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

Motivation

Methods

Results

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 0.8–2.5 x 1012

Mͽ

Are all satellites bound?

Why to estimate the MW halo mass?

How to estimate the MW halo mass?

Blitz 1990’s (HI)Dehnen&Binney 1998

~200 discrete tracersBattaglia, Helmi et al

2006

15kpc

Basic approach:a)Assemble a large and well defined set of distant kinematic tracers from SDSS DR6

blue Horizontal Branch Stars with 5% distances to D~60 kpcv ~ 10 km/s + Fe/H estimates

b)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)

Selection of the “clean” BHB samplePre-selected by color

(Yanny et al 2000) Measure Balmer line

profile parameters (cf Sirko et al 2004, Xue, Rix et

al 2008)

identification >90%

Distances 5-10%Stars are metal

poor

solid line---BHB Stardotted line---Blue Straggler

starSEGUE Survey SpectraLine Shape

Parameters

2400 halo BHB stars

Spatial, velocity and [Fe/H] distributions of BHBs

velocity distribution

metallicity distribution

velocity dispersion

spatial distribution

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 /Vcir|r) to give Vcir,obs(r), and ultimately Mvir

How to estimate the MW halo mass?

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.]

Red dots are halo BHB stars , while Black dots are simulated halo stars

Vesc(r) Vesc(r)Vcir(r) Vcir(r)

Mhalo ~ 2 × 1012 MͽMhalo ~ 1012 Mͽ

P(Vlos/Vcir)

Comparison of P(Vl.o.s/Vcir) in radial bin [15.0,20.0] kpc

Psim(Vlos, / Vcir),

Pobs(Vl.o.s,/Vcir) if vcir(obs)=180km/s

Construct estimate of Vcir (r)

P(Vlos/Vcir, obs) = P(Vlos/Vcir, sim)

Vcir(r) derived by Jeans Equation

First, relate σlos,obs(r) to σr(r)

Then, use Jeans Equation for

Use observed (photometric) halo profile ρ*~r-3.5

Estimate Vcir(r)

1. radially anisotropic case, β=0.37 (simulations)2. radially isotropic case, β=0.0

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 Mͽ

Result Robust measurement (2sims+Jeans Eq.)

M (r<60 kpc) = 4.0±0.7×1011 Mͽ

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 Mͽ

consistent with previous estimates, but more precise

Imply (high) 40% of baryons end up as stars

LMC and other satellites marginally bound

V3D,LMC=378 km/s +- 18km/s(Besla et al 2007)

Thank You