Environmental effects in the interaction and merging of galaxies in zCOSMOS survey

The zCOSMOS-bright 10k spectroscopic sample reveals a strong environmental dependence of close kinematic galaxy pair fractions in the redshift range 0.2 < z < 1. The fraction of close pairs is three times higher in the top density quartile than in the lowest one. This environmental variation in pair fractions will translate into merger fractions since merger timescales are shown, based on Millennium simulation catalogs, to be largely independent of environment. While galactic properties of close kinematic pairs (morphologies and star formation rates) may seem to be non-representative of an underlying galaxy population, they can be explained by taking into account well-known effects of environment, and changes caused by interactions. The latter is responsible for an increase of irregular galaxies in pairs by a factor of 50-75%, with a disproportionate increase in the number of irregular-irregular pairs (4-8 times), due to disturbance of about 15% of the disk galaxies in pairs. Another sign of interaction is an observed boost in specific star formation rate (factor 2-4) for the closest pairs. While significant for paired galaxies, this triggered star-formation due to interactions represents only about 5% of the integrated star-formation activity in our volume-limited sample. Although majority of close kinematic pairs are in dense environments, the effects of interactions appear to be strongest in the lower density environments. This may introduce strong biases into observational studies of mergers, especially those based on morphological criteria. Relative excess of post-starburst galaxies observed in paired galaxies (factor sim2) as well as excess of AGNs (factor of over 2), linked with environmental dependence of the pair fractions could indicate that early phases of interactions and merging are plausible candidates for environmental quenching, observed in the global galaxy populations.

Environmental effects in the interaction and merging of galaxies in zCOSMOS

(Abridged) We analyze the environments and galactic properties (morphologies and star-formation histories) of a sample of 153 close kinematic pairs in the redshift range 0.2 < z < 1 identified in the zCOSMOS-bright 10k spectroscopic sample of galaxies. Correcting for projection effects, the fraction of close kinematic pairs is three times higher in the top density quartile than in the lowest one. This translates to a three times higher merger rate because the merger timescales are shown, from mock catalogues based on the Millennium simulation, to be largely independent of environment once the same corrections for projection is applied. We then examine the morphologies and stellar populations of galaxies in the pairs, comparing them to control samples that are carefully matched in environment so as to remove as much as possible the well-known effects of environment on the properties of the parent population of galaxies. Once the environment is properly taken into account in this way, we find that the early-late morphology mix is the same as for the parent population, but that the fraction of irregular galaxies is boosted by 50-75%, with a disproportionate increase in the number of irregular-irregular pairs (factor of 4-8 times), due to the disturbance of disk galaxies. Future dry-mergers, involving elliptical galaxies comprise less than 5% of all close kinematic pairs. In the closest pairs, there is a boost in the specific star-formation rates of star-forming galaxies of a factor of 2-4, and there is also evidence for an increased incidence of post star-burst galaxies. Although significant for the galaxies involved, the excess star-formation associated with pairs represents only about 5% of the integrated star-formation activity in the parent sample. Although most pair galaxies are in dense environments, the effects of interaction appear to be largest in the lower density environments.