The Accretion History of the Milky Way

The Accretion History of the Milky Way Julio F. Navarro The Milky Way as seen by COBE Collaborators Mario Abadi Amina Helmi Matthias Steinmetz Ken Ken Freeman Andres Meza

The Hierarchical Formation of a Disk Galaxy Stellar disks disks form form after after the the dissipative collapse of of gas gas onto onto thin, thin, centrifugally supported structures. Spheroids form form subsequently as as a consequence of of mergers. Galaxy morphology is is a transient, evolving feature in in the the lifetime of of a galaxy. Steinmetz & Navarro Navarro 2000 2000

The Hierarchical Formation of the Milky Way Galaxies are assembled hierarchically; i.e. most are likely to have experienced an early period of active merging activity are there any relicts of this early merging history? In Milky Way-like galaxies this is followed by a relatively quiescent accretion epoch that led to the formation of the thin disk but merging is still ongoing, as demonstrated by the Sagittarius dwarf Galactic disks date from the time of the last major merger and form from the inside out. why are there thin disk stars in the solar neighborhood as old as the Galaxy?

Age-Angular Momentum Distribution in in the the Solar Neighbourhood Age Age From a catalogue of of ~13,000 stars stars within ~100 ~100 pc pc of of the the Sun Sun Specific Specific Angular Angular Momentum

Dynamical components of of the simulated galaxy Specific Angular Momentum Specific Angular Momentum Binding Binding Energy Energy Abadi Abadi et et al al 2003 2003

Dynamical components of of a simulated galaxy Abadi Orbital Orbital Circularity Abadi et et al al 2003 2003

Dynamical components of of a simulated galaxy Non-rotating spheroid thick thick disk disk thin thin disk disk Orbital Orbital Circularity Abadi Abadi et et al al 2003 2003

The Star Formation History of of the Simulated Galaxy Mass Fraction Mass Fraction The The thin thin disk disk contains aa significant number number of of old old stars stars (15% (15% are are older older than than 10 10 Gyrs) Gyrs) >90% >90% of of old old stars stars in in the the disk disk are are the the result result of of satellite satellite accretion events events The The thick thick disk disk is is not not an an early early thin thin disk disk thickened by by aa minor minor merger merger but but actually actually the the accumulated debris debris from from satellite satellite accretion events events Age Age in in Gyr Gyr Abadi Abadi et et al al 2003 2003

A disk made up of tidal debris: edge-on view

A disk made up of tidal debris: face-on view

Satellite orbit circularization Satellites on on roughly coplanar orbits are are brought into into the the plane and and circularized by by dynamical friction Meza, Meza, Navarro, Navarro, Abadi, Abadi, Steinmetz Steinmetz 2004 2004

Satellite Orbit Circularization R apocenter R pericenter Satellites orbits orbits are are generally circularized by by dynamical friction friction Meza, Meza, Navarro Navarro et et al al

Tidal Debris in the Milky Way Disk? Tidal Tidal debris debris is is usually usually assumed to to contribute to to the the spheroidal component of of the the galaxy like the the Sagittarius stream but it it may may also also contribute to to the the Galactic Galactic disk(s) disk(s)

Signatures of ongoing disruption: transient tidal arcs

Ring around the Galaxy Yanny Yanny et et al al 2003 2003 Newberg et et al al 2003 2003

Ring around the Galaxy: a tidal arc? Helmi, Helmi, Meza, Meza, Navarro, Navarro, Steinmetz, Steinmetz, Eke Eke 2003 2003

Ring around the Galaxy: a tidal shell? Helmi, Helmi, Meza, Meza, Navarro, Navarro, Steinmetz, Steinmetz, Eke Eke 2003 2003

Ring around the Galaxy: a tidal arc? Crane Crane et et al al 2003 2003

Ring around the Galaxy: its progenitor? Ibata Ibataetetalal2003 2003

Are there further examples of accretion onto the Milky Way disk? Tidal relicts are are most easily identified in in samples of of stars that minimize the the contribution of of the the young thin disk: metal poor stars stars above or or below the the Galactic plane stars at at large Galactocentric distances

Metal-Poor Stars near the Sun Sun They are are less rotationallysupported than stars like the the Sun The The rotation rotation speed speed distribution distribution of of metal-poor metal-poor stars stars and and the the three three canonical canonical components components of of the the Milky Milky Way Way Beers et et al al 2000 2000

Metal-Poor Stars near the Sun Sun Ratio of alpha elements/fe relative to solar Ratio of alpha elements/fe relative to solar Their chemical composition is is enhanced in in α-elements relative to to the the Sun. Iron Iron abundance relative relative to to solar solar Gratton et et al al 2003 2003

Tidal debris in the disk of the Milky Way ωcen

Arcturus: a night-sky witness of of the merging history of of the Milky Way? In In the the early early 70s, 70s, Eggen identified a number of of bright stars stars with with spatial motions similar to to Arcturus. The The Arcturus group is is one one of of the the several moving groups (coherent dynamical structures in in the the solar solar neighbourhood) that that Eggen identified and and interpreted as as the the late late stages in in the the disruption of of old old open open clusters.

