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This document consists of two very different projects but the common thread is in the interest of magnetic fields. It describes the effect of magnetic fields in two Interstellar Medium regions in the Galaxy. Electromagnetic force is one of the four fundamental forces in physics. It is not known where magnetic field has initially risen in the Universe, but what is certain is that it has significant effect in the dynamics of star formation and galaxy formation. The studies aim to better understand the effects of field in an active star forming region and in the halo of the Galaxy. We observed the HI 21 cm spectral line via the Zeeman effect in attempt to detect line-of-sight magnetic field strengths in both of the projects. For the star forming region project in Chapter 2, towards the Eagle Nebula, an upper limit of the field strength was determined. From the observational results, physical conditions of the region were modeled. For the second project in Chapter 3, we attempted to detect magnetic fields via Zeeman effect towards non galactic disk objects. All of the observed positions have radial velocities that cannot be explained by the simple galactic rotation. Hence, they are considered to be non galactic disk sources and often grouped as High Velocity Clouds. With a unique observational technique and analysis, we derived the best fit line-of-sight magnetic fields. A particular interest to us is the Smith Cloud. From the detection of magnetic field, we attempted to estimate the density of the ambient medium in the halo, which will be useful for studying the galaxy formation.

The interstellar medium (ISM) of our Galaxy, and of others, is pervaded by ultra low-density gas and dust, as well as magnetic fields. Embedded magnetic fields have been known to play an important role in the structure and dynamics of the ISM. However, the ability to accurately quantify these fields has plagued astronomers for many decades. Unfortunately, the experimental techniques for measuring the strength and direction of magnetic fields are few, and they are observationally challenging. The only direct method of measuring the magnetic field is through the Zeeman effect. The goal of this dissertation is to expand upon the current observational studies and understanding of the effects of interstellar magnetic fields across various regions of the Galaxy. Zeeman effect observations of magnetic fields in two dynamically diverse environments in the Milky Way are presented: (1) An OH and HI absorption line study of envelopes of molecular clouds distributed throughout the Galaxy, and (2) A study of OH absorption lines toward the Galactic center region in the vicinity of the supermassive black hole Sgr A*. We have executed the first systematic observational survey designed to determine the role of magnetic fields in the inter-core regions of molecular clouds. Observations of extragalactic continuum sources that lie along the line-of-sight passing through Galactic molecular clouds were studied using the Arecibo telescope. OH Zeeman effect observations were combined with estimates of column density to allow for computation of the mass-to-flux ratio, a measurement of the gravitational to magnetic energies within a cloud. We find that molecular clouds are slightly subcritical overall. However, individual measurements yield the first evidence for magnetically subcritical molecular gas. Jansky VLA observations of 18 cm OH absorption lines were used to determine the strength of the line-of-sight magnetic field in the Galactic center region. This study yields no clear detections of the magnetic field and results that differ from a similar study by Killeen, Lo, & Crutcher (1992). Our results suggest magnetic fields no more than a few microgauss in strength.

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