ULTRAVIOLET AND RADIO OBSERVATIONS OF MILKY-WAY HALO GAS

High-resolution IUE absorption-line and 21 cm emission-line spectra have been obtained for 56 lines of sight through the Galactic disk and halo. For each line of sight, we present absorption spectra from both high-ionization species (e.g., C IV and Si IV) and low-ionization species (e.g., Si II) that cover a wide range in sensitivity. The stellar Si III lambda-1294 line is also included to aid in the evaluation of stellar contamination problems. The IUE data have a resolution of 25 km s-1 while the 21 cm data have a 1 km s-1 resolution. The data are intended for use as an atlas of interstellar line data through the Galactic disk and halo. Measurements of the interstellar velocities and H I column densities from the 21 cm emission and Ly-alpha-absorption are also included. We analyze the problem of contamination of the interstellar Ly-alpha-absorption line by stellar Ly-alpha-absorption and use this information to reevaluate the vertical distribution of H I. Assuming an exponential distribution, the scale height, midplane density, and logarithmic column density perpendicular to the plane are H = 0.51 +/- 0.10 kpc, n(0) = 0.15 cm-3, and log N(infinity) = 20.38 +/- 0.04. The medium is quite patchy. We present a new method for determining lower limits on the vertical distribution of gas by including information on the velocity structure in the gas. For stars with Absolute value of z > 1 kpc, a vertical column of H I between 0.5 and 2 x 10(19) cm-2 is seen in emission at velocities outside those seen in absorption along most sight lines in the sample. Comparison of the emission and absorption velocity limits provides information on the location of gas above the Galactic plane. In the low halo (i.e., Absolute value of z < 1 kpc) there are approximately equal numbers of positive and negative velocity clouds. However, in the more distant halo (Absolute value of z > 1 kpc) the gas has few additional positive velocity components and numerous additional negative velocity components. The data indicate that peculiar velocities exceeding 40 km s-1 are common in halo gas beyond about Absolute value of z > 1 kpc, while the gas nearer to the plane has smaller peculiar motions. The data for individual sight lines are discussed. Several lines of sight in our sample are in the direction of H I high-velocity clouds (HVCs), but none of the HVCs are seen in absorption. Although many of the inner Galaxy sight lines pass through the estimated position of the 3 kpc expanding arm, the presence of the arm is not clearly revealed in absorption. One of our target stars is projected against H I features usually associated with the Galactic nucleus, but does not show them in absorption. The nuclear features must lie beyond the star, HD 168941, d > 6.3 kpc from the Sun, and z > 0.70 kpc from the midplane.