Transcript triaxial

The asymmetric structure of the
Galactic halo ----progress report
Xu Yan
Supervisor: Deng LiCai
 Motivation
 Observational data
 Theoretical model
 Result and analysis
 Conclusion
Motivation
 A central goal of star counts is to tell us how the Galaxy
functions and how it was assembled.(Binney 2004)
 SDSS is five-color(u’,g’,r’,i’,z’) wind-band CCD system with
wavelength coverage from 3000 to 11000Å, DR4 imaging
catalogue covers 6670 square degrees.Complete to 95%
detection repeatability for point sources below limiting
apparent magnitude 22m, 22.m2, 22.m2, 21.m3,
20.m5 .(www.sdss.org)
 The standard model, Bahcall(1986)
 Blitz(1991) proposed that a rotation triaxial spheriod is the
source of the large-scale asymmetries observed in the L-V
diagram of the HI distribution in the outer Milky Way.
Observational data
Theoretical model
Ai (m1 , m2 , l , b)d
m2

m1
0
  dm dR R  i (r , M )i ( M )d
2
AT  Ad  Atd  As
Bahcall(1986)
d (r, M )  exp[  z / H (M )  x / h]
z  R  sin( b)  z0
t  r02  R 2  2  R  r0  cos(b)  cos(l )
x1  x  cos( )  y  sin(  )
y1   x  sin(  )  y  cos( )
r  x32  ( y 3 / p ) 2  ( z 3 / q ) 2
a0n  R0n
 (r )  n n
a0  r
 V band luminosity function of Robin(1986) is adopted in our
model, which provides two dimensional distribution in both
luminosity and spectral type good enough up to 12m.
 Fainter than 12m, I band luminosity function based on HST
deep photometry (Zheng 2004) are applied.
 Cox(1999), I band to V band, g=V+0.53(B-V)-0.075(Chen
2001) is adopted here.
 CMR (Girardi 2004)
thin disk, thick disk, halo: 4.5, 11, 13Gyr,
z= 0.19, 0.004,0.0012
Result and analysis
 Fit surface density
 Fit distribution in (g-r)
2
(
R

S
)
2   i i
Ri  Si
i
(0,60),(180,60)
(30,65),(330,65)
(60,60),(300,60)
(90,60),(270,60)
(120,60),(240,60)
(150,60)

Starcounts of the observational data of the Milky Way adopted in the
present work show large-scale asymmetric in space.

The phenomenon may be caused by two reasons. One possibility is
orign halo itself has asymmetric structure.

Triaxial dark halo is not individual case in extragalactic system.
Mazzei, P. \& Curir A. (2001) analysized that triaxial dark halo effect
star formation of bar trigger and feedback process after formation of
disk stars. They indicated that dark halo has important effect
toevolution of barion matter. And star formation rate not only has
relation to total mass of dark halo but also to dynamical state of dark
halo.Then if it is normal to form a asymmetric star halo in the
asymmetric gravitational field?

Newberg(2005) selected F star (therefore turnoff stars) data in 8
SDSS stripes from DR3 to map the distribution of stars in real space.
They selected fields that most likely contain only halo star. Sky areas
near any previously identified over density caused by star streams
were also avoided. After compared observational data with the results
of 5 triaxial models, they obtained the best fit parameter set for an
oblate ellipsoid with its major axis pointed to 50°-70° from the line of
sight of Galactic center from the Sun, and with a 4°-6° inclination
with respect to the Galactic plane. The minor axis z measures 65\% of
major axis. The intermediate axis y is about 75\% of the major axis
 Another source to cause triaxiality of the halo may be large-scale star
streams. In the histroy of formation and evolution, the halo of the Milky
Way experience accrete and merge process. Wyse (2005) claimed that
merger events and accretions of satellite galaxies in the early history of
the Galaxy do play some role in the evolution of the Milky Way. It might
especially be the case for the outer halo to be dominated by
substructures that are likely the remnants of interactions.
 Also using DR4 data Jurić et al (2005) analyzed the structure of the
Galaxy using the photometric parallax method. They claimed that there
are significant over-densities in (5 < Z/KPC < 15), with the over-density
peaked in the direction of constellation Virgo, and the whole overdense region covers about one thousand square degrees of sky. They
selected sky field of the overdensity and compared sky field on the
other side of meridian in figure 24 of Jurić et al(2005). Stars of 0.2 <g-r
< 0.8 and 18 < r < 21.5 of the two fields are compared on CMD,
residual fluxs are integrated to estimate the surface bright and
luminosity of the overdensity.

Based on this result, we calculate the increased star number by this
overdensity on each square degree. The magnitude threshold of Jurić et al
(2005) is 18 to 21.5mag. Assume the distance of the overdensity is 10KPC just
like the estimated value adpoted Jurić in et al(2005).
 Then lower and upper limit of r band absolute magnitude of stars is
from 6.5 to 3mag. From transfer relation r=V-0.49(B-V)+0.11, V band limit
magnitudes can be obtained. Relation of V and (B-V) also obtained from
Cox(1999). Then mass - V magnitude relation (Reid 2002) is used to calculate
mass of luminousest 1.44793M☉and faintest star 0.809622M☉ that can be
observed in the magnitude threshold. Integrating luminosity-mass ratio weighed
by m* IMF (Reid 2002), increased star number per square degree can be
calculated:
N  Ltot /( c  ( L / m)m(m)dm)

The number 614 stars per square degree which is enough to provide the extra
part of asymmetric structure. But there is still a problem. If the triaxial orign halo
is realexist, it will can not seperate from virgo star stream by the selection
criterion in figure 24 of Jurić et al(2005). So the difference of star counts in
CMD which is used to estimate surface brightness and luminosity will be
contaminated by the overdensity profile of asymmetric orign halo.
conclusion
 It can be found that stellar projected surface density of halo
star of L from 180 to 360 is significantly higher than that of
L from 0 to 180 in the scope of SDSS magnitude limit.
 Fit observational stellar projected surface density using
triaxial halo density is much better than fitting of axis
symmetric model.
 Parameter combinations of best fit triaxial model are that
power law index is from
2 to 2.5, θis from 55 to 75, ξ,φare less than 0,
p, q are 0.5, 0.7 or 0.6, 0.8. It is unexpected that p is less
than q.
 Though the projected surface number density profile of the
12 groups can be fitted well by triaxial halo model. But it
can't be concluded that the asymmetry must reflect
thetriaxiality of the origin halo of Milky Way.
 How much is the propotion of star stream on the whole star
halo? If the star streams are the reason of the triaxiality or
they are only the disturbance to triaxial profile? We can't
obtain clear answer from present data.
 So further work will be push forward significantly by
spectroscopic sky survey projects such SEGUE, LAMOST,
GAIA and south sky survey.