Metallicity gradient in the mILKY WAY

Metallicity gradient in the mILKY WAY

mAE GUILLOT

Astronomy 1302 – Stars and Galaxies, Louisiana State University

Supervisor - Natalie Hinkel

Baton Rouge, May 31st 2026

background

Background

Metallicity

What is it ? And how is it mesured ?

• “Metals”: elements heavier than H and He.

• Metallicity, [X/H] (chemical abundance ratio), for any metal X, measures the abundance of metals in objects

• [Fe/H] compares a star's iron-to-hydrogen ratio to the Sun's. Unit: dex

background

Background

Metallicity

What is it ? And how is it mesured ?

• Metallicity gradient : how metal abundance changes as you move outward from the galactic center

• Milky Way's : roughly −0.06 dex/kpc

• For every kiloparsec outward, metal abundance drops by 10⁻⁰·⁰⁶.

• The center is richer; the outer disk is poorer

Zeljko Ivezic, University of Washington, SDSS-II Collaboration

background

Background

Metallicity

What is it ? And how is it mesured ?

• Stars are born from clouds of gas. If that gas was already enriched by previous generations of stars, new stars will be metal-rich. If the gas is pristine, new stars will be metal-poor

• Metallicity tells you how chemically evolved the gas was when a star formed

Image: NASA, ESA, CSA, STScI, Webb ERO Production Team

Background

Milky way's structure

• Galactic disk: the thin disk (young, metal-rich stars) and the thick disk (older, more metal-poor stars)

• Galactic bulge: the oldest and most metal-rich component of the Galaxy, marking the high-metallicity end of the radial gradient

• Halo : ancient, metal-poor

Metallicity gradient in the Milky Way → result of the Galaxy forming from the inside out. The center formed first and enriched itself early, while the outer regions formed later from gas that hadn't yet been processed by stars.

Observing the Metallicity gradient

How do we mesure metallicity across 100,000 light years?

NASA/ESA/AURA/Caltech

ESO/M. Montargès et al.

National Aeronautics and Space Administration

Cepheid Variable Stars(& RR Lyrae)

Open Clusters

Planetary Nebulae

Tracers are used : objects whose properties are well understood and that can be studied across large distancesUsing spectroscopy mostly

Observing the Metallicity gradient

How do we mesure metallicity across 100,000 light years?

ESO/M. Montargès et al.

• Bright, young, and act as precise standard candles, allowing us to accurately map chemical composition alongside distance

• Known period-luminosity → exact distance calculations

Cepheid Variable Stars

Observing the Metallicity gradient

How do we mesure metallicity across 100,000 light years?

NASA/ESA/AURA/Caltech

• Young-to-intermediate age stellar populations located primarily in the thin disk

• Clusters form from the same molecular cloud → identical ages and chemical compositions, more precise metallicity measurements

Open Clusters

Observing the Metallicity gradient

How do we mesure metallicity across 100,000 light years?

National Aeronautics and Space Administration

• Bright, widely distributed across the disk and age

• Their emission lines provide accurate measurements of chemical abundances (especially O) in the gas, reflecting the composition of the ISM at the time their progenitors formed : they are acting as "fossil record"

Planetary Nebulae

Observations

Astronomical Society of Australia 2016

This figure is showing how different parts of the Milky Way formed at different times:Inner + high regions → dominated by old stars Disk (especially inner disk) → lots of younger stars The galaxy built up chemically over time, not all at once

Inside out formation

Physical Origin

-13 Gyr

-6 GYR/NOW

-10/8 GYR

Intermediate region

outer disk

Close to center

Moderate star formation at intermediate radii. Enrichment of the ISM

Gas arrives from pristine intergalactic reserver. Star formation is slower, gas less processed

Gas collapses first at the center. Rapid star formation

Metallicity

high

low

moderate

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CONCLUSION

The metallicity gradient is a direct fingerprint of inside-out formation. The Galaxy built itself from the center outward, and that chemical record is still visible today.

~−0.06 dex/kpc

multiple tracers confirm this consistent negative disk gradient

bulge

metal-rich, marks the Galaxy's earliest, most chemically enriched region.

inside out

formation explains the shape of the gradient and the bulge's dominant metallicity.

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THANKS!

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