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Table 1

Observed spiral arm parameters at the Perseus, Norma, and Crux tangencies.

Spiral arm Perseus Norma Crux1
tangency
longitude l = 336.̊0−338.̊0 327.̊0−329.̊5 304.̊9−305.̊9
Emissions: Vpeak (km s -1 )
[C ii]–WIM −126 −99.5 −52
[C ii] (molecular)2 −113 −91.5 −35
12CO −117 −91 −32.5
H i −118 −92 −38
Emissions: Peak brightness T mb (K)
[C ii]–WIM 0.56 0.35 ~0.94
[C ii](molecular) 0.58 0.35 5.0
12CO 3 2.03 2.12 4.8
H i 70 118 106
Galactocentric distance of emission layers (kpc) 4
[C ii]–WIM 3.45 4.57 6.59
12CO (mid-plane) 3.60 4.76 7.20
H i 3.58 4.74 7.01

Notes.

(1)

This longitude range is offset from the true tangency by ~2°. Our data are only available for this longitude range.

(2)

This identification with H2 gas is used just to distinguish it from the compressed WIM. However, in addition to excitation by H2 , this component may include some [C ii] excited by atomic H or electrons in the diffuse ionized medium.

(3)

Uncorrected for main-beam efficiency.

(4)

From the Galactic center as determined from the VLSR and Galactic rotation velocity.

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