Numerical Cosmology & Galaxy Formation

Transcription

1 Numerical Cosmology & Galaxy Formation Lecture 13: Example simulations Isolated galaxies, mergers & zooms Benjamin Moster 1

2 Outline of the lecture course Lecture 1: Motivation & Historical Overview Lecture 2: Review of Cosmology Lecture 3: Generating initial conditions Lecture 4: Gravity algorithms Lecture 5: Time integration & parallelization Lecture 6: Hydro schemes - Grid codes Lecture 7: Hydro schemes - Particle codes Lecture 8: Radiative cooling, photo heating Lecture 9: Subresolution physics Lecture 10: Halo and subhalo finders Lecture 11: Semi-analytic models Lecture 12: Example simulations: cosmological boxes Lecture 13: Example simulations: mergers and isolated galaxies 2

3 Cosmological simulations Motivation: Full hydrodynamic simulation of a cosmological volume Statistical sample of galaxies that can be compared to observations However: extreme computational cost Therefore: only low resolution possible Cannot study formation of individual galaxies in detail Morphology is almost impossible to follow Alternative: simulate individual galaxies at high resolution 3

4 Why simulate isolated disc systems? Set up individual system(s) and simulate/study those Advantage: much higher resolution (more particles per system) Nice discs can be studied (often hard in cosmological runs) Disadvantage: Cosmological background is neglected (no infall, etc) Stars Gas 4

5 Many early collisionless simulations of isolated galaxies First hydro simulation of galaxy formation by Katz & Gunn 1991 No cosmological initial conditions Collapse of isolated constant density perturbation Dark matter to gas ratio: 10:1 Possible to create dark matter halo + thin stellar disc Disc has exponential surface density profile Flat rotation curve The first hydro simulations 5

6 The first hydro simulations 6

7 The first hydro simulations 7

8 The first hydro simulations 8

9 The first hydro simulations 9

10 The first hydro simulations 10

11 The first hydro simulations 11

12 Merger simulations have been used early-on to understand morphology (Toomre & Toomre 1972) E.g. Negroponte & White 1983 run 30 simulations of merging disc-halo galaxy pairs. Collisionless merger simulations Analyse tails and bridges: transfer of mass, energy, angular momentum If elliptical galaxies are remnants of binary major mergers: initial orbits must have had low angular momenta Observed galaxies must be prolate 12

13 Collisionless merger simulations 13

14 Collisionless merger simulations 14

15 Large set of simulations of minor mergers E.g. Velázquez & White 1999 study merging satellites and the kinematic changes induced to the main galaxy Heating/thickening rates for disc differ for prograde and retrograde orbits Collisionless merger simulations Massive bulge is able to reduce heating Typical thickening for Milky-Way like galaxies: pc 15

16 Collisionless merger simulations 16

17 Collisionless merger simulations 17

18 Collisionless merger simulations Also collisionless simulations with no live DM partices DM halo is modelled as a static potential (e.g. NFW profile) Allows much higher resolution in the stellar component (e.g. for satellites) Helmi & de Zeeuw 2000 perform such simulations for the accretion of satellite galaxies onto Milky Way like galaxies They find that phase space coordinates do not change much By identifying stars with the same PS coordinates disrupted satellites can be identified However, transfer of energy & angular momentum to halo neglected!!! 18

19 Collisionless merger simulations 19

20 Hydrodynamic merger simulations Including gas in merger simulations is relatively cheap Can study many gas dynamical effects in great detail Can study the effect of feedback and tune the parameters Mihos & Hernquist 1996 study starbursts in major mergers Strong gaseous inflows during the final stages of the merger Gravitational torques from the host galaxy Good match to ultraluminous infrared galaxies Internal structure of galaxies more important than orbits 20

21 Hydrodynamic merger simulations 21

22 Hydrodynamic merger simulations 22

23 Hydrodynamic merger simulations Springel, Di Matteo & Hernquist 2005 use major merger simulations to study effect of AGN feedback Interplay between starbursts and AGN activity when tidal interactions trigger intense nuclear inflows AGN activity expels gas from centre Star formation rate drops after merger Remnant then becomes red and dead Black holes grow during mergers 23

24 Hydrodynamic merger simulations 24

25 Hydrodynamic merger simulations 25

26 Hydrodynamic merger simulations 26

27 Hydrodynamic merger simulations Hopkins et al ran a large suite of merger simulations Comparison of different merger stages to observed galaxies 27

28 Hydrodynamic merger simulations 28

29 Hydrodynamic merger simulations 29

30 Hydrodynamic merger simulations 30

31 Hydrodynamic merger simulations 31

32 Hydrodynamic merger simulations 32

33 Hydrodynamic merger simulations 33

34 Hydrodynamic merger simulations 34

35 Zoom simulations Increasing the resolution of the box becomes very expensive Alternative: just increase the resolution in the area of interest: 1) Run low resolution simulation and identify interesting object(s) 2) trace back particles of that object to initial positions in ICs 3) resample this area with more particles and rerun the simulation Disadvantage: only one system simulated - no statistics ICs z=0 zoom on halo 35

36 Zoom simulations Introduced by Katz & White 1993 No star formation yet Cluster is formed by flow along a set of intersecting filaments Gas is not isothermal X-ray profile similar to Virgo 36

37 Zoom simulations Then the challenge to form disc galaxies started Governato+ 2004,2010, Okamoto+2008, Guedes+11, Stinson+2013, etc Effective feedback mechanisms (and correct hydro) lead to the formation of disc galaxies Low angular momentum gas is removed from centre through winds Overcooling problem is solved (stellar masses are ok) Early quenching of Milky-Way like galaxies solved with early feedback Still not enough resolution to reliably study morphology 37

38 Zoom simulations 38

39 Zoom simulations 39

40 Zoom simulations 40

41 Zoom simulations 41

42 Zoom simulations 42

43 Zoom simulations 43

44 Up next Lecture 1: Motivation & Historical Overview Lecture 2: Review of Cosmology Lecture 3: Generating initial conditions Lecture 4: Gravity algorithms Lecture 5: Time integration & parallelization Lecture 6: Hydro schemes - Grid codes Lecture 7: Hydro schemes - Particle codes Lecture 8: Radiative cooling, photo heating Lecture 9: Subresolution physics Lecture 10: Halo and subhalo finders Lecture 11: Semi-analytic models Lecture 12: Example simulations: cosmological boxes Lecture 13: Example simulations: mergers and isolated galaxies 44

45 Up next WS 2016/17: Astrophysical fluid dynamics Fluid equations Hydrostatic equilibrium Sound waves Blast waves Fluid instabilities Viscous Flows Accretion discs And more numerics 45