Ben Moore, Institute for Theoretical Physics, University of Zurich +Oscar Agertz, Roman Teyssier+

1. Obergurgl, December 2009 Galaxy formation: is the end in sight? Ben Moore, Institute for Theoretical Physics, University of Zurich +Oscar Agertz, Roman Teyssier+

3. GHALO: A billion particle simulation of the dark matter distribution surrounding a galaxy. 3 million cpu hours with the parallel gravity code pkdgrav (Stadel et al 2008) 50 parsec, 1000Mo resolution, 100,000 substructures

4. GHALO: A billion particle simulation of the dark matter distribution surrounding a galaxy. 3 million cpu hours with the parallel gravity code pkdgrav (Stadel et al 2008) 50 parsec, 1000Mo resolution, 100,000 substructures

5. What is the origin of morphology? There is a large diversity in the galaxy population. Most stars are in disk galaxies, most galaxies are dSph/dE. What about S0, E, dIrr, LSB, Polar Rings, Bars…… Many of the observed galaxies start off as disks and undergo transformation between classes

6. Malin1 Malin2 200 kpc

7. Moore & Parker 2007 Malin1 has hardly had time to rotate once! How can stars form ~100kpc from the center?

8. The Cartwheel Ring Galaxy

9. Mapelli, Moore et al 2008 MNRAS, 383, 1223 Very low surface brightness galaxies could be the evolved state of ring galaxies T=100Myr=Cartwheel 200kpc T=1Gyr=Malin1

11. Here are some of the key baryonic components of galaxies that we need to simulate correctly

12. Evrard, Summers & Davis 1994 A few hundred SPH particles per ‘glob’

13. Steinmetz & Navarro 1998 1000-10000 SPH particles

14. Abadi et al 2002 Most of stars in a massive spheroid 50,000 SPH particles Baryons far too concentrated

15. Governato et al 2006 10^5 SPH particles in total, 500pc resolution Better match with T-F. Huge spheroid, disk is unresolved single phase cold gas

16. No thin disk dominated system, no morphological detail, no spiral patterns, bulge/spheroid dominated systems.

17. What is resolved and what is put in by hand? The dark matter distribution is resolved to about 0.5% of the virial radius with 1 million particles – that’s about ~1kpc for the MW. The shock heating of baryons and cooling processes are well resolved. Multiphase flows are not followed correctly with SPH (Agertz et al 2008) – almost all galaxy formation studies have used SPH. SPH AMR

18. T=0.2 T=0.5 T=0.8 GRID SPH

19. Hydro/star formation simulations have reached 10^5Mo per gas particle and gravitational force resolution of 0.5kpc, little else is resolved, not even the disk scale height and certainly not the 10pc molecular disk. SPH disks are smooth featureless blobs of single phase gas – no molecular clouds, no spiral patterns – but lets see the next talk?! Star particles put in places where the gas density is high (sets a maximum radius) – but stars form in molecular clouds and the molecular cloud mass function depends on gravitational instability which varies with radius.

20. Simulating star formation in disk galaxies:the present status - 2009 • The ISM is under-resolved • To radially resolve a galactic disk such as the Milky Way can be done using ∆x∼0.5 kpc. At this resolution the whole scale height is within one resolution element. The true disk stability is not captured as both the density and velocity structure (gas and stellar dispersion) is numerically affected. This still allows for the disk to have the correct global characteristics such as gas and stellar mass compositions, thin and thick disk, and at high resolution, realistic spiral structure. In this case a statistical star formation recipe based on the local gas density and free fall time is well motivated both theoretically and observationally • Satellites remain unresolved (the gas doesn’t reach the density threshold) This is possible but the relevant variables (star formation threshold and efficiency) must be understood robustly!

21. Simulating star formation in disk galaxies:the future - 2015 • The ISM is fully resolved • This means that the scale height of all ISM components are resolved using at least 10 resolution elements (Romeo 1994). This translates into at least ∆x∼1 pc. If this is not satisfied the true disk stability will not be modelled accurately. At such a resolution, star formation occurs in their natural sites i.e. massive clouds such as GMCs. This treatment is the goal of most simulations but is due to their computational load beyond the capabilities of modern simulations attempting to study the assembly and evolution of large spiral galaxies to z = 0. In addition, as the star formation sites become resolved the codes need to incorporate the radiative feedback in order to accurately treat the life-times of the GMC structures (Murray et al. 2009). Pandora’s box (new small scale physics must be treated)! Computationally impossible in a cosmological context today

