HWR

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IV Milky Way / Local Group Tomography
The stellar distribution in the Milky Way is not
smooth. What can it tell us bout its formation
history?

• Hans-Walter Rix
• MPI for Astronomy
• Heidelberg

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Substructure Signposts of Hierarchical
Formation

• The motions of stars (or groups) still reflect
  their formation history after many dynamical
  periods.
• In collissionless systems, the phase-space
  density/distribution is preserved.
• Phase mixing may lead to a smooth appearance in r
  or v space.

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2. Seeing Galaxies 2D, 6D or (the right) 3D
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..but all is well in phase space(e.g. Helmi, de
Zeeuw 2000)
Also holds true if the overall potential changes
adiabatically (Penarrubbia et al 2005) Scattering
off sub-structure to be checked!
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Questions

• Is there direct evidence for such sub-structure?
• In all galaxies? In the Milky Way?
• What is the mass spectrum of pieces?
• Is hierarchical accretion still going on?
• Can we use the streams to measure the
  gravitational potential?
• How tightly is chemical enrichment coupled to
  kinematics (i.e. to formation episode)?

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• Is (sub-)structure of the phase-space
  distribution observable in galaxies with
  unresolved stellar populations?

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Tomography of Unresolved Galaxies?NGC 4473
data-modelCappellari, de Zeeuw et al SAURON
2D-binned data
V s h3 h4
Symmetrized data
Axisymmetric model
Are the V-shaped velocity and high major-axis
dispersion produced by a counter rotating stellar
component?
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Schwarzschild's approach
Observed galaxy image
images of model orbits

• Compute all orbits possible in a given galaxy
• The goal is to find the combination of orbits
  that actually appear in the galaxy ? dynamical
  model
• But images alone don't contain enough information

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NGC 4473 orbital structureCappellari, de Zeeuw
in prep.
Counter-rotating stars
Main galaxy rotation
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Are spiral galaxies smooth?Lets step back and
look at M31
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Probing the Halo of M31 with SDSSZucker et al
2004
Advantage large volume-filling factor
easy Disadvatage 3D information limited
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M 31 Status QuoLewis et al 2004
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4. Substructure in the Milky Way Halo

• How to find it?
• Status quo
• How to interpret what has been found?

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A clear case the Sagittarius stream
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The density of turn-off colored stars in the
SDSS equatorial stripe
Galactic Plane
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Hess diagrams as diagnostic tools
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The Wilky Ways Low-Latitude Ring(Monoceros,
Tri/And, CMa, etc)
Kicked out (of the plane) or Dragged in
(disrupting satellite)?
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THE FIRST SCENARIO TIDALLY DISRUPTING DWARF
GALAXY
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THE SECOND SCENARIO THE MILKY WAY WARP
(Momany et al 2004)
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Low-Latitude Stellar Overdensities in the MW

• Is it a tidal stream? -- external
• Can all pieces be fit as originating from one
  disrupted entity?
• Is there a parent?
• Is a warp (or more complex response to a
  perturbation)? internal
• Discriminants
• Kinematics disk/warp-like
• Spatial distribution
• Chemical composition diff. star-formation
  history

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Modelling the Low-Latitude Ring(Penarrubbia,
Rix, et al. 2005)

• Question can all overdensities be attributed to
  one stream?
• Approach
• semi-analytic point orbit (incl. dynamical
  friction)
• full N-body realization

Best prograde semi-analytic orbit
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The Wilky Ways Low-Latitude Ring(Monoceros,
Tri/And, CMa, etc)
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Spatial Distribution wide z-range little R
range
? ? Not a warp
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Results of the stream modeling

• The location of all known over-densities at low
  latitude (trailing tidal tails of a disrupted satellite
• Geometry excludes warp
• Satellite orbit prograde, very low ellipticity
  (0.1-0.05) at low inclination (20o-5o)
• Parent location not well determined
• metallicity gradient of debris suggest l250
• Orbit model suggests distance dsun12 kpc
• Parent satellite mass 2x108-109Msun

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Does the stream have a parent galaxy?
Selecting stars with red giant colors and taking
apparent magnitude as a distance proxy
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Geometry/Kinematic of Low-Latitude Overdensities

• Orbit near-circular
• Orbit prograde
• Several wraps needed to explain observations
• Extends -10 kpc out of the plane
• At nearly the same R!
• Parent satellite towards CMa plausible

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Martinez-Delgado, Rix, et al 2004 (see also
Bellazzini et al 2004)
saturation
mV,0 24 (l,b)(240,-8)
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Density of MS stars towards CMa as a function of
distance (app. magnitude)
Depth of CMa r1/20.85kpc _at_ RGC13kpc
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What is the density profile of CMa?
galactic plane
Butler, Martinez-Delgado, Rix 05 (in prep.)
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• Narrow MS (15 depth)
• High-contrast (3)
• Two distinct (age?) populations
• Distance 8kpc

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• Buit lifes never easy!
• CMa may not be the point of maximal density

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Proper Motions of Canis Majoris
WCMa-49-15 km/s
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What would the Milky Ways response be to such a
disrupting satellite?
below
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SDSSSEGUE Sky Coverage
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NearTerm Future Astrometry

• PRIMA differential astrometry with VLTI
• 2008
• 10mas _at_ 17 mag across 30

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6. GAIA 2012---
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Summary

• Sub-structure exists (may even be pervasive)
• The observed parts were created recently (z
• We still have to learn how to best find it
• Quantitatively
• Objectively
• Milky Way seems to be surrounded by at least two
  large streams
• parent of the low-latitude stream is probably
  near Canis Majoris
• Impact of those streams on the Milky Way is
  considerable
• Milky Way subject to quite intense gravitational
  noise
• SEGUE (SDSS-II) and GAIA can revolutionize the
  field.
• The existing analysis tools for these data are
  still rudimentary