Contents

9.2.2 CO lines in emission9.2.3 The MW’s circular-speed curve

CO lines in emission 12CO molecular (2.6mm rotation lines) High angular resolution of CO survey (Table 9.2) Strongly confined to the galactic plane Review Dame et al. 1987(1) more patchy than HI emission(2) prominent features (envelopes, molecular ring, gas dis

tribution with R)(3) R>R0 : HI dominant (spiral arms)(4) ~25 times smaller between spiral arms than in the ar

ms (Digel et al. 1996)

*A composite CO survey of the Entire Milky Way (Dame et al. 1987)

Large-scale CO surveys 1.2m telescope in NW and Chile 10-30°wide in lat. & 0.5°angular resolution- molecular clouds in the inner arms(molecular ring)- spiral arms and local emissions- mean surface density within 1kpc: 1.3 M⊙/pc2

*A composite CO survey of the Entire Milky Way (Dame et al. 1987)

Complete CO survey?

Circular-speed curve 12CO & HI emission key to determining Vc(R) Velocity map radial mass distribution or potential Case 1 : R < R0

- estimate the terminal velocity, V(t)los(l), from (l,v) plot

- V(t)los(l), are good agreement between HI and CO

- small-scale variations with l ~ 7km/s affected by spiral structure?

- difference at fixed l ~ N(0, 4.1km/s) ISC’s random velocity dispersion Burton 1992

- accuracy ~ 10km/s at ± l Table 9.3

)sin()()()( 101)( lRvlvrv clost

tc

*The velocity field of the outer Galaxy (Brand & Blitz 1993)

Present the velocity filed of the outer parts of the Galaxy 2-3kpc from the Sun (inner) and about 17kpc (outer) HII regions/reflection nebulae (photometric distance) &

associated molecular clouds (radial velocities) Found non-circular motions and streams from velocity

residuals The galactic velocity field is axisymmetric

*The velocity field of the outer Galaxy (Brand & Blitz 1993)

Circular-speed curve Case 2 : R > R0 (beyond solar circle)- we cannot determine the Galactocentric Radius, R.

- distance (d) from the Sun obtain R from below eq.

(1) young cluster by main-sequence fitting cluster velocity from associated molecular clouds (Brand & Blitz 1993)

(2) Cepheid variables distance & velocity (Pont et al. 1997)(3) Planetary nebulae and Carbon stars (Fich & Tremaine 1991) Interstellar absorption give rise to errors in the d this error bias the circular-speed curve

)sin()]()([)( 00 lRRRlvlos

ldRdRR cos2 022

02

Circular-speed curve Binney define W,

- Errors in W SIMPLY reflect errors in vlos

- peculiar velocities of tracers >> measurement errors

Error in d error in R measured frequence difference W/R0

In particular, tracers that all lie on a ring of radius R1 (assume) annulus width depends on the magnitude of the errors

)]()([sin 00 RRRl

vW los

RRvconstRif c )( .,)(

Circular-speed curve Brand & Blitz (1993), W Vs. R/R0 Binney & Dehnen (1997), Vc Vs. R/R0

Keplerian rotation? ...rule out

02R R

1)( at

RRvc

Light would be a good tracer of Mass

*The outer rotation curve of the Milky Way ( Binney & Dehnen 1997)

*The outer rotation curve of the Milky Way ( Binney & Dehnen 1997)

Typical plot of Vc(R) of our Galaxy

(Brand & Blitz 1993)

Photometric error?