Stellar Migration in Stellar Migration in Disks and Disk Disks and Disk

OutskirtsOutskirtsVictor P. Debattista

R. Roškar, S. Loebman, P. Yoachim, A. Brooks, G. Stinson, T. Kaufmann, T. Quinn, J. Wadsley, C. Brook, F. Governato, J. Dalcanton

Sellwood & Binney (2002) demonstrate that spirals

can migrate stars over large distances at the CR

without heating

Migration can be inward or outward directed

Galactic archaeology in which strata are jumbled

Migration can be quantified by considering low

density regions

Stellar Migration

Simple Picture of Galaxy Formation

gasdark matterNFW, MW c

e.g. Fall & Efstathiou (1980), Dalcanton et al. (1997)Also viscous evolution - Lin & Pringle (1987), Ferguson & Clarke (2001)

Approximate disk formation via dissipational collapse after last major merger using SPH code GASOLINE

• 1M particles in each component, Mvir = 1012 M

• 10% baryons by mass • ~ 106 M per DM particle• initially 1.4x105 M per gas 4.6x104 M per star

• 50 pc force resolution for baryons, 100 pc for DM• star formation and feedback (Stinson et al. 2007) including feedback from SN Ia, II and stellar winds

Advantages:• fully self-consistent evolution• no ad-hoc assumptions about the disk• full modeling of dynamical processes• star formation and feedback allow for direct comparisons with observations (age, metallicity, SFR etc.)

Simulations

Model Galaxy Properties

Steady heating~ t0.53

Velo

cit

y D

isp

ers

ion

[km

/s]

Age [Gyr]

Data: Geneva-Copenhagen survey (Holmberg 2009)

Age [Gyr]Age [Gyr]

Simulation

Steady heating~ t0.35

60% truncated10% pure expo.

Pohlen & Trujillo 2006

van der Kruit 1979

Consistent with observations of de Jong (1996), Bell & de Jong (2000), MacArthur (2004)

But why is there an upturn?

• stellar disk is clearly extended beyond end of SF• if populated by stars formed in-situ, there shouldn’t be an age minimum

Outer disk is populated by material from the inner disk

Roškar et al. 2008

Very few particles beyond the break actually formed there

(plots show particles on mostly circular orbits)

Radial Migration Populates Outer DiskR

fin

al

Rform

(Roškar et al. 2008a)

Very few of the stars beyond the ``break’’ actually born there

Most particles in the outer disk still retain nearly circular orbits

RbRb

All

Circular

Outer Disk Kinematics

J/Jc(E)

Outer disk only

a) Fourier expansion -> b) power spectrum -> c) identify patterns/resonances

a)

b)

c)

Confirming Role of Spirals

HST star count profiles

Rbr the same for all pops

Age

Comparison to NGC 4244

Possible interpretations

Break just formed

Break has never changed

Radial migration forces stars to adopt a common break radius

de Jong et al. 2007

simulation HST star count profiles

Rbr the same for all pops

Age

Comparison to NGC 4244

de Jong et al. 2007

Stacking Galaxies

Azzollini et al. 2008

No breaks

Breaks Anti-breaks

All rest frame ~ u-g

> 1010Msun < 1010Msun

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 et al. 2010

Reconstructing the SF History

Except for small radii, assuming that a star was born where it currently is leads to gross errors in the local SFR

MeasuredActual

How much are SFH determinations in NGC 300 influenced by radial mixing?• simulation with lower total mass -> spirals of smaller amplitude -> less migration• could be influenced by interactions (might be the case with M33)

MW ~1012 Msun

NGC 300 ~ 1011 Msun

(Gogarten et al. 2010)

Outer Disk Contamination

Roskar et al. 2010

The outer disk is contaminated also by stars forming in situ, in the warped region.

Sanchez-Blazquez et al. 2009

Martinez-Serrano et al. 2009

Roskar et al. 2010

Could significantly affect:• outer disk surface density• age distribution• metallicityBut - migration still dominates

Predicting What Anti-Centre Survey Might Find

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

Mean

ag

e

IntermediateYoun

g

Old

Predicting What Anti-Centre Survey Might Find

[Fe/H] continues to decline with radius: some of the oldest disk stars present here. The solar neighborhood, instead, is contaminated by younger, more metal rich, stars, increasing the mean.

“Solar” Neighborhood

Most of the stars in the solar neighborhood (7 < R < 9 kpc) formed elsewhere. The metal poor ones come from a wide range of radii, including from outside the solar neighborhood, while the metal rich ones come from inside the solar radius.

Rform

“Solar” Neighborhood

Assuming stars remain in situ, we find a significant gradient and low dispersion in AMR. But AMR is flattened & broadened by migration, which is true also for stars on circular orbits. Note: increase in metallicity of old stars and increasing scatter with age

Age

[Fe/

H]

Edvardsson et al. 1993, Nordstrm et al. 2004, Haywood 2008

• A “thick” disk detected in star counts as a function of height in the solar neighborhood

• Also considered distinct in metallicity, velocity dispersion and age

• Various formation hypotheses… accretion of satellites, heating by minor mergers, rapid formation after the last major merger…

Two components

(Gilmore & Reid 1983)

Thin disk

Thick disk Sun

Thick Disk Formation

Effect on Vertical Direction

If vertical action is conserved by migration then stars moving outwards puff up and vice-versa

Is there such a thing as a dynamically distinct thick disk?

Schoenrich & Binney 2009

Sales+ 2009

Caruana 2009

Thin disk

Thick disk

Testing Thick Disk Formation

See also Amina’s talk

z

Metal poor + slow rotation

Metal rich + fast rotation

In transition region, V should correlate with [Fe/H]

Decreasing metallicity and vrot with height

Thick Disk: An SDSS Puzzle

Ivezic+ 2008

Stars form with a single expo profile but quickly develop a second component

More stars formed in situ at high ; smaller variation in radial density across migration scale

EXPECT less of a “bimodality” in vertical density

= 0.039 = 0.1

Stars are born on circular orbits and hence have no correlation between V and [Fe/H]. Heating brings stars into the local volume creating a correlation. Migration shuffles [Fe/H] and erases the correlation

Profiles of mean metallicity changes for a chemical selection. <[/Fe]> separates for kine. selection

Bensby+2003, 2005; Feltzing 2006

0.1 dexSee Venn’s talk

Predicting What LAMOST Might Find

Separating stars by [O/Fe], the -enhanced (older) population should have no correlation of V versus [Fe/H] while the lower [O/Fe] (younger) stars will show a correlation

Migration complicates Galactic archaeology

and needs to be quantified to decipher the

clues to galaxy formation inherent in

observational data

Migration can account for a wide range of

observational data, in both the Milky Way and

external galaxies.

Faint regions of disk galaxies contain

important clues to past migration and are

prime targets for quantifying migration rates

Conclusions