HWRPrinceton 2005

IV: Milky Way / Local Group Tomography

Hans-Walter RixMPI for Astronomy

Heidelberg

The stellar distribution in the Milky Way is not smooth.

What can it tell us bout its formation history?

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1. “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

P. Harding

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Bullock, Kravtsov, Weinberg 2002

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..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!

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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

Symmetrized data

Axisymmetric model

Are the V-shaped velocity and high major-axis dispersion produced by a counter rotating stellar component?

V h3 h4

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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

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NGC 4473: orbital structureCappellari, de Zeeuw in prep.

Counter-rotating starsMain galaxy rotation

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• 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

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Probing the Halo of M31 with SDSSZucker et al 2004

SDSS Scan of M31: 45 sq. deg. in 3 hrAnd 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

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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

Majewski et al 2003

Ibata et al 1995

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Newberg, Yanny et al 2002

The density of “turn-off” colored stars

in the SDSS equatorial stripe

Sagittarius Stream

Pal 5

Gala

ctic Pla

ne

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SEGUE (unpublished)

“Hess diagrams” as diagnostic tools

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The Wilky Way’s 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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Galaxy ESO 510-13. Conselice et al 2003

THE SECOND SCENARIO: THE MILKY WAY WARP (Momany et al 2004)

15 kpc

1.5 kpc

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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)

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

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The Wilky Way’s Low-Latitude Ring(Monoceros, Tri/And, CMa, etc…)

Penarrubia, Rix et al 2005

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Penarrubia, Rix et al 2005

Spatial Distribution:

wide z-range; little R range

? Not a warp Peñarrubia 2004

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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

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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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Thin Disk Main Seq.

ThickDisk

Halo

Can Maj Main Seq.

Martinez-Delgado, Rix et al 2005

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saturation

V,0 ~ 24

(l,b)=(240,-8)

Martinez-Delgado, Rix, et al 2004

(see also Bellazzini et al 2004)

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Density of MS stars towards CMa as a function of distance (app. magnitude)

Martinez-Delgado, Rix et al 2005

Depth of CMa: r1/2~0.85kpc @ RGC~13kpc

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What is the density profile of CMa?

Butler, Martinez-Delgado, Rix ’05 (in prep.)

galactic plane

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Thin Disk Main Seq.

ThickDisk

Halo

Can Maj Main Seq.

Martinez-Delgado, Rix et al 2005

•Narrow MS (15% depth)

•High-contrast (>3)

•Two distinct (age?) populations

•Distance 8kpc

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• Buit life’s never easy!

• CMa may not be the point of maximal density

Rocha-Pinto et al 2005

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Proper Motions of “Canis Majoris”

Rocha-Pinto et al 2005

Dinescu et al 2005

Warp motion

WCMa=-49+-15 km/s

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What would the Milky Way’s response be to such a disrupting satellite?

sun

(CMa) satellitebelow

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SDSS+SEGUE Sky Coverage

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Near–Term Future Astrometry

• PRIMA: differential astrometry with VLTI– 2008– 10as @ 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<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