Structure and star formation in NGC 925

Abstract

We present the results from an optical study of the stellar and star formation properties of NGC 925 using the WIYN 3.5 m telescope. Images in B, V, R, and Hα reveal a galaxy that is fraught with asymmetries. From isophote fits we discover that the bar center is not coincident with the center of the outer isophotes or with the dynamical center (from Paper I in this series). Cuts across the spiral arms reveal that the northern arms are distinctly different from the southern arm. The southern arm not only appears more coherent, but the peaks in stellar and Hα emission are found to be coincident with those of the H I distribution, while no such consistency is present in the northern disk. We also examine the gas surface density criterion for massive star formation in NGC 925, and we find that its behavior is more consistent with that for irregular galaxies than with late-type spirals. In particular, star formation persists beyond the radius at which the gas surface density falls below the predicted critical value for star formation for late-type spirals. Such properties are characteristic of Magellanic spirals but are present at a less dramatic level in NGC 925, a late-type spiral.