The H I halo in the inner galaxy

There is 21 cm emission from corotating H I in the inner Galaxy to |z| ≥ 1000 pc from the plane. Over most of the inner Galaxy more than 10% of the H I emission at the subcentral points comes from |z| > 500 pc. High-z H I is not present ⪉3 kpc from the galactic center, but appears suddenly near R = 3.5 kpc, in qualitative agreement with Bregman's model for a wind from the bulge. The scale height and surface density of the halo are approximately constant over 4 ≤ R ≤ 8 kpc. In this region nH(z) has a long exponential tail with a scale height ≡500 pc and a density of 0.012 cm⁻³ at |z| = 750 pc.