Substructure of Dark Matter in the Simulations and the Observed Universe

1. 22.1.2008, Tuorla Observatory 1 Dark Matter Substructure in the Simulations and Observed Universe P. Nurmi

2. 22.1.2008, Tuorla Observatory 2 Pure N-body vs. Hydro Collisionless N-body (DM only) simulations ( accurate solution to an idealized problem) - m is WIMP and is distributed as N particles - problems in the center of galaxies where baryons dominate - only gravity - high resolution - no free parameters (ICs taken from CMB) Hydrodynamical simulations (approximate solution to a more realistic problem) - computationally expensive, relatively low resolution - complicated (SPH and grid codes often disagrees) - important physical processes typically act on scales far below resolution and are implemented through uncertain functions and free parameters

3. 22.1.2008, Tuorla Observatory 3 Cosmological N-body Simulations Our simulations: 6 different simulations with 3 different resolutions and 2 different simulation codes (AMIGA and GADGET-2): Louhi: Cray XT4

4. 22.1.2008, Tuorla Observatory 4 Subhalo-galaxy connection? For large halos M tot 10 13 - 10 15 M Sun /h: Main halo= massive elliptical galaxy Substructure = normal galaxies For small halos M tot 10 11 - 10 13 M Sun /h: Main halo = typical spiral galaxy Substructure = dwarf galaxy 5-10% of total mass are in substructures dN/dm~ m -1.8 0.1

5. 22.1.2008, Tuorla Observatory 5 Substructure in the DM (only) simulations? Two sets of slides: 1. Z-evolution of all halos in the 40 Mpc/h simulation. An interesting region is shown with several merger events. 2. Zoom of substructure in the 20 Mpc/h simulation of a system with 2.4 10 13 M Sun /h and containing 275 subhalos. Subhalo masses are between 10 9 M Sun /h and 10 11 M Sun /h.

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9. 22.1.2008, Tuorla Observatory 9 Mass accretion history of subhalos: Zentner & Bullock, ApJ, 598, 2005 (semi-analytic) Most accreted subhalos are destroyed! Some general results confirmed by many studies: 1. Most of the mass is accreted in large ~10 11 Msun subhalos 2. Majority of accreted systems are destroyed before z=0 3. Surviving substructure is generally young .

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12. 22.1.2008, Tuorla Observatory 12 Dynamical and physical evolution of subhalos Tidal effects: - mainhalo-subhalo encounters, subhalo-subhalo encounters Depends on the halo profile and halo masses - Leads to massloss, profile changes etc. Dynamical friction: Dynamical friction arises because of the wake of particles that grow behind the motion of particle due to gravitational focusing. - Orbital changes

13. 22.1.2008, Tuorla Observatory 13 Some problems concerning substructure Overmerging, a problem related to resolution (White (1976), van Kampen (1995)) Abundance of CDM structure match galaxy abundance in clusters, but not in local group satellites (Moore at al. (1999)) Spatial distribution of subhalos: they are too far from the center (Diemand (2004)) Some improvement by selecting subhalos according to mass (or circular velocity) before accretion: (Nagai & Kravtsov (2005), Conroy at al. (2006))

14. 22.1.2008, Tuorla Observatory 14 Large-scale galaxy clustering Two-point correlation functions calculated from the halos in CDM-simulations and galaxies from SDSS agree very well (Conroy et al. 2006, ApJ 647)-> dots = SDSS, solid line = ART simulations 512 in (80 Mpc/h) Similar results from Virgo Consortium simulations in larger scales (Springel et al. 2005, Nature, 435)-> 2160 in (500 Mpc/h) Also the galaxy formation physics incorporated in the SPH simulation give a good account of observed galaxy clustering (Weinberg et al. 2005, ApJ 601). [144 in (50 Mpc/h) cube]

15. 22.1.2008, Tuorla Observatory 15 Comparison between SDSS galaxy data and our simulations ? Abell 2151: The Hercules Galaxy Cluster SDSS DR5 data CDM simulations Typical halo with several subhalos (galaxies) ? R vir The 2.5-meter SDSS survey telescope

16. 22.1.2008, Tuorla Observatory 16 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.

17. 22.1.2008, Tuorla Observatory 17 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 m r (lim)=17.5. This gives us three luminosity limits for galaxies that are included in the analysis.

18. 22.1.2008, Tuorla Observatory 18 Comparison 1: Richness ?

