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

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, Hayden, Haywood, Kubryck, McMillan, Minchev, Roskar, Sanders, Sellwood, Schönrich The Dynamical Universe For All, Lund Observatory, 6 February 2018

Simplest picture Inside-out disk growth: L z Low gas settles first Stars form: 1. Low angular Orbit determined by of the gas L z momentum gas settles Fe Galactic radius

Simplest picture Inside-out disk growth: L z Low gas settles first Stars form: 1. Low angular L z Orbit determined by of the gas momentum gas settles In parallel: Stars evolve → Chemical enrichment of the gas Fe Z Galactic radius

Simplest picture Inside-out disk growth: L z L z Low gas settles first 2. Higher Higher gas settles later angular momentum Stars form: 1. Low angular gas L z Orbit determined by of the gas momentum gas settles In parallel: Stars evolve → Chemical enrichment of the gas Fe Z Galactic radius

Simplest picture Inside-out disk growth: L z L z Low gas settles first 2. Higher Higher gas settles later angular momentum Stars form: 1. Low angular gas L z Orbit determined by of the gas momentum gas settles In parallel: Stars evolve → Chemical enrichment of the gas Fe Z+ Z Galactic radius

Simplest picture Inside-out disk growth: L z L z Low gas settles first 2. Higher Higher gas settles later angular momentum Stars form: 1. Low angular gas L z Orbit determined by of the gas momentum gas settles In parallel: Stars evolve → Chemical enrichment of the gas → Metallicity gradient in the disk → At given radius: clear age – metallicity relation Fe Z+ Z Galactic radius

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

Simplest picture Expect a tight [Fe/H]–τ relation time Sanders & Binney (2015) Fe Z+ Z Galactic radius Solar neighborhood

Simplest picture Observe large [Fe/H]–τ scatter Expect a tight [Fe/H]–τ relation time Edvardsson+ (1993) Sanders & Binney (2015) Fe Z+ Z Galactic radius Solar neighborhood

What is the next simplest picture? ● As before: A tight relation [ Fe / H ]−τ− R b ● But: the angular momentum (or radius) evolves in time

What is the next simplest picture? ● As before: A tight relation [ Fe / H ]−τ− R b ● But: the angular momentum (or radius) evolves in time Stars migrate

Migration mechanism: orbit scatter near co-rotation Selwood & Binney 2002: spiral arms co-rotation

Migration mechanism: orbit scatter near co-rotation Selwood & Binney 2002: spiral arms co-rotation

Migration mechanism: orbit scatter near co-rotation Selwood & Binney 2002: spiral arms co-rotation Radial migration ~ angular momentum diffusion

Implications of radial migration

Implications of radial migration ● : tight age – metallicity t migr ≫ t Hubble

Implications of radial migration ● : tight age – metallicity t migr ≫ t Hubble t migr ≪ t Hubble ● : Orbits are scrambled – Formation/dynamics memory loss – Leads towards exponential disk profiles (Herpich+ 2017) – Radial gradients smoothed out

Implications of radial migration ● : tight age – metallicity t migr ≫ t Hubble t migr ≪ t Hubble ● : Orbits are scrambled – Formation/dynamics memory loss – Leads towards exponential disk profiles (Herpich+ 2017) – Radial gradients smoothed out t migr ∼ t Hubble ● : ? – How much memory does the Galactic disk keep? – Impact on the dynamics?

Implications of radial migration ● : tight age – metallicity t migr ≫ t Hubble t migr ≪ t Hubble ● : Orbits are scrambled – Formation/dynamics memory loss – Leads towards exponential disk profiles (Herpich+ 2017) – Radial gradients smoothed out t migr ∼ t Hubble ● : ? – How much memory does the Galactic disk keep? – Impact on the dynamics? To test this, we need to quantify the strength of radial migration, globally

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

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 Ness+ (2016) adapted from Ness et al. (2016) # stars # stars

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 Ness+ (2016) Genovali+ (2014) adapted from Ness et al. (2016) # stars # stars # stars

A global measure: a simple model p ([ Fe / H ] , τ∣ R , ⃗ p m ) Sanders & Binney (2015)

A global measure: a simple model p ([ Fe / H ] , τ∣ R , ⃗ p m ) Sanders & Binney (2015) ● Radial migration p ( R ∣ R b , τ) τ σ=σ 0 √ 12 Gyr

A global measure: a simple model p ([ Fe / H ] , τ∣ R , ⃗ p m ) Sanders & Binney (2015) ● Radial migration p ( R ∣ R b , τ) ● Birth radius R b = f ([ Fe / H ] , τ)

A global measure: a simple model p ([ Fe / H ] , τ∣ R , ⃗ p m ) Bovy+ (2014) Sanders & Binney (2015) ● Radial migration p ( R ∣ R b , τ) ● Birth radius R b = f ([ Fe / H ] , τ) ● Age distribution p (τ)

A global measure: a simple model p ([ Fe / H ] , τ∣ R , ⃗ p m ) Sanders & Binney (2015) ● Radial migration p ( R ∣ R b , τ) ● Birth radius R b = f ([ Fe / H ] , τ) ● Age distribution p (τ) ● Birth distribution p ( R b ∣τ)

A global measure: a simple model p ([ Fe / H ] , τ∣ R , ⃗ p m ) Sanders & Binney (2015) ● Radial migration p ( R ∣ R b , τ) ● Birth radius R b = f ([ Fe / H ] , τ) ● Age distribution p (τ) ● Birth distribution p ( R b ∣τ) Sampling (MCMC) Diffusion coefficient p ( ⃗ p m ∣[ Fe / H ] , τ , R ) → radial migration strength emcee (Foreman-Mackey+2013)

Radial migration efficiency Star formation (at face value...) efficiency Inside-out formation Radius where ISM metallicity Work in progress (Frankel+ in prep) is solar Enrichment timescale ISM metallicity gradient at solar radius Radial migration efficiency

Radial migration efficiency (at face value...) ISM metallicity gradient at Work in progress solar radius (Frankel+ in prep) Over a Hubble time, stars have on average migrated Radial migration of about 4-5 kpc. efficiency → 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.

What sets the radial structure of the Milky Way disk? To first order...

What sets the radial structure of the Milky Way disk? To first order... ● Orbits of stars in the Milky Way depend upon: ● The initial angular momentum of the gas ● The subsequent radial migration of stars

What sets the radial structure of the Milky Way disk? To first order... ● 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

What sets the radial structure of the Milky Way disk? To first order... ● 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 Work in progress ● Using global data { R, τ , [ Fe / H ]} APOGEE (Frankel+ in prep) and a formation and diffusion model: ⟨ | R − R b | ⟩≃ 4 kpc √ τ/ 12 Gyr