Population of Dark Matter Subhaloes

1. Population of Dark Matter Subhaloes Department of Astronomy - UniPD INAF - Observatory of Padova Carlo Giocoli prof. Giuseppe Tormen Blois

2. OUTLINE • Introduction: galaxyformation and dark matterclustering in a CDM-universe • N-bodysimulations • Satellite mass function • Subhalodefinition and mass-loss rate • Present-daysubhalo mass function

3. INTRODUCTION • Dark Energy (DE): unknown form of energy permeating all of space and increasing the expansion rate of the universe. • Dark Matter (DM): unknown weakly interacting elementary particle not emitting any radiation, whose presence can be inferred indirectly from gravitational effects on visible matter. • Baryons: “common” and visiblematter: hot and cold gas, stars …

4. INTRODUCTION Dark Matter: Cold, i.e. its velocity is non-relativistic (v«c) at all epochs relevant for structure formation. Non-Baryonic &Collisionless. DM density fluctuations grow with the expansion of the universe, become non-linear and form collapsed structures: dark matter haloes. • In the last twenty or so years, physicists have proposed different candidates for DM. Among these, two classes of particles are sufficiently promising to motivate major experimental search: • WIMPS – e.g. from supersymmetric extensions of the standard model (SUSY): neutralino. • Axion – to solve the strong-CP problem. Dimopulos 1990; Bertone et al. 2005; Giocoli, Pieri & Tormen 2008; Pieri, Bertone & Branchini 2008 Springel et al. 2005

5. INTRODUCTION Galaxies reside inside dark matter haloes, where baryons can shock, cool and eventually form stars (White & Rees 1978). The structure formation process is hierarchical: smaller haloes collapse first, and later merge to form larger systems. The cores of progenitor haloes may survive this process, and constitute the so-called substructure population of a halo.

6. INTRODUCTION Understanding the clusteringof DM is a fundamentaltopic in moderncosmology. Semi-analyticalmodelsofgalaxyformationprovidelinksbetweenobservations (galaxycolour, clustering, etc) and DM haloes and subhaloes. Semi analyticalmodels start from Monte Carlo mergertrees or, more realistically, fromN-bodynumericalsimulations. Halo and Subhalo Mass Functions are alsoimportanttoconstrain the g-rayemissionfrom DM particlesannihilation. Colberg & Diaferio (GIF – project) Kauffman & White 1993; Kauffmann et al. 1999; Springel et al. 2001; Diaferio et al. 2001; De Lucia et al. 2004, Gao et al. 2004, van den Bosch, Tormen, Giocoli2005, Giocoli, Pieri & Tormen 2008

7. N-BODY SIMULATIONS N-body simulations model the expanding universe as a system of DM particles in a large box, and evolve it in time under the action of its own gravity. They are used to study structure formation and clustering in the non-linear regime. • GIF(Kauffmanet al 1999)& GIF2(Gaoet al 2004)CosmologicalSimulations • ResimulatedGalaxyClusters(Dolaget al 2004 – DM run)

8. N-BODY SIMULATIONS - GIF2

9. MERGER HISTORY TREES • Halo Finder: • Follow each halo along its merging-history tree, and store all information about satellites accreted by the main halo progenitor at any z > z0 ; time main progenitor; satellites: progenitor haloes accreted by the main prog. z0 = [ 0, 0.5, 1, 2, 4 ]

10. Giocoli, Tormen & van den Bosch 2008 - MNRAS SATELLITE MASS FUNCTION (AKA unevolved subhalo mass function) fitting function van den Bosch, Tormen, Giocoli (2005) Giocoli, Tormen, van den Bosch (2008)

11. SATELLITE MASS FUNCTION The mass function of satellites accreted at all redshift is universal: • Independent on final (observation) redshift • Independent on final host halo mass.

12. SATELLITE MASS FUNCTION before and after the formation redshift zf (zf = earliest redshift when the main halo progenitor assemble half of its final mass) • Distribution slopes are identical, while normalisations can be obtained from the main halo progenitor mass distribution at zf(Sheth & Tormen 2004b). • More mass is accreted in satellites before the formation redshift (57%).

13. WHAT ABOUT THE evolved POPULATION? • Evolved: tidal stripping, gravitational heating and dynamical effects reduce the mass of satellites after they enter the environment of the host halo.

14. SATELLITE EVOLUTION

15. PRESENT-DAY SUBHALOES z=0 satellite self-bound mass

16. Giocoli, Tormen & van den Bosch 2008 - MNRAS PRESENT-DAY SUBHALOES broken universality correlation Satellites orbiting within the host halo lose (part of) their mass due to: gravitational heating and tidal effects.

17. Giocoli, Tormen & van den Bosch 2008 - MNRAS MASS LOSS RATE van den Bosch, Tormen & Giocoli (2005) propose a simple model for the satellite mass loss in a steady-state (dM/dt≈0) halo: yes Assuming the host halo in a steady-state: ok! Can we check this in numerical simulations?

18. Giocoli, Tormen & van den Bosch 2008 - MNRAS MASS LOSS RATE characteristic time-scale for subhalo mass loss at z= 0 • The fractional mass loss rate isconstant. • The slopeof the un-evolved mass functionispreserved. • Smallhaloes are denser and form at higherredshifts.

19. Why the universalityisbroken? • Comparedto massive haloes, smallerhaloesform at higherredshifts. • Smallerhaloesthusaccretesatellites at earliertimes. • Thesesatellitessuffer mass loss forlongertimes. • The time scale for mass loss rate isshorter at higherredshift. • Due tobotheffects, smallhaloespossessfewersubhaloestoday.

20. Thanks so muchfor the attention