Dynamical Substructure in the Solar Neighbourhood Arcturus a metal-poor, [Fe/H]~-0.6, presumably old old star star on on an an disk-like orbit orbit confined to to within ~ 1 kpc kpc of of the the Galactic plane with negligible radial velocity (near (near its its apocenter) Open Open circles: circles: metal-poor metal-poor stars stars of of Beers Beers et et al al 2000 2000 Filled Filled circles: circles: Eggen s Eggen s Arcturus Arcturus group group candidates candidates lagging the the LSR LSR rotation speed by by ~110 ~110 km/s km/s (more than than twice as as much as as the the canonical thick thick disk) disk) Navarro, Helmi Helmi & Freeman 2004 2004

Dynamical Substructure in the solar neighbourhood Stellar groupings that that lag lag the the LSR LSR by by a few few tens tens of of km/s-- such such as as the the Hercules group--is an an excess of of stars stars probably caused by by resonances induced by by the the Galactic bar bar (Dehnen 2000, Fux Fux 2001) Such resonances, as as well well as as perturbations by by the the spiral pattern may may cause some some moving groups, but but it it is is unlikely their their effect may may lead lead to to lags lags above ~50 ~50 km/s km/s relative to to the the LSR. LSR. The The rotation rotation speed speed distribution distribution of of metal-poor metal-poor stars stars decomposed decomposed in in the the three three canonical canonical components components of of the the Milky Milky Way Way The The Arcturus group lags lags the the LSR LSR by by 120-140 km/s km/s Navarro, Navarro, Helmi Helmi & Freeman Freeman 2004 2004

Dynamical Substructure in the Solar Neighbourhood The The Arcturus group appears as as a recognizable excess of of stars stars in in various compilations of of metal-poor stars, stars, such such as as the the samples of of Beers et et al al (2000, B00) B00) and and Gratton et et al al (2003, GCCLB). The The JJ z distribution z distribution of of stars stars in in various various compilations. compilations. Navarro, Navarro, Helmi Helmi & Freeman Freeman 2004 2004

Chemical abundance of Arcturus group candidates Stars Stars in in the the Arcturus group are are fairly metal-poor, and and trace trace a tight tight relation in in the the [α/fe] vs vs [Fe/H] plane, as as expected for for a population of of stars stars that that selfenriched to to a metallicity of of self- about 1/3 1/3 solar. Open Open circles: circles: Gratton Gratton et et al al 2003 2003 sample sample Filled Filled circles: circles: stars stars with with JJ z comparable z comparable to to Arcturus Arcturus Navarro, Helmi Helmi & Freeman 2004 2004

More Substructures in the Solar Neighbourhood? The The Beers et et al al catalog shows another possible excess of of stars stars at at slightly retrograde, very very eccentric orbits. This This peak peak has has been been associated with with the the unusual globular cluster ωcen, which might have have been been the the nucleus of of a disrupted dwarf galaxy. The The rotation rotation speed speed distribution distribution of of metal-poor metal-poor stars stars decomposed decomposed in in the the three three canonical canonical components components of of the the Milky Milky Way Way ωcen Meza, Meza, Navarro Navarro et et al al 2004 2004

Dynamical properties of disk debris on eccentric orbits The The satellite stars stars split split into into groups of of similar angular momentum and and binding energy at at each each pericentric passage. Meza, Meza, Navarro Navarro et et al al 2004 2004

Dynamical properties of disk debris Debris stars stars are are expected to to show show as as a symmetric excess of of stars stars with with large large galactocentric radial velocities, if if observed from from a radius between the the apocenter and and pericenter of of their their orbit. Meza, Meza, Navarro Navarro et et al al 2004 2004

Dynamical properties of ωcen group candidates Vertical Velocity Vertical Velocity Stars confined to to the the disk and with similar angular momentum as as ωcen appear to to have an an excess of of large galactocentric radial velocities. Galactocentric Galactocentric Radial Radial Velocity Velocity Meza, Meza, Navarro, et et al al 2004 2004

