The detailed distribution and kinematics of the atomic and the CO-bright molecular hydrogen in the disc of the Milky Way inside the Solar circle are derived under the assumptions of axisymmetry and pure circular motions .
We divide the Galactic disc into a series of rings , and assume that the gas in each ring is described by four parameters : its rotation velocity , velocity dispersion , midplane density and its scale height .
We fit these parameters to the Galactic H I and ^ { 12 } CO ( J=1-0 ) data by producing artificial H I and CO line-profiles and comparing them with the observations .
Our approach allows us to fit all parameters to the data simultaneously without assuming a-priori a radial profile for one of the parameters .
We present the distribution and kinematics of the H I and H _ { 2 } in both the approaching ( QIV ) and the receding ( QI ) regions of the Galaxy .
Our best-fit models reproduces remarkably well the observed H I and CO longitude-velocity diagrams up to a few degrees of distance from the midplane .
With the exception of the innermost 2.5 { kpc } , QI and QIV show very similar kinematics .
The rotation curves traced by the H I and H _ { 2 } follow closely each other , flattening beyond R = 6.5 { kpc } .
Both the H I and the H _ { 2 } surface densities show a ) a deep depression at 0.5 < R < 2.5 { kpc } , analogous to that shown by some nearby barred galaxies , b ) local overdensities that can be interpreted in terms of spiral arms or ring-like features in the disk .
The H I ( H _ { 2 } ) properties are fairly constant in the region outside the depression , with typical velocity dispersion of 8.9 \pm 1.1 ( 4.4 \pm 1.2 ) { km s } ^ { -1 } , density of 0.43 \pm 0.11 ( 0.42 \pm 0.22 ) { cm ^ { -3 } } and HWHM scale height of 202 \pm 28 ( 64 \pm 12 ) { pc } .
We also show that the H I opacity in the LAB data can be accounted for by using an ‘ effective ’ spin temperature of \sim 150 { K } : assuming an optically thin regime leads to underestimate the H I mass by about 30 \% .