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Preprint typeset using L ATEX style emulateapj v. 05/04/06 THE Hα GALAXY SURVEY. VIII. CLOSE COMPANIONS AND INTERACTIONS, AND THE DEFINITION OF STARBURSTS

By Johan H. Knapen and Philip A. James

Abstract

We consider the massive star formation properties, radial profiles, and atomic gas masses of those galaxies in our Hα Galaxy Survey that have close companion galaxies, in comparison with a matched control sample of galaxies without companions. Our analysis is based on Hα and continuum images of 327 disk galaxies that form a representative sample of the local Universe. We find that the presence of a close companion raises the star formation rate by a factor of just under two, while increasing hardly at all the equivalent width of the Hα emission. This means that although statistically galaxies with close companions form stars at a higher rate, they do this over extended periods of time, and not as bursts. We find no significant increase in the central concentration of the star formation as a result of the presence of a close companion. The fraction of truly interacting or merging galaxies is very small in the local Universe, at around 2%, and possibly 4 % of bright galaxies. Most of these interacting galaxies currently have unremarkable star formation properties. We also study the properties of the galaxies in the Survey with the most extreme values for star formation indicators such as rate, equivalent width, star formation rate per area, and gas depletio

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