How to find a group of galaxies.IIThe 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

Contents0) Introduction1) 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 simulations3) How to find a group • Groupfinders : from pairs to superclusters4) What kind of problems arise and how to overcome them5) How to enjoy results

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Intro: historical reference-------------------------------------------------

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

Galaxies interact in pairs & groups

Local Group of galaxies

Clusters

Clusters N=X… 10 000 galaxies

Shapley supercluster includes 33 Abell clusters

Has this classification some physical meaning? - in particular for groups and clusters (YES?)

Rich cluster of galaxies Abell 1689

Shapley supercluster of galaxies

Multi-nucleus cluster of galaxies :an evidence for group merging

CL0958-4702 Spitzer

Bullet cluster of galaxies –encounting clusters

APM and 2dFGRS sky coveridge

SDSS DR6

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

Redshift surveys we have used

• Abell (ACO) : 4072 + 1300 clusters

• 2dF GRS : 250 000 galaxies

• Sloan Survey (SDSS) : 10^6 galaxies

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

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

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

Number density of groups

Number density of galaxies in the 2dFGRS and SDSS DR5

Multiplicity function

The scaling of membershipfor individual groups

Scaling law of linking length for the SDSS DR5 North sample

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 subgroups4) 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

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

Distribution of velocity dispersion as a function of distance

Distribution of maximum projected size as a function of distance

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

FoF result for 2dF GRS at A933 cluster

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

Numerical simulation

• Pioneers Alar and Jury Toomre 1970ies

modelling of interacting galaxies

John Dubinsky : models of cluster formation

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

Welcome to our web site for groups and clusters

http://www.aai.ee/~erik/sdss

sdss dr5 group catalogue

Thank You!