19. 22.1.2008, Tuorla Observatory 19 Comparison 2a: Luminosity ? (all galaxies that have L > L lim (d) are included, for observations L group is corrected for invisible galaxies)

20. 22.1.2008, Tuorla Observatory 20 But what about small subhalos around Milky Way sized halos? A simple DM halo mass Luminosity correlation does not work anymore Too many subhalos if compared with observed dwarf galaxies Classical Dwarf Galaxy Problem: (Moore et al. 1999)

21. 22.1.2008, Tuorla Observatory 21 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) multimass simulation, m p =20900 M sun 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)

22. 22.1.2008, Tuorla Observatory 22 Can MW dwarfs be used at all for comparison? (Kroupa et al. 2005A&A, 431, 517 ) The shape of the observed distribution of Milky Way (MW) satellites is inconsistent with their being drawn from a cosmological sub-structure population with a confidence of 99.5 per cent. Most of the MW satellites therefore cannot be relate to dark-matter dominated satellites. If the MW dwarfs do indeed constitute the shining fraction of DM sub-structures, then their number- density distribution should be consistent with an isotropic (i.e. spherical) or oblate power-law radial parent distribution.

23. 22.1.2008, Tuorla Observatory 23 But also in simulations accretions are an-isotropic and large subhalos tend to be more accreted along the major axis of the host halo. Consistent if the major axis of MW halo is perpendicular to Galactic disk (Kang, Mao, Jing, Gao 2005) Great Disk (pancake) has thickness ~ 20kpc ~ perpedicular to the MW disk Can MW dwarfs be used at all for comparison?

24. 22.1.2008, Tuorla Observatory 24 Other Groups? (Karachentsev, AJ, 129, 178 , 2005) - Good targets (M31, M81, M83) - There is maybe some signal, but it is much weaker

25. 22.1.2008, Tuorla Observatory 25 Radial distribution of subhalos ? (Willman et al. MNRAS 353 (2004) 639-646 ) Incompleteness needs to be taken seriously! Radial distribution of the oldest subhalos in a Lambda+CDM simulation of a Milky Way-like galaxy possess a close match to the observed distribution of M31's satellites, which suggests that reionization may be an important factor controlling the observability of subhalos.

26. 22.1.2008, Tuorla Observatory 26 Observational signature of substructure 2. Gravitational Lensing: - Galaxy substructure may explain the flux ratio anomalies observed in multiply-imaged lensed QSOs - Milliarcsecond scale image splitting of quasars that are known to be splitted on arcsecond level (Zackrisson et al. 2008) One major problem is the density profile of small subhalos. 1. Satellite Galaxies of MW: Most massive DM subhalos are associated with luminous dSph satellites. Problem: most dark matter subhalos appear to have no optically luminous counterparts in the Local Group (missing satellite problem).

27. 22.1.2008, Tuorla Observatory 27 Is it possible to observe substructure by strong gravitational lensing ?

28. 22.1.2008, Tuorla Observatory 28 Is it possible to observe substructure by strong gravitational lensing ?

29. 22.1.2008, Tuorla Observatory 29 Observational signature of substructure 3. Dark Matter Annihilation: Because of their high phase-space densities, subhalos may be detectable via -rays from DM particle annihilation in their cores (Diemand, Kuhlen, & Madau 2006) (GLAST, VERITAS). 4. Tidal streams: Presence of a population of CDM clumps alters the phase-space structure of a globular cluster tidal stream. If the global Galactic potential is nearly spherical, this corresponds to a broadening of the stream from a thin great-circle stream into a wide band on the sky. (Ibata et al. 2002) (GC streams detectable by GAIA)

30. 22.1.2008, Tuorla Observatory 30 Observational signature of substructure 5. Signatures of long-term dynamical effects of subhalos to galaxies: Satellite-disk encounters of the kind expected in CDM models can induce morphological features in galactic disks that are similar to those being discovered in the Milky Way, M31, and in other nearby and distant disk galaxies. (Kazantsidis et al. 2007)

31. 22.1.2008, Tuorla Observatory 31 Conclusions Cdm models predict several close encounters of massive subhalos with the galactic disks since z<1. Unless a mechanism (gas accretion?) can somehow stabilize the disks to these violent gravitational encounters stellar disks as old and thin as the Milky Ways will have severe difficulties to survive typical satellite accretion within CDM. Kazantzidis, 2007, arXiv:0708.1949v1

32. 22.1.2008, Tuorla Observatory 32 Summary The galaxy-halo-subhalo-DM connection is not yet fully understood !