Laura Danly

A determination of the distance to the high-velocity cloud complex M

The high-velocity cloud complex M has been detected in absorption toward the halo star BD +38°2182 (z=4.4 kpc) with the International Ultraviolet Explorer (IUE) echelle spectrograph. Absorption is seen out to −120 km s −1 in the saturated resonance lines of C II, Si II, and O I. Data on highly ionized species of C IV and Si IV are too noisy to determine the presence or absence of absorption from highly ionized gas at the velocities of complex M. The BD +38°2182 data are compared to similar data for HD 93521, which lies 27 arcmin away, but at a z-distance of only 1.5 kpc

The kinematics of Milky Way halo gas. I. Observations of low-ionization species

Ultraviolet interstellar line day observed with the IUE toward 70 halo stars and four extragalactic sight lines are analyzed in a study of the large-scale kinematic properties of the Milky Way halo gas. The motions of the low-ionization gas is focused on. Large systematic velocities are found, and a pronounced asymmetry in the absorption characteristics of halo gas toward the Galactic poles is indicated. In the north, substantial amounts of material are falling toward the disk at velocities up to about 120 km/s in the most extreme case. Toward the south, low-ionization material shows no extreme or systematic motions. 97 refs.

THE IONIZATION CONDITIONS IN THE MILKY WAY HALO: INFALLING GAS TOWARD THE NORTH GALACTIC POLE

The observations of gas in the Milky Way are reviewed with an eye toward the theoretical predictions of the Galactic Fountain model for the production of halo gas. Data are shown which indicate significant variations in the ionization conditions in infalling halo gas in the northern galactic hemisphere. Besides being interesting in its own right, understanding the nature of Miky Way halo gas plays a critical role in interpreting QSO absorption lines in the study of galaxies at high redshift.

The formation of high‐ionization species associated with Galactic, intermediate velocity H I

We report first results on the photoproduction of the species H I, Al III, Si IV, and C IV kinematically associated with the Galactic intermediate‐velocity (IV) arch. We find that photoionization of solar abundance gas cannot reproduce the observed column densities of these species, while models using stellar radiation fields and the observed x‐ray background can reproduce the observed columns in gas having depleted metal abundances. The pattern of metal depletions required is consistent with their incorporation into standard interstellar grains. Limits on the intensity of the halo radiation field indicate that the material of the IV arch is smoothly distributed, having a local density ∼10−2 cm−3 and filling factor of order unity. These results suggest that photoionization plays a key role in the production of at least some of the high‐ionization species Si IV and C IV in the Galactic halo.

An absorption line detection of the High Velocity Cloud Complex M

The High Velocity Cloud Complex M has been detected in absorption toward the halo star BD +38 2182 with the International Ultraviolet Explorer (IUE) Echelle spectrograph. Absorption is seen out to −120 km/s in the saturated resonance lines of C II, Si II, and O I. The absorption line detection toward BD +38 2182, along with the non‐detection at v<−100 km/s toward the halo star HD 93521 which also lies in the direction of M II (27 arcmin away from BD +38 2182), places the distance to Complex M at 1.5

HIGH-RESOLUTION SPECTROSCOPY OF THE INTERSTELLAR MEDIUM

High-resolution spectroscopy of interstellar absorption and emission lines provides information on the physical conditions and dynamical evolution of the ISM in our Galaxy and other nearby galaxies. We describe several examples of applications, and briefly address the impact of ISM applications on instrument design.