The extent of the local \hi halo
F. J. Lockman, L. M. Hobbs, J. M. Shull;
ApJ, 1986, 301, 380
ABSTRACT:Forty-five high-latitude, OB stars have been observed in the Ly alpha and 21 cm lines of HI in an effort to map out the
vertical distribution and extent of the local HI halo.
The 25 stars for which a
reliable HI column density can be obtained from Ly alpha lie between 60 and 3100 pc
from the plane.
The principal result is that the total column density of HI
at an absolute value of z greater than 1 kpc is, on the average, 5 + or - 3 x 10 to
the 19th power/sq cm, or 15 percent of the total sub HI.
At relatively low z
the data toward some stars suggest a low effective scale height and fairly
high average foreground density, while toward others the effective scale
height is large and the average density is low.
This can be understood as the
result of irregularities in the interstellar medium.
A model with half of the
HI mass in clouds having radii of a few pc and a Gaussian vertical
distribution with sigma sub 2 = 135 pc, and half of the mass in an exponential component
with a scale height of 500 pc, gives a satisfactory fit to the data.
The
technique of comparing Ly alpha and 21 cm column densities is also used to discuss
the problem of estimating the distance to several possibly subluminous
stars.
KEYWORDS: halos, hydrogen, interstellar gas, lyman alpha radiation, radio sources (astronomy), luminosity, milky way galaxy, ultraviolet spectra, vertical distribution
PERSOKEY:21 cm, h_i, uv, absorption, milky way, ,
CODE: lockman86