample
• SkyMapper Southern Survey: First Data Release (DR1)
• The Most Metal-Poor Stars. II. Chemical Abundances of 190 Metal-Poor Stars Including 10 New Stars With [Fe/H] < -3.5
• Carbon-Enhanced Metal-Poor Star Frequencies in the Galaxy: Corrections for the Effect of Evolutionary Status on Carbon Abundances
• The Pristine survey I: Mining the Galaxy for the most metal-poor stars
• SkyMapper Southern Survey: Second Data Release (DR2)
• Bright Metal-Poor Stars from the Hamburg/ESO Survey. I. Selection and Follow-up Observations from 329 Fields
• The Sixth Data Release of the Radial Velocity Experiment (RAVE) -- II: Stellar Atmospheric Parameters, Chemical Abundances and Distances
• Dynamical Relics of the Ancient Galactic Halo
• The stellar content of the Hamburg/ESO survey. V. The metallicity distribution function of the Galactic halo
• High-Resolution Spectroscopic Study of Extremely Metal-Poor Star Candidates from the SkyMapper Survey
• Deriving Stellar Effective Temperatures of Metal-Poor Stars with the Excitation Potential Method
• Extremely metal-poor stars from the cosmic dawn in the bulge of the Milky Way
• SkyMapper Filter Set: Design and Fabrication of Large Scale Optical Filters
• The R-process Alliance: First Magellan/MIKE Release from the Southern Search for R-Process-enhanced Stars
• Deep SDSS optical spectroscopy of distant halo stars I. Atmospheric parameters and stellar metallicity distribution
• The Pristine Survey IV: Approaching the Galactic metallicity floor with the discovery of an ultra metal-poor star
• The Pristine survey VIII: The metallicity distribution function of the Milky Way halo down to the extremely metal-poor regime
• The SkyMapper DR1.1 Search for Extremely Metal-Poor Stars
• SkyMapper stellar parameters for Galactic Archaeology on a grand-scale
• The Pristine Inner Galaxy Survey (PIGS) II: Uncovering the most metal-poor populations in the inner Milky Way
• The Fractions of Inner- and Outer-Halo Stars in the Local Volume
• The GALAH survey: properties of the Galactic disk(s) in the solar neighbourhood
• A Blueprint for the Milky Way's Stellar Populations: The Power of Large Photometric and Astrometric Surveys
• Stellar metallicities from SkyMapper photometry I: A study of the Tucana II ultra-faint dwarf galaxy
• Metal-poor Stars Observed with the Automated Planet Finder Telescope. II. Chemodynamical Analysis of Six Low-Metallicity Stars in the Halo System of the Milky Way
• Milky Way tomography with the SkyMapper Southern Survey: I: Atmospheric parameters and distances of one million red giants
• Keck HIRES Spectroscopy of SkyMapper Commissioning Survey Candidate Extremely Metal-Poor Stars

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• GRUMPY: a simple framework for realistic forward-modelling of dwarf galaxies
• Stellar Metallicities from SkyMapper Photometry II: Precise photometric metallicities of $\sim$280,000 giant stars with [Fe/H] $<-0.75$ in the Milky Way
• High resolution spectroscopic follow-up of the most metal-poor candidates from SkyMapper DR1.1
• A-SLOTH reveals the nature of the first stars
• Mining S-PLUS for Metal-Poor Stars in the Milky Way
• First estimate of the local value of the baryonic streaming velocity
• Where are the extremely metal-poor stars in the Milky Way and Andromeda? Expectations from TNG50
• A machine learning approach to photometric metallicities of giant stars
• A second-generation star in a relic dwarf galaxy
• Capivara: A Spectral-based Segmentation Method for IFU Data Cubes
• Magellan/IMACS spectroscopy of Grus I: A low metallicity ultra-faint dwarf galaxy
• Infrared photometry and CaT spectroscopy of the most metal-poor in-situ globular cluster VVV-CL001
• Detailed Abundance Determination of Metal-Poor Stars with X-Shooter I. Unusual Chemistry in Halo Stars