( 東京 )
Nobuyuki Sakai, National Astronomical Observatory of JapanOctober 9, 2014@EVN Symp. 2014 in Italy
Direct Comparison between Astrometry
Results and an Analytic Dynamics Model
Context : Theory 1/9
Density wave theory Density wave theory (Lin & Shu 1964)(Lin & Shu 1964)
Recurrent and transient spiralRecurrent and transient spiral(e.g., Miller+1970; many papers)(e.g., Miller+1970; many papers)
©Ingo Berg Fujii et al. 2011:N-body simulation
・ Spiral arm is the most prominent structure in a disk galaxy.
・ However, nature, origin and evolution are still unknown.
Context : 1-D observation 2/9
Perseus arm(?)Sun
G.C.
(Burton 1973)
(Burton 1973)●Distribution of line-of-sight velocity is asymmetry in the MW.→ Effect of spiral arm ?
●The observed velocity field might be explained by gravitational perturbation (i.e., spiral arm) based on the density-wave theory.
Systematic non-circular motion might be explained by the density-wave theory !
3D non-circular motion (U, V, W) in Reid+09
G.C.
VU
Right-handedcoordinate system
Averaged non-circular motions with 8 sources in the Perseus arm (U_mean, V_mean) = (9.8+/-2.6, ー 17.8+/-2.5) km/s in Sakai et al. (2012) →Systematic inward motion and slower Gal. rotation
V
(to
war
d G
al.
rota
tion
) [k
m/s
]
U (toward G.C.) [km/s]
Inter (no) armOuter arm
Perseusarm
Flat rotation assumed.
Perseus arm
Choi et al. (2014) found the same tendency with 25 sources !
Context : 3-D3-D observation 3/9
Goals of this research 4/9
Churchwell et al. 2009
Perseus armSun
G.C.
①We compare astrometry (3D) results for the Perseus arm with an analytic gas dynamics model for deriving physical parameters of the spiral arm.
②Using the results, we make a suggestion toward an incoming astrometry (e.g., Gaia).
Ultimate Goal・ We understand nature, origin and evolution of the spiral arm especially for the Milky Way Galaxy.
Least Squares Method 5/9
χ^2χ^2 was calculated.was calculated.
ObservablesObservables(U, V) for 27 sources in the Perseus arm
Spiral model (density wave)Spiral model (density wave)・ Pinol-Ferrer+12 (analytic)
・ Equations of motion with gas friction term added.
Search Range
Ref.
Ωp Patternspeed
10 – 30km/s/kpc
Gies & Helsel 05
i Pitch angle
11.1 ° Sakai+14b in prep
Φa Relative Amp.
10-50 % Grosbol+04
Rref Reference 1 period
m Mode 2 or 4 Vallee 14
λ Frictionterm
1-30km/s/kpc
ε Artificialterm
1-30km/s/kpc
Preliminary Results 6/9
SearchRange
Solution
Ωp Patternspeed
10 – 30km/s/kpc
i Pitch angle
11.1 °
Φa Relative Amp.
10-50 %
Rref Reference 1 period
m Mode 2 or 4
λ Frictionterm
1-30km/s/kpc
ε Artificialterm
1-30km/s/kpc
SearchRange
Solution(error)
Ωp Patternspeed
10 – 30km/s/kpc
17.6(0.6)
i Pitch angle
11.1 ° 11.1°
Φa Relative Amp.
10-50 % 50%
Rref Reference 1 period 9.4 kpc
m Mode 2 or 4 2
λ Frictionterm
1-30km/s/kpc
16.8(2.2)
ε Artificialterm
1-30km/s/kpc
4.0(0.7)Rref
Χ^2
Rref = 9.4 +/- 0.1 (kpc)
Discussions : Offset bet. gas and starGas orbitsmodel
7/9
Spiral potential
Perseus arm ?
SunDirect comparison between astrometry and a model・ Suggestion :Stellar distribution as the bottom of the spiral potential is deviated from the dense gas region.------> Incoming astrometry (e.g., Gaia+VLBI) is important !
G.C.
Discussions : Ice Age Epochs
8/9
・ Ωp = 17.6 +/- 0.6 km/s/kpc ・ Ω_sun = 28.8 (Reid+14)・ m = 2 for main arms and 4 for gas arms (Churchwell+09)
---------> Period of an arm passage = 2π / m(Ω_sun – Ωp ) = 137 +/- 7 Myr
Shaviv & Veizer 2003
~140 Myr. ~140 Myr. ~140 Myr.
18O/16O
Veizer 2000
Conclusions 9/9
Astrometry results vs Gas dynamics model
We made two suggestions for the spiral arm based onthe comparison betw. astrometry results and a dynamics model.
①Offset betw. Gas and Star
②Relation betw. Ice ege epochs and arm passages
We can judge various Galactic dynamics models by incoming astrometry !
20
Thank you for your attention!
(c)Miyaji-san
This is not the end.It is not even the beginning of the end.But it is, perhaps,the end of the beginning.by Sir Winston Churchill
Top Related