1. SIMULATING THE GROUP FINDING IN THE DEEP2 SURVEY WITH PHOTOMETRIC REDSHIFT INFORMATION H.-Y. Baobab Liu 1, B.-C. Paul Hsieh 2, L.-H. Lin 3 1. NTU; 2. ASIAA; 3. UC Santa Cruz ABSTRACT We developed a modified friend-of-friend method for galaxy group/cluster finding by considering the probability of two objects to be physically associated based on their redshift information and projected position on the sky. This method is currently tested using the DEEP2 mock catalog. In the future work, we will use it to identify galaxy groups in DEEP2 survey, which has an averaged redshift ~1. INTRODUCTION In the previous work of DEEP2 group (Gerke et at., 2005), they identified galaxy groups using only galaxies with successful spectroscopic redshift. From DEEP2 mock catalog (Yan et al. 2004), we can see the total number of groups drops by an order of magnitude if we neglect those galaxies without spectroscopic redshift measurement ( After mask-making in FIGURE 1.). Now we want to combine the spectroscopic catalog with photometric redshift information( Magnitude-limited in FIGURE 1.) to obtain a group catalog with richness that is less affected by the selection effect and incompleteness problem. The improved richness can reduce the uncertainty for the study of the dependence of galaxy and galaxy group aproperties on the environment. (Gerke et al., 2005) After mask making, the total number of group DROP by around ONE ORDER OF MAGNITUDE, in comparison with magnitude-limited sample. THE MODIFIED FRIEND OF FRIEND(FOF) METHOD BASIC IDEA Instead of comparing the absolute separation of two galaxies to the linking length, whether the two galaxies belong to the same galaxy group is determined by their PROBABILITY OF BEING WITHIN LINKING LENGTH. FIGURE 2. illustrates our modification. PDF1 and PDF2 are the probability distributions of position of object1 and object2 respectively.ADVANTAGE I. Galaxies with and without spectroscopic redshift have EQUAL FOOTING IN THIS ALGORITHM. II. Spectroscopic data help to CONSTRAINT THE POSITION OF GROUPDISADVANTAGE The probability threshold has weak physical meaning, and is optimized empirically. Reference Gerke, et al.,2005, ApJ, 625:6-22 Yan, R., White, M., & Coil, A. C. 2004, ApJ, 607, 739 Linking Length: plan-of-sky :100 kpc line-of-sight: 3Mpc Probability threshold1%0.1%0.01%0.001% # of mock group159 # of reconstructed group 1393166 # of group not idenfied 15012695 # of extra identification 45695 # of identical group41012 FUTURE WORK Those parameters in our algorithm need further optimization. In addition, the weighting of physical quantities of each members in the groups, such as its position and and velocity, need to be modified to obtain the unbiased determination of center positions and velocity dispersions of galaxy groups/clusters. RESULT FIGURE 3. is our group finding result for DEEP2 mock catalog, projected to the RA and DEC coordinates on the sky. The reconstructed group and real group are correlated. And some reconstructed groups are found to be false detection. TALBE 1. shows our preliminary results of group finding in mock catalog with different parameters used in algorithm. Probability threshold is the P th as in FIGURE 2.. # of mock group is the total number of groups given in the DEEP2 mock catalog, and # of reconstructed group is the number of groups identified by our algorithm. # of Group not identified are the number of real group we didn’t find; while extra identification are the # of reconstruct groups which can’t be identified with real groups. Identical groups are those reconstructed groups with same number of members and members with a real group. MODIFICATION object1object2 LINKING LENGTH Mean position of object1 LINKING LENGTH Mean position of object2 PDF1PDF2(probability distribution) OLD LINKING CRITERIA V 12 : distance of two object V L : linking length NEW LINKING CRITERIA P : probability P th : threshold for probability FIGURE 1. FIGURE 2. TABLE 1. FIGURE 3.CONCLUSION We have developed a modified FOF algorithm which can be used to identify galaxy group/clusters using the photometric redshift and/or spectroscopic redshift information together with their projected coordinates on the sky. This group-finding method has been tested using the DEEP2 mock catalog. Our preliminary results, however, suggest that the false detection rate can be as high as 60% while the completeness is around 40%, which can be a result of wrong choices of the linking length and the probability threshold. We plan to address this issue in the near future.