How to find a group of galaxies. A new 2dF GRS group catalogue

1
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

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Intro why to search for groups and clusters of
galaxies

• To solve the principle problems of cosmology
• Spatial distribution of galaxy systems one of
  the main observational methods to determine
  cosmological parametres
• Spergel test in Princeton grow rate of rich
• clusters in the Universe as a solution
  between
• Dark Energy and Superstring cosmologies
• CDM models were tested by the abundance of rich
• clusters
• Obtain spatial distribution instead of redshift
  space - depression of God fingers
• To find the density field , large scale
  structure,
• dark matter, galaxy formation and evolution
  etc etc
• Galaxy cluster is an astrophysical laboratory

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How to find groups of galaxies

• 1) Differences between GROUPS and CLUSTERS of
  galaxies , in particular from the view of
  search methods
• 2) GROUPFINDERS group finding methods (short
  review of my earlier report )
• 3) Our method a modified cluster analysis
  alias friends-of-friends method (FoF) plus Dark
  Matter Halo test .
• 4) Majority of attention to group finders 

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GROUPFINDER METHODS

• Minimal spanning tree
• 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

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Observational data used

• 1) two degree field galaxy redshift survey (2dF
  GRS) final release 2004, N245000 gal., our
  sample N184000
• 2) Sloan DSS Data Release 3
• N550 000 gal., our sample N350 000

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The APM Galaxy Survey ( 1/10 th of the sky)
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APM and 2dFGRS sky coveridge
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2dF GRS slices in wedge diagram
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2dF GRS redshift distribution
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Table 1
The 2dF GRS samples used and the FoF
parameters ---------------------------------------
------------------------------------------------ S
ample Cat RA DEC Ngal
Ngroups Nsingle DV0 DR0 L0 a


km/s
Mpc/h --------------------------------------------
------------------------------------------- 2dF
GRS N A 147.5...223 -7...3 78067
9553 48861 750 0.3
2dF GRS S A 325...55 -37.5...-23
106328 12414 69299 750 0.3 2dF GRS
N B 147.5...223 -7...3 78067 10750
44134 200 0.25 414 1.46 2dF GRS S B
325...55 -37.5...-23 106328 14465 61344
200 0.25 414 1.46
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Total group luminosity vs distance
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Problems

• Flux limited sample involve selection effects
• depending on distance
• Density-luminosity effect
• Volume effect distant clusters are larger
• Evolutionary effect
• Merging of subgroups

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Our groupfinder description of steps

• 1) FoF with CONSTANT search radius
• alias linking length (LL) alias neighbourhood
  radius
• 2) preliminary FoF with variable LL scaling
  law for LL obtained from previous step
• 3) increasing shift of observed
• clusters to larger distances (about 80
  clusters, proposed by E. Saar) to determine
  scaling law for
• LL, starting scaling law from earlier step
• a) from cluster density LL (Rho)1/3
• b) direct determination of LL changing law
  applying minimal spanning tree method to find LL

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The main result of our method

• Selection effect involved by flux limited sample
• AND
• Density-lumiosity relation
• CANCEL each other out and in resulting
  scaling law
• LL slightly increase with distance (2
  times for our groups instead of 7 times for Eke
  etal groups
• from z0 to z0.2)
• Important for group selection dependence on
  distance
• is not the mean spatial density but intrinsic
• density of groups

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LL scaling law
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Results

• 7) Final FoF procedure with scaling ARCTAN law
• LL/LLo 1aATAN(D/L)
• D is distance, L and a are parametres
  found
• 8) Results Fig. 14, Tab 1 and 3
• 9) Comparison to other catalogues, advances of
  our catalogue
• a) intermediate version between Eke etal and
  Yang etal
• b) independent of models
• c) Eke etal distant groups are too large,
  nearby groups too small in comparison to our
  groups
• (includes filaments of superclusters)
• 10) Catalogue of GROUPS, GALAXIES of GROUPS and
  ISOLATED GALAXIES in the North and South
  galactic hemisphere (6 tables at all).
•  

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Table 3
Group catalogues based on the 2dF
GRS Sample Ngal Ngr(n gt 2) Ngr(n gt 4) zlim
mlim method in gr, ---------------------------
--------------------------------------------------
--------- Merchan 60000 2209
0.25 19.45 FoF mod 2002 Eke
191440 28877 7020 0.11 (med)
FoF mock 55 2004 Yang 151280 12096
2502 0.01-0.2 19.45 FoFmock
25 2004 Tago 184395 25215 4852
0.009-0.2 FoF modDMH 43
2004/05 ------------------------------------------
---------------------------------------------
revised catalogue in comparison to the published
in preprint
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Intrinsic median density of groups vs distance
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FoF result for 2dF GRS at A933 cluster
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FoF result at A1620 cluster
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Distribution of the 2dF GRS group distance
northern and southern areas
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2dF group spatial density vs distance
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The 2dF GRS group catalogue
--------------------------------------------------
--------------------------------------------------
--------------- id Ngal RA DEC
dist s_sky disp_v Rvir
Lmain Lobs Ltot
Mpc
Mpc km/s Mpc
L_sun --------------------------------------------
--------------------------------------------------
---------------------- 1 4 153.10069275
-3.51675248 229.78 0.6809 147.22 0.2472
0.602020E10 0.151832E11 0.194104E11 2 10
153.12361145 -3.71129107 176.59 0.8914
329.80 0.3551 0.290560E11 0.762542E11
0.893009E11 3 24 153.23248291 -2.66546297
131.60 0.9479 346.43 0.2673 0.239150E11
0.112659E12 0.123242E12