Star formation and the environment of nearby field galaxies

Abstract

We investigate the environmental dependence of galaxies with star formation from a volume-limited sample of 4782 nearby field galaxy spectra extracted from the 2dF Galaxy Redshift Survey final data release. The environment is characterized by the local spatial density of galaxies, estimated from the distance to the 5th nearest neighbour. Extensive simulations have been made to estimate correction factors for the local density due to sample incompleteness. We discriminate the galaxies in distinct spectral classes -- passive, star-forming, and short starburst galaxies -- by the use of the equivalent widths of [{\sc O ii}]$\lambda$3727 and H$\delta$. The frequency of galaxies of different classes are then evaluated as a function of the environment. We show that the fraction of star-forming galaxies decreases with increasing density, whereas passive galaxies present the opposite behaviour. The fraction of short starburst galaxies -- that suffered a starburst at $\sim$ 200 Myr ago -- do not present strong environmental dependence. The fraction of this class of galaxies is also approximately constant with galaxy luminosity, except for the faintest bins in the sample, where their fraction seems to increase. We find that the star-formation properties are affected in all range of densities present in our sample (that excludes clusters), what supports the idea that star-formation in galaxies is affected by the environment everywhere. We suggest that mechanisms like tidal interactions, which act in all environments, do play a relevant role on the star-formation in galaxies.