Astronomy and Astrophysics – Astrophysics
Scientific paper
1994-11-21
Astrophys.J.448:138-148,1995
Astronomy and Astrophysics
Astrophysics
17 pages, latex file without figures (uses aaspp style file). The postscipt version can be obtained by ftp:/astro.princeton.ed
Scientific paper
10.1086/175946
We present full modeling of tangent point emission of HI as seen in the 21 cm transition in the inner Galaxy ($R \simeq 3-8 \kpc$). The model used takes into account emission from a large path length along the line of sight, corresponding to an interval ($\Delta R$) of typically $\le 1 \kpc$ in galactic radius; and is parametrized by the scale height of the gas, the centroid in z, the rotation velocity and the velocity dispersion. These parameters are assumed to be constant over the interval $\Delta R$. This modeling is carried out for the 21 cm surveys of Weaver \& Williams(1974), Bania \& Lockman (1984), and Kerr et al. (1986) to measure these parameters. The terminal velocity values are found to be in good agreement with previous measurements. The velocity dispersion is constant with radius at the theoretically expected value of $9 \pm 1 \kms$. The Gaussian scale height of the HI layer increases with Galactocentric radius. The centroid of the layer deviates significantly from $z=0$. Apart from small local variations, the velocity dispersion $\sigma_v$, scale height $\sigma_z$ and the centroid $z_0$ show similar variations in the first and the fourth quadrants. On balancing the turbulent pressure support of the layer against the disk gravitational potential, we confirm that additional support is needed for the HI layer. The radial profile of the reduced midplane mass density is an exponential with a scale length of $3.3 \pm 0.3 \kpc$. This picture is consistent with a constant mass-to-light ratio of the disk, and extra pressure support for the HI layer which is constant with radius in the inner Galaxy.
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