A New Estimate of the Milky Way

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A New Estimate of the Milky Ways Dark Halo Mass

• Motivation
• Methods
• Results

• Xiangxiang Xue
• Hans-Walter Rix , G. Zhao, P. Re Fiorentin, T.
  Naab, M. Steinmetz, E. F. Bell, F. C. van den
  Bosch, T. C. Beers, R. Wilhelm, Y. S. Lee,
  C. Rockosi, B. Yanny,
• H. Newberg, X. Kang, M. C. Smith, D. P.
  Schneider

Dec 3 2008 KIAA-Cambridge Joint Workshop
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Why to estimate the MW halo mass?

• Milky Way properties scale with halo mass
• Mstar /Mhalo ? cooled baryon fraction
• Number of expected sub-halos
• The poorly known Galactic parameter
• Recent lit. values 0.82.5 x 1012 M?
• Are all satellites bound?

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How to estimate the MW halo mass?

• Basic approach
• Assemble a large and well defined set of distant
  kinematic tracers from SDSS DR6
• blue Horizontal Branch Stars with
• 5 distances to D60 kpc
• dv 10 km/s Fe/H estimates
• Compare to kinematics in simulated halos that
  have been scaled to different halo mass
• derive p(vlos) at different rgc
• model it to get vcir(r)

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Selection of the clean BHB sample

• Pre-selected by color (Yanny et al 2000)
• Measure Balmer line profile parameters
• (cf Sirko et al 2004, Xue, Rix et al 2008)

• identification gt90
• Distances 5-10
• Stars are metal poor

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Spatial, velocity and Fe/H distributions of
BHBs
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How to estimate the MW halo mass?

• Modelling the BHB kinematics with simulations
• make mock observations from within the
  output of the cosmological (Milky Way-like)
  galaxy simulations, and then match P(Vlos
  /Vcirr) to give Vcir,obs(r), and ultimately Mvir

• use simulations from two different groups
  (Steinmetz, Naab)
• same volume as SDSS DR6
• derive P(Vlos/Vcir, r) for simulated halo stars
• get P(Vlos/Vcir, r) for observed halo BHB stars
• matching the distributions gives estimate of
  Vcir,obs(r)
• also use good ole Jean Eq.

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Comparison of P(Vl.o.s/Vcir) in radial bin
15.0,20.0 kpc
Psim(Vlos, / Vcir),
? Construct estimate of Vcir (r)
Pobs(Vl.o.s,/Vcir) if vcir(obs)180km/s
P(Vlos/Vcir, obs) P(Vlos/Vcir, sim)
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• Vcir(r) derived by Jeans Equation

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Estimate the DM halo mass

• NFW DM halo Hernquist bulge exponential
  disk
• Rotation curve matches
• Both contracted and uncontracted halos match
• Mvir 1.0 0.3 1012 M?

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Result

• Robust measurement (2simsJeans Eq.)
  
• M (rlt60 kpc) 4.00.71011 M?
• Vcirc(R) is not constant but gently falling, and
  matches either contracted or uncontracted NFW
  profile
• If DM halo is NFW then
• Mvir (275kpc) 1.0 0.3 1012 M?
• consistent with previous estimates,
  but more precise
• Imply (high) 40 of baryons end up as stars
• LMC and other satellites marginally bound

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Thank You