Chemical abundance of ωcen group candidates Stars Stars in in the the ωcen group are are fairly metal-poor, and and trace trace a tight tight relation in in the the [α/fe] vs vs [Fe/H] plane, as as expected for for a population of of stars stars that that selfenriched to to a metallicity of of self- about 1/5 1/5 solar solar on on a longer timescale than than the the Arcturus group. It It may may be be that that *most* metal poor poor disk disk stars stars have have been been contributed by by various accretion events! Open Open circles: circles: Gratton Gratton et et al al 2003 2003 sample sample Filled Filled circles: circles: stars stars with with JJ z comparable z comparable to to ωcen ωcen Meza, Meza, Navarro Navarro et et al al 2004 2004

Further evidence above and below the plane? Gilmore, Wyse and and Norris (2002) report the the presence of of a substantial population of of stars stars lagging the the LSR LSR rotation by by about 100 100 km/s km/s at at ~3 ~3 kpc kpc above and and below the the Galactic plane. Arcturus is is in in all all likelihood the the solar solar neighbourhood extension of of such such population. Could this this secondary peak peak be be an an extension of of ωcen group? Gilmore, Gilmore, Wyse Wyse & Norris Norris 2002 2002 Navarro, Helmi Helmi & Freeman 2004 2004

Substructure in the Solar Neighborhood

Substructure in the Solar Neighbourhood: the example of Arcturus

Substructure in the Solar Neighbourhood

Arcturus stars in the Solar Neighbourhood

Stars Beyond Galaxies: Luminous Halos Around Galaxies Essentially all all stars stars beyond the the traditional luminous radius of of a galaxy (and (and not not in in satellites) originate in in past past accretion events The The outer outer luminous halo halo of of a galaxy holds important clues clues to to the the merging history of of a galaxy Abadi, Abadi, Navarro Navarro & Steinmetz Steinmetz 2005 2005

The Outer Surface Brightness Profile of Galaxies Outer halos halos appear as as an an excess of of light light over over extrapolations of of the the inner inner surface brightness profile of of a galaxy. The The outer outer halo halo is is well well approximated by by a Sersic law law The The same same law law describes very very well well the the profile of of all all accreted stars stars Abadi, Abadi, Navarro Navarro & Steinmetz Steinmetz 2005 2005

The Surface Brightness Profile of Accreted Stars Outer halos halos appear as as an an excess of of light light over over extrapolations of of the the inner inner surface brightness profile of of a galaxy. The The outer outer halo halo is is well well approximated by by a Sersic law law The The same same law law describes very very well well the the profile of of all all accreted stars stars Abadi, Abadi, Navarro Navarro & Steinmetz Steinmetz 2005 2005

The Outer Surface Brightness Profile of M31 Such Such outer outer halos halos have have been been detected around isolated spirals (edgeon, see see Zibetti et et al al on, 2004) and and M31 M31 (see (see also also Irwin et et al al 2005) The The outer outer spheroid may may be be used used to to estimate the the total total number of of accreted stars stars in in a galaxy! Guhatakurta et et al al 2005 2005

The Dynamics of Luminous Halos The The velocity dispersion tensor of of stars stars in in the the outer outer halo halo is is remarkably anisotropic: σ r2 r2 ~4 ~4 σ 2 t 2 t This, combined with with the the progressive steepening of of the the outer outer density profile, explains why why traditional modeling of of the the outer outer halo halo of of the the Milky Way Way favors truncated dark dark matter halos halos (see (see Battaglia et et al al 2005) Abadi, Abadi, Navarro Navarro & Steinmetz 2005 2005

The Dynamics of of the Milky Way s Luminous Halo The The velocity anisotropy, combined with with the the progressive steepening of of the the outer outer density profile explains why why traditional modeling of of the the outer outer halo halo of of the the Milky Way Way favors truncated dark dark matter halos halos Battaglia et et al al 2005 2005

Substructure in the Solar Neighbourhood

A RAVE to Unravel the Formation of the Milky Way

Pilot Project up to 279 nights of unscheduled bright-time at the 6dF (UK Schmidt) in 2003-2005 100 000 targeted stars 9<I<12, on the southern hemisphere no color selection ~120 fibres per field, 7 diameter Ca-triplet at 8400-8750Å at R=7000 RVs to 2 km/s

Typical Spectra

Current Status Phase I started on April 11th 2003 @ 7 nights/lunation >50000 spectra collected (as of March 05) expected: 70000 spectra by mid 2005 Data analysis RV pipeline finished metallicity pipeline in progress data verification in progress Data distribution data base, VO interfaces in progress internal data release v0.3 out first public data release envisioned for mid 2005 critical path: sky subtraction/wavelength calibration

The End