Stellar Migration in Disks and Disk Outskirts Victor P. Debattista R. Ro š kar, 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 gas dark matter NFW, 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, M vir = M  10% baryons by mass ~ 10 6 M  per DM particle initially 1.4x10 5 M  per gas 4.6x10 4 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 ~ t 0.53 Velocity Dispersion [km/s] Age [Gyr] Data: Geneva- Copenhagen survey (Holmberg 2009) Age [Gyr] Simulation Steady heating ~ t 0.35

60% truncated 10% 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 Ro š kar et al. 2008

Very few particles beyond the break actually formed there (plots show particles on mostly circular orbits) Radial Migration Populates Outer Disk R final R form

(Ro š kar 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 AllCircular 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 R br 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 R br the same for all pops Age Comparison to NGC 4244 de Jong et al. 2007

Stacking Galaxies Azzollini et al No breaks Breaks Anti-breaks All rest frame ~ u-g > M sun < M sun

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 Measured Actual

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 ~10 12 M sun NGC 300 ~ M sun (Gogarten et al. 2010)

Outer Disk Contamination Roskar et al The outer disk is contaminated also by stars forming in situ, in the warped region. Sanchez-Blazquez et al Martinez-Serrano et al. 2009

Roskar et al. 2010

Could significantly affect: outer disk surface density age distribution metallicity But - 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 age Intermediate Young 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. R form

“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 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 v rot 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 = = 0.1

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. separates for kine. selection Bensby+2003, 2005; Feltzing dex See 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