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HWR Princeton 2005 IV: Milky Way / Local Group Tomography Hans-Walter Rix MPI for Astronomy...
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![Page 1: 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.](https://reader035.fdocuments.us/reader035/viewer/2022062409/56649cb05503460f949743b6/html5/thumbnails/1.jpg)
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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HWRPrinceton 2005
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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HWRPrinceton 2005
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HWRPrinceton 2005
2. Seeing Galaxies: 2D, 6D or (the right) 3D
P. Harding
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HWRPrinceton 2005
Bullock, Kravtsov, Weinberg 2002
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HWRPrinceton 2005
..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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HWRPrinceton 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)?
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HWRPrinceton 2005
Is (sub-)structure of the phase-space distribution observable in galaxies with unresolved stellar populations?
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HWRPrinceton 2005
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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HWRPrinceton 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
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HWRPrinceton 2005
NGC 4473: orbital structureCappellari, de Zeeuw in prep.
Counter-rotating starsMain galaxy rotation
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HWRPrinceton 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
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HWRPrinceton 2005
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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HWRPrinceton 2005
M 31 Status QuoLewis et al 2004
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HWRPrinceton 2005
4. Substructure in the Milky Way Halo
• How to find it?
• Status quo
• How to interpret what has been found?
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HWRPrinceton 2005
A clear case: the Sagittarius stream
Majewski et al 2003
Ibata et al 1995
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HWRPrinceton 2005
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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HWRPrinceton 2005
SEGUE (unpublished)
“Hess diagrams” as diagnostic tools
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HWRPrinceton 2005
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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HWRPrinceton 2005
THE FIRST SCENARIO: TIDALLY DISRUPTING DWARF GALAXY
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HWRPrinceton 2005
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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HWRPrinceton 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
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HWRPrinceton 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
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HWRPrinceton 2005
The Wilky Way’s Low-Latitude Ring(Monoceros, Tri/And, CMa, etc…)
Penarrubia, Rix et al 2005
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HWRPrinceton 2005
Penarrubia, Rix et al 2005
Spatial Distribution:
wide z-range; little R range
? Not a warp Peñarrubia 2004
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HWRPrinceton 2005
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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HWRPrinceton 2005
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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HWRPrinceton 2005
Thin Disk Main Seq.
ThickDisk
Halo
Can Maj Main Seq.
Martinez-Delgado, Rix et al 2005
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HWRPrinceton 2005
saturation
V,0 ~ 24
(l,b)=(240,-8)
Martinez-Delgado, Rix, et al 2004
(see also Bellazzini et al 2004)
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HWRPrinceton 2005
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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HWRPrinceton 2005
What is the density profile of CMa?
Butler, Martinez-Delgado, Rix ’05 (in prep.)
galactic plane
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HWRPrinceton 2005
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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HWRPrinceton 2005
• Buit life’s never easy!
• CMa may not be the point of maximal density
Rocha-Pinto et al 2005
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HWRPrinceton 2005
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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HWRPrinceton 2005
What would the Milky Way’s response be to such a disrupting satellite?
sun
(CMa) satellitebelow
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HWRPrinceton 2005
SDSS+SEGUE Sky Coverage
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HWRPrinceton 2005
Near–Term Future Astrometry
• PRIMA: differential astrometry with VLTI– 2008– 10as @ 17 mag across 30”
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HWRPrinceton 2005
6. GAIA 2012---
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HWRPrinceton 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