Victor P. Debattista R. Roškar, S. Loebman, P. Yoachim, D. Radburn-Smith, A. Clarke,

T. Quinn, J. Wadsley, Z. Ivezic, J. Dalcanton

σ

B-V Dehnen & Binney 1998

ΔR ≈ √2 σR/κ ≈ 1.3 kpc

Rform Rfinal

Negligible heating associated with the migration

Full Near circular ∆L

L

Radius

σR

ΔJR = (Ωp - Ω) ΔL/κ

ΔE = Ωp ΔL

Sellwood & Binney 2002

60% truncated 10% pure expo.

Pohlen & Trujillo 2006

van der Kruit 1979

Structural properties of break consistent with those observed

Gas+SF+feedback simulations

Disk (to break radius) grows from the inside out

10 kpc = 2.6 Rd

Roškar et al. 2008a

Very few of the stars beyond the break actually born there

Steady heating ~ t0.53 Ve

locity

Dispe

rsion

[km/s

]

Age [Gyr]

Data: Geneva-Copenhagen survey (Holmberg 2009)

Age [Gyr]Age [Gyr]

Simulation

Steady heating ~ t0.35

Inward migrators ahead of the spiral while outward migrators are behind it

Roškar+ 12

HST star count profiles

Rbr the same for all pops

Age

NGC 4244

Possible interpretations

Break just formed

Break has never changed

Radial migration forces stars to adopt a common break radius

de Jong+ 2007

simulation

Outer Disk Stellar Pops Migration is basically a random walk, so older stars get to larger distance from formation

Expect the mean age of disk stars to increase beyond the break

The variation of outer disk scale-length for different age stellar bins constrain migration rates

Mea

n ag

e

Intermediate Young

Old

NGC 7793

Radburn-Smith+ 12

Break radius

NGC 7793

Radburn-Smith+ 12

NGC 6155

Break at 34” (Pohlen & Trujillo 2006)

Data using VIRUS-P on 2.7-m HJS telescope 1.7’ x 1.7’ fov. Rebin data to S/N ~45 beyond break

Fit the spectra using GANDALF (Sarzi et al. 2006) assuming exponential SFH and varying metallicity

Yoachim+ 10

Metallicity Gradients

Metallicity gradient of old stars does not reflect that of gas from which they formed

SFH usually obtained by considering stars within an annulus or small region

Drastic difference between ‘observed’ and ‘real’ SFH due to radial migration

(NGC 300 mosaic from the ANGST HST survey, courtesy of Stephanie Gogarten)

Inferred

Roškar et al. 08b

Measured

Actual

Implications for Galactic Archaeology

Rform

Roškar+ 08 Schoenrich & Binney 09ab

Migration can give rise to a thick disc with density, kinematic, metallicity and abundance patterns not inconsistent with the observed

Loebman+ 11 Roškar+ 13

Ruchti+ 11

Bland-Hawthorn+ 10

Disrupting Clusters

Fujii & Baba 12

See also poster by Martinez-Barbosa

In 2 Gyr the cluster goes from 7.4 kpc to 10.6 kpc

Other Migration Mechanism: Bar Driven

Final

Initial

Face-on azimuthally averaged profile

Initial disk

Inner exp Break radius

Outer exp

Edge-on profile

Debattista+ 06 (also Foyle+ 08, Minchev & Famaey 10)

Rbr

Brunetti+ 11

Munoz-Mateo+S4G 13

Ask the Star Clusters M

ean

age

Star clusters are an ideal probe of this age upturn in the Milky Way.

The upturn is predicted to occur at the break. This will hold for disc clusters but not for any clusters forming in the warp (migration does not lead to stars leaving the plane of the disc)

  Contrary to decades of assumption, a mechanism for mixing stars in Galactic radius without heating exists: scattering at corotation off transient spirals. This can substantially alter stellar populations in disks.

  A large fraction of stars in outer disks probably formed at smaller radii and migrated outwards. There is a growing body of evidence that disks get increasingly old outwards of the break.

  Migration may also be the explanation for at least part of the thick disk. Many of the properties and trends of the thick disk can be qualitatively matched by migration.

  (Open) star clusters should experience migration in the same way as stars, but those moving inwards may be more prone to disruption while those moving outwards are more likely to survive. Clusters provide a very good way to trace the migration history of the outer disc.

Conclusions

Nuclear Clusters in Galaxies and the Role of the Environment Lorentz Center June 30 – July 4, 2014