a new estimate of the milky way’s dark halo mass
DESCRIPTION
A New Estimate of the Milky Way’s 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, - PowerPoint PPT PresentationTRANSCRIPT
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 tracers
Battaglia, 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 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 >90%
Distances 5-10%Stars are metal
poor
solid line---BHB Star
dotted line---Blue Straggler star
SEGUE 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