Minor Galaxy Interactions: Star Formation Rates and Galaxy Properties

Abstract

We study star formation in a sample of 1204 galaxies in minor (| \Delta m_z | \geq 2) pairs and compact groups, drawn from the Sloan Digital Sky Survey Data Release 5 (SDSS DR5). We analyze an analogous sample of 2409 galaxies in major (| \Delta m_z | < 2$) pairs and compact groups to ensure that our selection reproduces previous results, and we use a ``field'' sample of 65,570 galaxies for comparison. Our major and minor pairs samples include only galaxies in spectroscopically confirmed pairs, where the recessional velocity separation $\Delta V < 500$ km/s and the projected spatial separation $\Delta D < 50$ kpc/h. The relative magnitude (a proxy for the mass ratio) of the pair is an important parameter in the effectiveness of the tidally triggered star formation in minor interactions. As expected, the secondary galaxies in minor pairs show evidence for tidally triggered star formation, whereas the primary galaxies in the minor pairs do not. The galaxy color is also an important parameter in the effectiveness of triggered star formation in the major galaxy pairs. In the major pairs sample, there is a correlation between the specific H$\alpha$ star formation rate (SSFR) and $\Delta D$ in the blue primary and blue secondary galaxies; for the red primary and red secondary galaxies, there is none. Galaxies in pairs have a higher mean SSFR at every absolute magnitude compared to matched sets of field galaxies, and the relative increase in mean SSFR becomes larger with decreasing intrinsic luminosity. We also detect a significantly increased AGN fraction in the pair galaxies compared to matched sets of field galaxies.