1. How to find a group of galaxies.II The Sloan Survey case Erik Tago (and J.Einasto,E.Saar,E.Tempel, M.Einasto,J.Vennik,V.Müller Tartu Observatory,Potsdam Observatory) Tuorla-Tartu meeting Tuorla Observatory, Oct 4-5, 2007

2. Contents 0) Introduction 1) What is a group of galaxies : theoretical and operational points of view Hierarchy: from a single galaxy to supercluster and beyond The non-island Universe : interacting galaxies 2) Where from to search for groups ? Observations: galaxy and redshift surveys Numerical simulations 3) How to find a group Groupfinders : from pairs to superclusters 4) What kind of problems arise and how to overcome them 5) How to enjoy results

3. How to find a group of galaxies. A new 2dF GRS group catalogue Erik Tago,Jaan Einasto, Enn Saar, Maret Einasto, Ivan Suhhonenko, Mihkel Jõeveer, Jaan Vennik, Pekka Heinämäki, Douglas Tucker Tartu Observatory, Tuorla Observatory, Fermilab Tartu-Tuorla seminar June 20-21 2005 Tõravere Intro: historical reference -------------------------------------------------

4. Hierarchy in the world of galaxies What is a group isn't a trivial question Hierarchy : from single galaxy – through pair, group, cluster, supercluster -- to supercluster-void network Is there a really single galaxy ? May-be among giant galaxies. In general I prefer an answer – NO Pairs of galaxies : some examples Milky Way /Andromeda Interacting pairs : M51, the Antennae, the Mice Groups N=3…X Local group N=3 + 40

5. Galaxies interact in pairs & groups

6. Local Group of galaxies

7. Clusters C lusters N=X… 10 000 galaxies Shapley supercluster includes 33 Abell clusters Has this classification some physical meaning? - in particular for groups and clusters (YES?)

8. Rich cluster of galaxies Abell 1689

9. Shapley supercluster of galaxies

10. Multi-nucleus cluster of galaxies : an evidence for group merging CL0958-4702 Spitzer

11. Bullet cluster of galaxies – encounting clusters

12. APM and 2dFGRS sky coveridge

13. SDSS DR6

14. GROUPFINDER METHODS Cell count Cluster analysis, FoF method, percolation Minimal spanning tree Matched filter method Adaptive filter method Kernel based methods Voronoi tesselation Wavelet transform Maximum brightest cluster galaxies etc

15. Redshift surveys we have used Abell (ACO) : 4072 + 1300 clusters 2dF GRS : 250 000 galaxies Sloan Survey (SDSS) : 10^6 galaxies

16. Groupfinders and catalogues: a few reacent of them AUTHOR+etal sample no. of gr. (N>=2) (N>=4) Eke 2005 2dFGRS 28877 7020 Merchan 2002 “ 2209 Yang 2004 “ 12096 2502 Tago 2005 “ 25215 4852 Merchan 2005 SDSS DR3 10864 Goto 2005 DR2 335 Weinmann 2006 DR2 16012 3720 Berlind 2007 DR3 4119 Tago 2007 DR5 50362 9454

17. A groupfinder and a catalogue: our case We use Friends of Friends (FoF) groupfinder (cluster analysis) applying linking length (LL) scaling a) using simulated clusters b) calibrating observed groups by shifting to higher distances

18. The LL scaling recipe Perform preliminary FoF Select of initial nearby groups Shift the groups step by step to larger distances and recalculatate their properties reject the group members which do not satisfy visibility conditions for the catalogue luminosity window using Minimal Spanning Tree method determine new LL which is needed to link reduced group at new distance Find LL law, and using this perform final FoF

19. Number density of groups

20. Number density of galaxies in the 2dFGRS and SDSS DR5

21. Multiplicity function

22. The scaling of membership for individual groups

23. Scaling law of linking length for the SDSS DR5 North sample

24. What kind of problems ? Not a 3D space but 2D+redshift Distorsions in redshift space In redshift space we can see 1) fingers of God due to dynamical dispersion in groups and clusters ( 10… 1500 km/s) ? 2) substructures ; multimodal velocity distribution 3) contamination by fore and background groups and galaxies, merging of subgroups 4) Selection effects depending on distance due to flux limited samples : a) number density decrease b) richness decrease c) volume effect – distant clusters are larger Evolutionary effect Luminosity-density relation in groups and clusters

25. How to ovecome the problems ? Do not study groups (in this paper not applied) Restrict samples from low (SDSS incomplete r=14.5) and high redshift (17.7) Luminosity corrected by weight Perform FoF in two direction – radial and transversal - assuming some ratio of them (6 …12) Linking Length scaling

26. Distribution of velocity dispersion as a function of distance

27. Distribution of maximum projected size as a function of distance

28. Sky distribution of groups : by Berlind (+), Tago (O - group,o- pair)

29. FoF result for 2dF GRS at A933 cluster

30. Clusters of galaxies In redshift space we see 1) fingers of God due to dynamical dispersion among member galaxies ( Abell clusters sigma_V 100… 1500 km/s) 2) substructures ; multimodal velocity distribution 3) contamination by fore and background groups and galaxies

31. Numerical simulation Pioneers Alar and Jury Toomre 1970ies modelling of interacting galaxies John Dubinsky : models of cluster formation

32. To enjoy your own results You have to compare with other results and find that your results being better This is the case. You have to share your results with others: NEXT SLIDE

33. Welcome to our web site for groups and clusters http://www.aai.ee/~erik/sdss sdss dr5 group catalogue Thank You!