What sets the radial structure of the Milky Way disk? Neige Frankel - - PowerPoint PPT Presentation

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What sets the radial structure of the Milky Way disk? Neige Frankel - - PowerPoint PPT Presentation

Sep 29, 2022 100 likes •454 views

What sets the radial structure of the Milky Way disk? Neige Frankel (Heidelberg) Hans-Walter Rix (Heidelberg) Yuan-Sen Ting (IAS/Princeton/Carnegie) Relevant contributions to the field: Bensby, Binney, Bovy, Chiappini, Feltzing, Girardi,

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SLIDE 1

What sets the radial structure

  • f the Milky Way disk?

The Dynamical Universe For All, Lund Observatory, 6 February 2018

Neige Frankel (Heidelberg) Hans-Walter Rix (Heidelberg) Yuan-Sen Ting (IAS/Princeton/Carnegie)

Relevant contributions to the field: Bensby, Binney, Bovy, Chiappini, Feltzing, Girardi, Hayden, Haywood, Kubryck, McMillan, Minchev, Roskar, Sanders, Sellwood, Schönrich

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SLIDE 2

Simplest picture

  • 1. Low angular

momentum gas settles Galactic radius Fe

Inside-out disk growth:

Low gas settles first

Stars form:

Orbit determined by of the gas Lz Lz

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SLIDE 3

Simplest picture

Fe Z Galactic radius

  • 1. Low angular

momentum gas settles

Inside-out disk growth:

Low gas settles first

Stars form:

Orbit determined by of the gas

In parallel:

Stars evolve → Chemical enrichment of the gas Lz Lz

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SLIDE 4

Simplest picture

  • 2. Higher

angular momentum gas Fe Z Galactic radius

  • 1. Low angular

momentum gas settles

Inside-out disk growth:

Low gas settles first Higher gas settles later

Stars form:

Orbit determined by of the gas

In parallel:

Stars evolve → Chemical enrichment of the gas LzLz Lz

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SLIDE 5

Simplest picture

Fe Z Z+ Galactic radius

  • 2. Higher

angular momentum gas

  • 1. Low angular

momentum gas settles

Inside-out disk growth:

Low gas settles first Higher gas settles later

Stars form:

Orbit determined by of the gas

In parallel:

Stars evolve → Chemical enrichment of the gas LzLz Lz

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SLIDE 6

Simplest picture

Fe Z Z+ Galactic radius

  • 2. Higher

angular momentum gas

  • 1. Low angular

momentum gas settles

Inside-out disk growth:

Low gas settles first Higher gas settles later

Stars form:

Orbit determined by of the gas

In parallel:

Stars evolve → Chemical enrichment of the gas

→ Metallicity gradient in the disk → At given radius: clear age – metallicity relation

LzLz Lz

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SLIDE 7

Simplest picture

Fe Z Z+ Sanders & Binney (2015) Galactic radius time

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SLIDE 8

Simplest picture

Fe Z Z+ Solar neighborhood Sanders & Binney (2015) Galactic radius

Expect a tight [Fe/H]–τ relation

time

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SLIDE 9

Simplest picture

Fe Z Z+ Solar neighborhood Galactic radius Edvardsson+ (1993)

Observe large [Fe/H]–τ scatter

Sanders & Binney (2015)

Expect a tight [Fe/H]–τ relation

time

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SLIDE 10

What is the next simplest picture?

  • As before: A tight relation
  • But: the angular momentum (or radius) evolves

in time [Fe/H ]−τ−Rb

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SLIDE 11

What is the next simplest picture?

  • As before: A tight relation
  • But: the angular momentum (or radius) evolves

in time

Stars migrate

[Fe/H ]−τ−Rb

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SLIDE 12

Migration mechanism:

  • rbit scatter near co-rotation

Selwood & Binney 2002: spiral arms

co-rotation

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SLIDE 13

Migration mechanism:

  • rbit scatter near co-rotation

Selwood & Binney 2002: spiral arms

co-rotation

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SLIDE 14

Migration mechanism:

  • rbit scatter near co-rotation

Selwood & Binney 2002: spiral arms

Radial migration ~ angular momentum diffusion

co-rotation

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SLIDE 15

Implications of radial migration

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SLIDE 16

Implications of radial migration

  • : tight age – metallicity

t migr≫t Hubble

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SLIDE 17

Implications of radial migration

  • : Orbits are scrambled

– Formation/dynamics memory loss – Leads towards exponential disk profiles (Herpich+ 2017) – Radial gradients smoothed out

t migr≪t Hubble

  • : tight age – metallicity

t migr≫t Hubble

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SLIDE 18

Implications of radial migration

  • : Orbits are scrambled

– Formation/dynamics memory loss – Leads towards exponential disk profiles (Herpich+ 2017) – Radial gradients smoothed out

  • : ?

– How much memory does the Galactic disk keep? – Impact on the dynamics?

t migr∼t Hubble t migr≪t Hubble

  • : tight age – metallicity

t migr≫t Hubble

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SLIDE 19

Implications of radial migration

  • : Orbits are scrambled

– Formation/dynamics memory loss – Leads towards exponential disk profiles (Herpich+ 2017) – Radial gradients smoothed out

To test this, we need to quantify the strength of radial migration, globally

  • : ?

