The Distribution and Kinematics of Neutral

Hydrogen Gas and the Radio Continuum

Emission in Barred Spiral Galaxies

By Marc Philip Ondrechen

Under the supervision of Professor Robert Kennicutt

ABSTRACT

This dissertation presents VLA radio synthesis observations of three barred spiral galaxies, NGC 1097, NGC 1365 and M83. Neutral hydrogen spectral line observations were made, as well as continuum observations at wavelengths of 6 cm and 20 cm.

The continuum observations of the bars of NGC 1097 and M83 reveal that the radio emission is coincident with the linear dust lanes in the bars, that it is non-thermal in origin, and is moderately polarized. These data show that the dust lanes are generated by shocks, confirming a major theoretical predication of bar structure.

Spectral line observations of the neutral hydrogen gas reveal many interesting properties of these galaxies. Direct observation of the gas flow in the bar of NGC 1097 has confirmed the presence of non-circular motions, of similar magnitude to those predicted based on the presence of the bar and its associated shocks. Non-circular motions in the spiral arms of NGC 1097 and NGC 1365 were also found. These appear to be due to the presence of spiral density waves in the disks of these galaxies, and are of similar magnitude to those observed in M81, as well-studied example of a galaxy with a density wave. The rotation curves of all three galaxies are flat, as is the case in most observed spiral galaxies.

The distribution of neutral hydrogen gas in the three galaxies has two common properties. In each galaxy, the peak surface densities of hydrogen gas and the optical spiral arms are coincident. Also, there are rings of gas at radii just larger than the bar lengths in each galaxy, with the greatest concentration of gas occurring at the ends of the bars. This may indicate the location of corotation in the galaxies. There is an elliptical "ring" of gas in M83, extending in radius from 7' to 12'. The plane of the neutral hydrogen gas in NGC 1365 appears to be warped from a radius of 5' outward.