Dark Matter Substructure in the Simulations and Observed Universe

1. Dark Matter Substructure in the Simulations and Observed Universe P. Nurmi

2. Pure N-body vs. Hydro Collisionless N-body (DM only) simulations (accurate solution to an idealized problem) - Ωm 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. Louhi: Cray XT4 Cosmological N-body Simulations Our simulations: 6 different simulations with 3 different resolutions and 2 different simulation codes (AMIGA and GADGET-2):

4. Subhalo-galaxy connection? For large halos Mtot≈ 1013 - 1015 MSun/h: Main halo= massive “elliptical” galaxy Substructure = normal galaxies For small halos Mtot≈ 1011 - 1013 MSun/h: Main halo = typical spiral galaxy Substructure = dwarf galaxy dN/dm~m-1.8±0.1 5-10% of total mass are in substructures

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 2.4·1013 MSun/h and containing 275 subhalos. Subhalo masses are between 109 MSun/h and 1011 MSun/h.

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: • Most of the mass is accreted in large ~1011Msun subhalos 2. Majority of accreted systems are destroyed before z=0 3. Surviving substructure is generally young.

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. 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. 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]

15. The 2.5-meter SDSS survey telescope Comparison between SDSS galaxy data and our simulations ? SDSS DR5 data ΛCDM simulations Rvir ? Typical halo with several subhalos (galaxies) Abell 2151: The Hercules Galaxy Cluster

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. 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.

18. Comparison 1: Richness ?

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

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

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)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)

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. Can MW dwarfs be used at all for comparison? • Great Disk (pancake) has thickness ~ 20kpc • ~ perpedicular to the MW disk 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)

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

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. Observational signature of substructure 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”). 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.

27. Is it possible to observe substructure by strong gravitational lensing ?

28. Is it possible to observe substructure by strong gravitational lensing ?

29. Observational signature of substructure 3. Dark Matter Annihilation: Because of their high phase-space densities, subhalos may be detectable via γ-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. 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. 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 Way’s will have severe difficulties to survive typical satellite accretion within ΛCDM.

32. Summary • The galaxy-halo-subhalo-DM connection is not yet fully understood !