– How much memory does the Galactic disk keep? – Impact on the dynamics?

t migr∼t Hubble t migr≪t Hubble

  • : tight age – metallicity

t migr≫t Hubble

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SLIDE 20

A global measure: APOGEE data

  • APOGEE: NIR → sees through dust: MW disk,

spectrosopic survey → metallicity

  • 20,000 Red clump giants → distances (Bovy+ 2014)
  • Ness+ (2016) → Ages calibrated to asteroseismology
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SLIDE 21

A global measure: APOGEE data

  • APOGEE: NIR → sees through dust: MW disk,

spectrosopic survey → metallicity

  • 20,000 Red clump giants → distances (Bovy+ 2014)
  • Ness+ (2016) → Ages calibrated to asteroseismology

adapted from Ness et al. (2016)

# stars # stars Ness+ (2016)

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SLIDE 22

A global measure: APOGEE data

adapted from Ness et al. (2016)

# stars # stars # stars Genovali+ (2014) Ness+ (2016)

  • APOGEE: NIR → sees through dust: MW disk,

spectrosopic survey → metallicity

  • 20,000 Red clump giants → distances (Bovy+ 2014)
  • Ness+ (2016) → Ages calibrated to asteroseismology
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SLIDE 23

A global measure: a simple model

p([Fe/H], τ∣R ,⃗ pm)

Sanders & Binney (2015)

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SLIDE 24

A global measure: a simple model

Sanders & Binney (2015)

p([Fe/H], τ∣R ,⃗ pm)

p(R∣Rb, τ)

  • Radial migration

σ=σ0√ τ 12Gyr

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SLIDE 25

A global measure: a simple model

Sanders & Binney (2015)

p([Fe/H], τ∣R ,⃗ pm)

p(R∣Rb, τ) Rb=f ([Fe/ H], τ)

  • Radial migration
  • Birth radius
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SLIDE 26

A global measure: a simple model

Sanders & Binney (2015) Bovy+ (2014)

p([Fe/H], τ∣R ,⃗ pm)

p(R∣Rb, τ) p(τ)

  • Radial migration
  • Birth radius
  • Age distribution

Rb=f ([Fe/ H], τ)

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SLIDE 27

A global measure: a simple model

Sanders & Binney (2015)

p([Fe/H], τ∣R ,⃗ pm)

p(R∣Rb, τ) p(τ) p(Rb∣τ)

  • Radial migration
  • Birth radius
  • Age distribution
  • Birth distribution

Rb=f ([Fe/ H], τ)

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SLIDE 28

A global measure: a simple model

Sanders & Binney (2015)

p(R∣Rb, τ) p(τ) p(Rb∣τ)

Diffusion coefficient → radial migration strength

p(⃗ pm∣[Fe/H ], τ, R) p([Fe/H], τ∣R ,⃗ pm)

  • Radial migration
  • Birth radius
  • Age distribution
  • Birth distribution

Sampling (MCMC)

Rb=f ([Fe/ H], τ)

emcee (Foreman-Mackey+2013)

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SLIDE 29

Radial migration efficiency

Star formation efficiency Inside-out formation Radius where ISM metallicity is solar Enrichment timescale ISM metallicity gradient at solar radius Radial migration efficiency

(at face value...)

Work in progress (Frankel+ in prep)

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SLIDE 30

Radial migration efficiency

Over a Hubble time, stars have on average migrated

  • f about 4-5 kpc.

→ on a scale larger than

the half-mass radius of the Milky Way!

Radial migration is very important, but not asymptotically strong to erase all gradients.

(at face value...)

Radial migration efficiency ISM metallicity gradient at solar radius Work in progress (Frankel+ in prep)

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SLIDE 31

What sets the radial structure

  • f the Milky Way disk?

To first order...

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SLIDE 32

What sets the radial structure

  • f the Milky Way disk?
  • Orbits of stars in the Milky Way depend upon:
  • The initial angular momentum of the gas
  • The subsequent radial migration of stars

To first order...

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SLIDE 33

What sets the radial structure

  • f the Milky Way disk?
  • Orbits of stars in the Milky Way depend upon:
  • The initial angular momentum of the gas
  • The subsequent radial migration of stars
  • For the first time, we can measure a global,

averaged, radial migration efficiency in the Milky Way disk

To first order...

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SLIDE 34

What sets the radial structure

  • f the Milky Way disk?
  • Orbits of stars in the Milky Way depend upon:
  • The initial angular momentum of the gas
  • The subsequent radial migration of stars
  • For the first time, we can measure a global,

averaged, radial migration efficiency in the Milky Way disk

  • Using global data

and a formation and diffusion model:

{R, τ,[Fe/H]}APOGEE

⟨|R−Rb|⟩≃4 kpc√τ/12Gyr

To first order... Work in progress (Frankel+ in prep)