Galaxy-galaxy interactions in different environments

ALONSO, M. S.; TISSERA, P. B.; LAMBAS, D. G.; COLDWELL, G.

Pucón, Chile

Congreso; XI IAU Regional Latin American Meeting of Astronomy; 2005

Sociedad Chilena de Astronomía

We analyse the star formation rates derived from photometric and spectroscopic data of galaxies in pairs in different environments selected from the 2dF Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey (SDSS). The two samples comprise several thousand pairs, suitable to explore in detail the dependence of star formation activity on orbital parameters and global environment. In order to characterise environment, we computed the projected local density estimator, Sigma, from the distance to the 5^{th} nearest neighbour of galaxy pairs, brighter than M_r = -20.5 and M_b = -19.3 for the SDSS and 2dFGRS, respectively, and within a fixed velocity interval of 1000 km s^{-1}. Star formation activity is derived through the eta parameter in 2dFGRS, and the star formation rate normalised to the total mass in stars, SFR/M* in SDSS (Brinchmann et al. 2004). Our analysis of SDSS pairs confirms previous results found with the 2dFGRS regarding the projected distance r_p = 100 kpc h^{-1} and a relative velocity V = 350 km s^{-1} thresholds for interactions to induce significant star formation activity. We found an increase of star formation activity in galaxies in pairs for smaller projected separations and relative velocities in all environments. However, in high density regions, galaxies have to be closer to statistically show enhanced star formation activity with respect to galaxies without a close companion (see Alonso et al. 2006 for details).