Mapping the Milky Way’s halo with BHB and K giant stars. Xiangxiang Xue (Max-Planck-Institute for Astronomy)
Mapping the Milky Way’s halo with BHB and K giant stars
Xiangxiang Xue (Max-Planck-Institute for Astronomy)
collaborators: Hans-Walter Rix, Heather Morrison, Paul Harding, Real(Zhibo) Ma, Bill Janesh, Timothy C. Beers, Young Sun Lee, Eric F. Bell, Constance Rockosi, Brian Yanny, James S. Bullock and Kathryn Johnston
need kinematic tracers with r(α,δ,D), vlos and [Fe/H] to
Mvir (Halo) vs. e.g. Mstar -->> baryon fraction in stars
Scale expected number of dark matter sub-halos
Dynamics of the local group -->> M31 infall, LMC bound?
Signposts of hierarchical formation
for that, we need tracers which are luminous and have good distances:
BHB stars(Mg~0.7) have good distances, but are very old.
K-giants (-3<Mr<1) are more representative, but distances are less precise.
BHB stars in SDSS （Xue et al. 2008, 2011,Deason et al. 2012)
So, they have precise distances ~ 5%, but metallicities are only approximate.
SEGUE K giants
with Gaussian errors
Prior (=external) information
Results for 3 stars L(DM)
no [Fe/H] prior
no lum. prior
How good are the distances?
The impact of priors
BHBs with distance precision of 5%
r~60 kpc, δv~7 km/s, [Fe/H]
derive p(vlos) at different r
model it to get Vcir(r)
Results:(Xue, Rix et al 2008)
Xue et al 2012 (in prep)
no Sgr stream r>15kpc
is defined as distance between two stars in 4-dimension space (α,δ,d,vlos)(Starkenburg et al. 2009)
Compare to smooth model to quantify the substructure.
Results: (Xue, Rix et al. 2011)
BHB stars are overrepresented in the oldest sub-populations of the stellar halo
turn to K giants !!
in less metal poor stars
in K giants (vs BHB)
in distant halo
Sgr dominates substructure signal