1. Interacting Galaxies in SDSS DR4 plus Do-Gyun Kim 1,2 Changbom Park 1 Yun-Young Choi 3 1 Korea Institute for Advanced Study, Seoul, Korea, 2 Yonsei University, Seodaemun-Gu, Seoul, Korea, 3 ARCSEC, Sejong University Seoul, Korea

2. Abstract We identify the galaxies showing perturbed features in SDSS DR4 plus by visual inspection to estimate the current merging/interacting rate. From 8991 target galaxies brighter than -20 Mag with 0.025<z<0.055, we extract more than 3000 galaxies with interacting features. The categories of interacting features are non- axisymmetry, off-centered core, bridge and tail and shell. The companions are bounded by ΔM<=1.5 mag in r band and Δz<=0.001(Δv<=300km/s). All galaxies are also categorized by their morphologies. From the relation of galaxy interaction rate(GIR) and projected distance to companion, we find that some amount of galaxies basically have interacting features in spite of the absence of close companion. Removing that basic overestimation of GIR, we derive ongoing GIR as 3.73% and this value is in good agreement to previous researches. We also find that GIR start to increase when the companion locates nearer than its the virial radius. The variances of GIR along the velocity difference and local density are not significant.

3. Introduction With the rise of hierarchical merging scenario and evidences of environmental effects to galaxies, determining the evolution parameter of galaxy merging rate and current interacting rate become to be so important. Since Toomre and Toomre (1972) first introduced the paradigm of galaxy merging, there have been many observational evidences indicating galaxy interaction like Arp’s peculiar galaxy catalog. In this research, we select interacting galaxies showing at least one of the 4 kinds of morphological features such as non-axisymmetry, off-centered core, shell and bridge & tail and investigate the variation of the fraction of interacting galaxies on environmental parameters.

4. Target Selection The data set used in this study is a volume limited sample (Choi et al. 2007) with Mr<-18.5. The targets to be investigated are 8991 galaxies brighter than Mr=-20. Their neighbors are found among those with Mr<-18.5, allowing 1.5 mag difference. Choi et al. (2007)

5. Finding Interacting Galaxies Δv < 300km/s (to be conservative) Perturbed features 1.Non-Axisymmetric body 2.Off-centered core 3.Bridge & Tail 4.Shell Iterative visual inspection Consideration of morphology of each galaxy (as Early and Late type) 1234 Park et al. (2008)

6. Result ~3000 galaxies have interacting features. Most galaxies with perturbing features are non-axisymmetric. Shell tend to appear much more frequently in E type target as expected. L type showed much more fraction in Off-centered core category

7. Result The GIR increases when two galaxy approaches to each other. There are basic levels of GIR. This indicates that some galaxies already have interacting feature in spite of the big spatial separation. The GIR of Late type target start to increase at about Rp=400kpc while that location of Early type target is about Rp=200kpc. When we see GIR as a function of pair separation normalized by the virial radius of companion, particularly the GIR of LL pair starts to increase when the companion come inside a few times of its virial radius. For the case of the other pair types, virial radius is the starting point of increment of GIR. Target Companion

8. Current Galaxy Interacting Rate Genel et al. (2009) After subtracting the basic level of GIR, we derived current GIR. This value is 3.73% and shows good agreement with previous researches.

9. References Choi, Y. Y., Park, C., & Vogeley, M. S. 2007, ApJ, 658, 884 Genel, S., Genzel, R., Bouché, N., Naab, T., & Sternberg, A. 2008, arXiv:0812.3154 Park, C., & Choi, Y.-Y. 2008, arXiv:0809.2156 Toomre, A., & Toomre, J. 1972, ApJ, 178, 623