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zCOSMOS 20k: A high quality group catalog

zCOSMOS 20k: A high quality group catalog. Christian Knobel (ETH Zurich). Simon Lilly (ETH Zurich), Angela Iovino, Valentina Presotto ( INAF Brera) + zCOSMOS Team. COSMOS Meeting, Honolulu 10.06.2010. Basic strategy. z. I AB ≤ 22.5. spec. phot. x. Spectroscopic component.

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zCOSMOS 20k: A high quality group catalog

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  1. zCOSMOS 20k:A high quality group catalog Christian Knobel (ETH Zurich) Simon Lilly (ETH Zurich), Angela Iovino, Valentina Presotto (INAF Brera)+ zCOSMOS Team COSMOS Meeting, Honolulu 10.06.2010

  2. Basic strategy z IAB ≤ 22.5 spec phot x

  3. Spectroscopic component Friends-of-friends (FOF) Basic group catalog Voronoi (VDM) 1-way-matched sample (1WM) pure but less complete subset

  4. 10k catalog Published & publicly available: Knobel, Lilly, Iovino, Cucciati + zCOSMOS team et al. (2009) • 800 groups, 2310 group galaxies • 502 groups for N ≥ 5 Sample: • Applications of the catalog: • Role of groups in the density field (Kovac et al. 2010) • Color as a function of environment (Iovino et al. 2010) • Morphology as function of group environment (Kovac et al. 2009) • AGN in groups (Silverman et al. 2009) • Contribution to lensing analysis (Anguita et al. 2009, Faure et al. in preparation)

  5. 20k spectroscopic catalog 20k FOF 10k 20k Groups: 800 1681 Members: 2310 5102 N ≥ 5: 102 213 1WM 20k mocks 10k N ≥ 5 N ≥ 2 N ≥ 10

  6. 20k spectroscopic catalog Properties/features: • for N ≥ 3: ≳ 85 % complete ≳ 80 % pure • for N = 2 completeness & purity ~5-10 % lower • group purity parameter (GRP) 1WM • group robustness • velocity dispersion (for N ≥ 5) • flux (abs. mag.) limited richness • mock calibrated mass („fudge mass“) very high confidence subsamples

  7. Including photo-z Photometric component σz Δz = |zgr – zphot| rgr Δr

  8. Including photo-z Assigning probabilties Δz Δr 5 ≤ N ≤ 9 σz rgr f empirical fraction f( , ,N) |Δz| / σphot Δr / rgr 2 ≤ N ≤ 4 f N ≥ 10 f |Δz| / σphot Δr / rgr Δr / rgr |Δz| / σphot

  9. Including photo-z Assigning probabilties Δr Δz Δr Δz Scheme of estimating probabilities: rgr σz rgr σz • Estimate fraction f( , ,N) empirically by the mocks using only galaxies associated to a single group • Assign probabilities to all galaxies: p = f( , ,N) • For galaxies associated to more than one group, the probability must be modified:

  10. Including photo-z Basic strategy rel. median Nest real groups Nreal Nreal Estimated richness: rel. quartiles Nreal

  11. Including photo-z Most massive galaxy How to determine the most massive (= central?, dominant?) galaxy in a group? • Introduce probability of a spectroscopic member to be associated to a group • Straightforward scheme to compute probability of each member (spec AND phot) to be the most massive: Sort galaxies in descending order after M such that Mi-1 ≥ Mi ≥ Mi+1 :

  12. Most massive galaxy Most massive galaxy 5 ≤ N ≤ 9 Most groups have a clearly identifiable „most massive galaxy“ # galaxies 3 ≤ N ≤ 4 pM # galaxies N ≥ 10 # galaxies pM pM

  13. Group center Group center Only spectroscopic component: geometrical mean stellar mass weighted voronoi vol. weighted voronoi vol. & stellar mass weighted Spec + phot components: geometrical mean Stellar mass weighted Voronoi vol. weighted Voronoi vol. & stellar mass weighted Used by Alexis

  14. Group center Spec + phot components: Selecting the position of the galaxy with the largest… probability voronoi volume probability * stellar mass voronoi volume

  15. Future work (zCOSMOS) Future work/applications within zCOSMOS • Analyzing central/satellite/isolated galaxies • Optical/Xray group selection comparison • Masses of optical groups (group-galaxy cross-correlation, weak lensing, N(z)-σ relation,…) • Optical/spectroscopic properties of Xray selected group members • Investigating passives (and actives?) around log M = 10.2 as f(env) • distinction between mass‐quenching and environment quenching • "Super‐group" stacked spectra, looking for radial dependence etc of quenching ages etc. If you have other ideas/suggestions you are welcome to bring them in!

  16. Summary • 20k group catalog with ~1,600 groups and ~5,100 spectroscopic members • Overall high completeness and high purity • We are able to select extremely pure subsamples • We are able to assign probabilities to photometric galaxies with IAB < 22.5 (or IAB < 24) to be members of spectroscopic groups • Complete membership for IAB < 22.5 • We are able to find for each group the most massive member at high confidence • We can investigate the central-satellite issues • Combining spec and phot components yields improved group properties such as group centers

  17. Appendix

  18. Completeness Completeness for phot 5 ≤ N ≤ 9 2 ≤ N ≤ 4 completeness completeness ≥ probability ≥ probability N ≥ 10 completeness ≥ probability

  19. Group robustness Group robustness One-to-one correspondence Method to find robust groups: increase or decrease linking length by 20% Consider the increase or decline of the richness N „too small“ (fragmented) „too big“ (over-merged) No association

  20. Group robustness Group robustness 1-1 correspondence Subsample of groups …exhibiting less than 40% change in N by the 20% change of the linking lenght … GRP ≥ 0.8 „too small“ (fragmented) „too big“ (over-merged) No association

  21. 20k spectroscopic catalog Interloper fraction 10k 20k g2 completeness purity

  22. Mass completeness 20k

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