Galaxy groups in ΛCDM simulations and SDSS DR5

1. Galaxy groups in ΛCDM simulations and SDSS DR5 P. Nurmi, P. Heinämäki, S. Niemi, J. Holopainen Tuorla Observatory E. Saar, M. Einasto, E. Tempel, J. Einasto Tartu Observatory V.J. Martínez Observatori Astronòmic, Universitat de València Louhi: Cray XT4 The 2.5-meter SDSS survey telescope

2. The main idea of the study ? • To compare the properties of galaxy groups in SDSS DR5 and ΛCDM in different volume limited samples • Richness = N of galaxies in the galaxy group • Luminosity functions of galaxy groups • Velocity dispersion • Virial radius • Maximum projected linear size

3. The main idea of the study ? SDSS DR5 data ΛCDM simulations Rvir ? Typical halo with several subhalos (galaxies) Abell 2151: The Hercules Galaxy Cluster

4. Scientific context: large-scale galaxy clustering ? • Two-point correlation functions calculated from the halos in ΛCDM-simulations and galaxies from SDSS agree very well (Conroy et al. 2006, ApJ 647)-> dots = SDSS, solid line = ART simulations 512³ in (80 Mpc/h)³ • Similar results from Virgo Consortium simulations in larger scales (Springel et al. 2005, Nature, 435)-> 2160³ in (500 Mpc/h)³ • Also the galaxy formation physics incorporated in the SPH simulation give a good account of observed galaxy clustering (Weinberg et al. 2005, ApJ 601). [144³ in (50 Mpc/h)³ cube]

5. Scientific context: small-scale galaxy clustering -> missing dwarf problem ? • Basically all cosmological simulations predict that there are at least one order of magnitude more small subhalos (dwarf galaxies) around Milky Way like galaxies than what is observed (e.g. Via Lactea simulation Diemand et al. 2007, ApJ 657)->234 million particles in (90 Mpc/h)³ multimass simulation, mp=20900 Msun • Recently discovered (from SDSS data) ultra-faint dwarfs with M/L~1000 help to solve this discrepancy, but not fully (factor of 4 difference). However, If reionization occurred around redshift 9 − 14 , and dwarf galaxy formation was strongly suppressed thereafter, the circular velocity function of Milky Way satellite galaxies approximately matches that of CDM subhalos in Via Lactea simulation. (Simon and Geha 2007, astro-ph. 0706.0516)

6. Scientific context: large-scale galaxy clustering ? • Multiplicity function measurements provide one of the key constraints on the relation between galaxy populations and dark matter halos. • The closest study similar to us is Berlind et al. (2006, ApJSS, 167), where they used N-body simulations to find the best linking lengths (projected and line-of-sight) that would find galaxy groups in SDSS data in the best way. They also used different recipes to populate halos with galaxies were different. The main output is their calculated multiplicity function: • All three multiplicity functions are well fitted by power-law relations, with best-fit slopes of -2.72±0.16, -2.48±0.14, and -2.49±0.28.

7. Scientific context: our contribution ? • Although the idea of galaxy-halo connection is well justified , the question how tight the connection is and what properties are related, remains open. • Especially, here we study the connection between galaxy groups and halo-subhalo populations. This question is closely related to the question how galaxies and galaxy groups are formed and structured. • We also study the properties of galaxy groups in detail and compare velocity dispersions, virial radius values and maximum projected linear sizes (not yet ready).

8. Cosmological N-body Simulations The parameters of the cosmological model astro-ph/0603449 v1: March 20, 2006 Wilkinson Microwave Anisotropy Probe (WMAP) Three Year Results: Implications for Cosmology

9. Cosmological N-body Simulations Our simulations: 6 different simulations with 3 different resolutions and 2 different simulation codes (AMIGA and GADGET-2):

10. How to populate halos with galaxies(a major problem to DM-simulations) ? • We can use a simplified procedure (varying M/L function) that is based on the analytical fit that gives luminosity when halo mass is given (Vale & Ostriker 2004, MNRAS, 353). • We test if this is statistically satisfied by using another method in which suitable galaxies that resemble DM halos and subhalos are selected from the Millenium run semi-analytic galaxy catalogue.

11. SDSS DR5 galaxy group sample • Observational ingredient is based on the galaxy group catalogue by Tago et al. 2007). • From this data we select three volume limited samples based on the group distance; d<100 Mpc/h, d<200 Mpc/h and d<300 Mpc/h; and SDSS completeness limit mr(lim)=17.5. This gives us three luminosity limits for galaxies that are included in the analysis.

12. Comparison 1: Richness ?

13. Comparison 2a: Luminosity ?(all galaxies that have L > Llim(d) are included, for observations Lgroup is corrected for invisible galaxies)

14. Comparison 2b: Luminosity ?(all galaxies are included, lower limit for Lgroup is chosen)

15. Comparison 3: Boundness ?(simulations) Fractions N ≥ 2:

16. Summary • The galaxy-halo-subhalo connection is very strong beyond dwarf galaxy mass region. • If we assume that galaxy groups in Tago et al. (2007) always resemble halo-subhalo systems in the ΛCDM-simulations, then their multiplicity functions agree very well. • Also, the agreement with group luminosity functions is good (at least in large samples that include galaxy groups with large luminosities). • The simple analytical approach to populate halos with galaxies works surprisingly well in a statistical study.