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IV: Milky Way / Local Group Tomography

IV: Milky Way / Local Group Tomography. Hans-Walter Rix MPI for Astronomy Heidelberg. The stellar distribution in the Milky Way is not smooth. What can it tell us bout its formation history?. “Substructure”: Signposts of Hierarchical Formation.

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IV: Milky Way / Local Group Tomography

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  1. IV: Milky Way / Local Group Tomography Hans-Walter Rix MPI for Astronomy Heidelberg The stellar distribution in the Milky Way is not smooth. What can it tell us bout its formation history?

  2. “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.

  3. P. Harding 2. Seeing Galaxies: 2D, 6D or (the right) 3D

  4. Bullock, Kravtsov, Weinberg 2002

  5. initial 12 Gyrs later ..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!

  6. 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)?

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

  8. 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?

  9. Schwarzschild's approach • 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 Observed galaxy image images of model orbits

  10. NGC 4473: orbital structureCappellari, de Zeeuw in prep. Counter-rotating stars Main galaxy rotation

  11. Step 2: plot their spatial distribution Ferguson et al 2004 • Step 1: select stars in color-magnitude space blue RGB (= metal poor) red RGB (= metal rich) Are spiral galaxies smooth?Let’s step back and look at M31

  12. And NE And V SDSS Scan of M31: 45 sq. deg. in 3 hr 20 kpc And IX Spatial Density of Probable M31 RGB Stars by Color Probing the Halo of M31 with SDSSZucker et al 2004 Advantage: large volume-filling factor easy Disadvatage: 3D information limited

  13. M 31 Status QuoLewis et al 2004

  14. 4. Substructure in the Milky Way Halo • How to find it? • Status quo • How to interpret what has been found?

  15. Ibata et al 1995 Majewski et al 2003 A clear case: the Sagittarius stream

  16. Newberg, Yanny et al 2002 Sagittarius Stream Pal 5 The density of “turn-off” colored stars in the SDSS equatorial stripe Galactic Plane

  17. SEGUE (unpublished) “Hess diagrams” as diagnostic tools

  18. The Wilky Way’s Low-Latitude Ring(Monoceros, Tri/And, CMa, etc…) Kicked out (of the plane) or Dragged in (disrupting satellite)?

  19. THE FIRST SCENARIO: TIDALLY DISRUPTING DWARF GALAXY

  20. 15 kpc 1.5 kpc Galaxy ESO 510-13. Conselice et al 2003 THE SECOND SCENARIO: THE MILKY WAY WARP (Momany et al 2004)

  21. 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

  22. Best retrograde orbit 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

  23. Penarrubia, Rix et al 2005 The Wilky Way’s Low-Latitude Ring(Monoceros, Tri/And, CMa, etc…)

  24. Penarrubia, Rix et al 2005 Peñarrubia 2004 Spatial Distribution: wide z-range; little R range  ? Not a warp

  25. Results of the stream modeling • The location of all known over-densities at low latitude (<30o) can be matched as leading and 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 l=250 • Orbit model suggests distance dsun=12 kpc • Parent satellite mass 2x108-109Msun

  26. Does the stream have a parent galaxy? Selecting stars with red giant colors and taking apparent magnitude as a distance proxy

  27. Martinez-Delgado, Rix et al 2005 Can Maj Main Seq. ThickDisk Halo Thin Disk Main Seq.

  28. Martinez-Delgado, Rix, et al 2004 (see also Bellazzini et al 2004) saturation mV,0 ~ 24 (l,b)=(240,-8)

  29. Martinez-Delgado, Rix et al 2005 Density of MS stars towards CMa as a function of distance (app. magnitude) Depth of CMa: r1/2~0.85kpc @ RGC~13kpc

  30. What is the density profile of CMa? galactic plane Butler, Martinez-Delgado, Rix ’05 (in prep.)

  31. Martinez-Delgado, Rix et al 2005 Can Maj Main Seq. ThickDisk Halo Thin Disk Main Seq. • Narrow MS (15% depth) • High-contrast (>3) • Two distinct (age?) populations • Distance 8kpc

  32. Rocha-Pinto et al 2005 • Buit life’s never easy! • CMa may not be the point of maximal density

  33. Rocha-Pinto et al 2005 Warp motion Dinescu et al 2005 Proper Motions of “Canis Majoris” WCMa=-49+-15 km/s

  34. sun (CMa) satellite What would the Milky Way’s response be to such a disrupting satellite? below

  35. SDSS+SEGUE Sky Coverage

  36. Near–Term Future Astrometry • PRIMA: differential astrometry with VLTI • 2008 • 10mas @ 17 mag across 30”

  37. 6. GAIA 2012---

  38. Summary • Sub-structure exists (may even be pervasive) • The observed parts were created recently (z<0.5) • 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

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