HWR Princeton 2005 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?

HWR Princeton “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.

HWR Princeton 2005

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

HWR Princeton 2005 Bullock, Kravtsov, Weinberg 2002

HWR Princeton but all is well in phase space… (e.g. Helmi, de Zeeuw 2000) initial 12 Gyrs later Also holds true if the overall potential changes adiabatically (Penarrubbia et al 2005) Scattering off sub-structure to be checked!

HWR Princeton 2005 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)?

HWR Princeton 2005 Is (sub-)structure of the phase- space distribution observable in galaxies with unresolved stellar populations?

HWR Princeton 2005 Tomography of Unresolved Galaxies? NGC 4473: data-model Cappellari, de Zeeuw et al SAURON 2D-binned data Symmetrized data Axisymmetric model Are the V-shaped velocity and high major-axis dispersion produced by a counter rotating stellar component? V  h 3 h 4

HWR Princeton 2005 Schwarzschild's approach images of model orbits Observed galaxy image 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

HWR Princeton 2005 NGC 4473: orbital structure Cappellari, de Zeeuw in prep. Counter-rotating stars Main galaxy rotation

HWR Princeton 2005 Step 1: select stars in color-magnitude space Are spiral galaxies smooth? Let’s step back and look at M31 Step 2: plot their spatial distribution red RGB (= metal rich) blue RGB (= metal poor) Ferguson et al 2004

HWR Princeton 2005 Probing the Halo of M31 with SDSS Zucker et al 2004 SDSS Scan of M31: 45 sq. deg. in 3 hr And V And IX And NE Spatial Density of Probable M31 RGB Stars by Color 20 kpc Advantage: large volume-filling factor easy Disadvatage: 3D information limited

HWR Princeton 2005 M 31 Status Quo Lewis et al 2004

HWR Princeton Substructure in the Milky Way Halo How to find it? Status quo How to interpret what has been found?

HWR Princeton 2005 A clear case: the Sagittarius stream Majewski et al 2003 Ibata et al 1995

HWR Princeton 2005 Newberg, Yanny et al 2002 The density of “turn-off” colored stars in the SDSS equatorial stripe Sagittarius Stream Pal 5 Galactic Plane

HWR Princeton 2005 SEGUE (unpublished) “Hess diagrams” as diagnostic tools

HWR Princeton 2005 The Wilky Way’s Low-Latitude Ring (Monoceros, Tri/And, CMa, etc…) Kicked out (of the plane) or Dragged in (disrupting satellite)?

HWR Princeton 2005 THE FIRST SCENARIO: TIDALLY DISRUPTING DWARF GALAXY

HWR Princeton 2005 Galaxy ESO Conselice et al 2003 THE SECOND SCENARIO: THE MILKY WAY WARP (Momany et al 2004) 15 kpc 1.5 kpc

HWR Princeton 2005 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

HWR Princeton 2005 Modelling the Low-Latitude Ring (Penarrubbia, Rix, et al. 2005) Best prograde semi-analytic orbit Best retrograde orbit Question: can all overdensities be attributed to one stream? Approach: semi-analytic point orbit (incl. dynamical friction) full N-body realization

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

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

HWR Princeton 2005 Results of the stream modeling The location of all known over-densities at low latitude (<30 o ) can be matched as leading and trailing tidal tails of a disrupted satellite –Geometry excludes warp Satellite orbit prograde, very low ellipticity (~ ) at low inclination (20 o +-5 o ) “Parent” location not well determined –metallicity gradient of debris suggest l=250 –Orbit model suggests distance d sun =12 kpc –Parent satellite mass 2x M sun

HWR Princeton 2005 Does the stream have a parent galaxy? Selecting stars with red giant colors and taking apparent magnitude as a distance proxy

HWR Princeton 2005 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

HWR Princeton 2005 Thin Disk Main Seq. Thick Disk Halo Can Maj Main Seq. Martinez-Delgado, Rix et al 2005

HWR Princeton 2005 saturation  V,0 ~ 24 (l,b)=(240,-8) Martinez-Delgado, Rix, et al 2004 (see also Bellazzini et al 2004)

HWR Princeton 2005 Density of MS stars towards CMa as a function of distance (app. magnitude) Martinez-Delgado, Rix et al 2005 Depth of CMa: r 1/2 R GC ~13kpc

HWR Princeton 2005 What is the density profile of CMa? Butler, Martinez-Delgado, Rix ’05 (in prep.) galactic plane

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

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

HWR Princeton 2005 Proper Motions of “Canis Majoris” Rocha-Pinto et al 2005 Dinescu et al 2005 Warp motion W CMa = km/s

HWR Princeton 2005 What would the Milky Way’s response be to such a disrupting satellite? sun (CMa) satellite below

HWR Princeton 2005 SDSS+SEGUE Sky Coverage

HWR Princeton 2005 Near–Term Future Astrometry PRIMA: differential astrometry with VLTI –2008 –10  17 mag across 30”

HWR Princeton GAIA

HWR Princeton 2005 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