Interactions of galaxies outside clusters and massive groups
  We investigate the dependence of physical properties of galaxies on small and large scale density environment. The galaxy population consists of mainly passively evolving galaxies in comparatively low density regions of Sloan Digital Sky Survey (SDSS). We adopt (i) local density, $\rho_{20}$, derived using adaptive smoothing kernel, (ii) projected distance, $r_p$, to the nearest neighbor galaxy and (iii) the morphology of the nearest neighbor galaxy as various definitions of environment parameters of every galaxy in our sample. In order to detect long-range interaction effects we divide galaxy interactions into four cases depending on morphology of target and neighbor galaxies. This study builds upon an earlier study by Park & Choi (2009) by including improved definitions of target and neighbor galaxies thus enabling us to better understand the effect of "the nearest neighbor" interaction on target galaxy. We report that the impact of interaction on galaxy properties is detectable at least out to the pair separation corresponding to the virial radius of (the neighbor) galaxies. This turn out to be mostly between 210 and 360 $h^{-1}$kpc for galaxies included in our study. We report that early type fraction, for isolated galaxies with $r_p > r_{vir,nei}$ are almost ignorant of the background density and, has a very weak density dependence for closed pairs. Star formation activity of a a galaxy is found to be crucially dependent on neighbor galaxy morphology. We find star formation activity parameters and structure parameters of galaxies to be independent of the large scale background density. We also exhibit that changing the absolute magnitude of the neighbor galaxies does not affect significantly the star formation activity of those target galaxies whose morphology and luminosities are fixed. 
Jaswant K. Yadav, Xuelei Chen
astro-ph.GA astro-ph.CO