Statistic Properties Of Isolated Close Galaxpairs In SDSS

Interactions between galaxies are an important process in the evolution ofgalaxies with cosmic time. Simulations show that gravitational force betweeninteraction galaxies could affect various galaxy properties such as luminosities，colors，star formation, metallicities and morphologies. In general, paired galaxieshave bluer color, higher star formation rate, and irregular morphologies, which isevidence by a series of statistical studies in galaxy pairs. However people usuallycannot reach the same conclusions, which is partly because of the limitations ofsample size or the differences of the statistical methods involved. Based on the workof Allam et al.2004of SDSS DR2photometric data, we have assembled a large, highquality catalogue of8792merging galaxies (Photometric Galaxy Sample, PGS) whichare characterized by small projected separations and the absence of interferentialgalaxies. And we have also compiled a subsample from PGS containing2779merginggalaxies with spectroscopic data from MPA/JHU database as the Spectroscopic GalaxySample (SGS) of this work.%as well as a fair non-paired galaxies sample for control.We have detected several differences between the SGS and a Control Galaxy Sample(CGS) matched in redshift, luminosity and concentration index.First, the difference of SDSS Petrosian g-r color distribution. There are a muchhigher fraction of extremely blue galaxies in SGS than CGS, which could be regardedas a clear signs of interaction-induced star formation. There are also much moreextremely red galaxies in SGS, which seems not due to the color-density relation ofpaired galaxies because they are field galaxies in our SGS.And we demonstrate thatthis difference is at least partly due to the fact that SDSS pipeline does a poor job ofgalaxy photometry for closely separated pairs.Second, we also detected the difference between the color gradients (fibrecolors-global colors). This indicates that the star formation induced by interactiontends to be centrally concentrated, which is consistent with several simulations. Third, we also find the evidence for gas inflow onto the centres of galaxies%caused by galaxy-galaxy interaction, which comes from a clear offset of theluminosity-metallicity relation and mass-metallicity relation towards lowermetallicities in SGS compared with our non-paired CGS.Finally, we established ONE isolated close pair with two components bothmassive blue galaxies as a example of wet merger and ONE example of dry mergerbetween two massive red galaxies. We consider them as an insight of the galaxiesevolution (from blue cloud to red sequence) scenarios introduced by Faber et al. in2007, and we believe that red galaxies may have been undergoing a mix-quenching(neither early quenching or late quenching) process involves early mass assemblyand star formation, followed by quenching and further (but limited) dry merger alongred sequence.