22. Agertz et al 2008: AMR simulations of isolated disks at high resolution can resolve the formation and evolution of molecular clouds. Gravitational instability and MC interactions sets a minimum floor to the disk velocity dispersion. A 1 kpc region of the disk

23. Agertz et al ( 2008) “The observed dispersion velocity is generated through molecular cloud formation and subsequent gravitational encounters”

25. What is the state of the art in cosmological simulations of galaxy formation? We can resolve the ISM to ~50-200pc, enough to follow giant molecular cloud complexes. AMR simulations – Agertz, Teyssier & Moore

26. Feedback • SNII: 10^51 ergs dumped thermally after ~30 Myr. These are the M>8 Msun stars. Given a typical IMF, this occurs for 10 % of the mass of a given population and 10 % of that mass is returned as enriched material. We keep track of the young stars that form and turn off cooling in these regions allowing for the hot pockets of gas to develop into a blastwave. If this is not done the under-resolved medium unphysically radiates away that energy (McKee & Ostriker, Thacker & Couchman) • SNI: As a star of mass 1<M<8 Msun exits the main sequence we calculate the fraction of them being in a binary system and hence is eligible for a type I event. We enriched the ISM with the outcoming metals and Sn energy. • Winds: A population of stars loose 30-40% of its mass in winds. We return this mass to the gas component at the end of a stars life.

27. SN Feedback Thermal Dump z=10

28. SN Feedback Thermal Dump and Delayed Cooling z=10

29. The complex gas flows into a dark matter halo with a forming disk galaxy at a redshift z=3. R=temperature, G=metals and B=density. (Agertz, Teyssier & Moore 2009). One can clearly distinguish the cold pristine gas streams in blue connecting directly onto the edge of the disk, the shock heated gas in red surrounding the disk and metal rich gas in green being stripped from smaller galaxies interacting with the hot halo and cold streams of gas. The disk and the interacting satellites stand out since they are cold, dense and metal rich.

30. The complex gas flows into a dark matter halo forming a disk galaxy R=temperature, G=metals and B=density Agertz et al (2009)

33. Elmegreen et al 2009: Clumpy high redshift galaxies – chains, clusters etc. Clumpy (10^8Mo), high star-formation rates, extended over ~10kpc radii ACS images (Elmegreen et al)

35. No direct evidence for cold infalling gas…very hard to detect. NGC 4650A

36. Maccio’, Moore, Stadel (2005) serendipitous detection of a polar ring galaxy in a cosmological hydro-simulation. Evidence for cold accretion on sub-L* scales.

38. We managed to make a nice looking disk at z=3, but what about the evolution to z=0?

41. Parameter study • Resolution • Star formation threshold • Star formation efficiency • Inclusion of SN 1 events and wind mass-loss • For realistic choices we always get nice disk galaxies. • Due to regulation (balance of star formation and destructive feedback), a resolved disk is less sensitive to the actual choice of parameters. • If the disk is under-resolved, the efficiency sets the bulge to disk ration. The threshold for star formation must be set low enough to avoid “missed star formation events” in the outer disk. • Analytical derivations of required resolution given a star formation recipe will be provided!

42. Gas temperature Gas density Stars Gas metallicity The disk, SR5n1e1ML Gas density Gas temperature Stars Gas metallicity • Thin extended disk of stars and gas • Thick stellar disk and bulge • Hot pockets of gas from supernovae • Massive hot gaseous halo • Gravitational instability: we resolve spiral structure • Stars preferentially form in the arms • Galactic fountain • Realistic metallicity gradient Regardless of parameter choice, all disks get roughly the same global properties. Disks with low B/D ratios are obtained when star formation is resolved at large disk radii and when the efficiency per free fall time is kept below 2%. Detailed properties are unconstrained.

43. The Kennicutt-Schmidt law Bigiel (2009), fig. 15. Compilation using THINGS data + other Our AMR simulations – different colours are different star formation efficiencies

44. Gas temperature Gas density Gas density Gas temperature Stars Gas metallicity Stars Gas metallicity The disk, SR6n01e1ML No dominating bulge! Sb galaxy where the bulge forms from a buckled bar.

45. The rotation curve of the simulated galaxy with low star formation efficiency matches well that of the Milky Way.

46. Compare SDSS stellar mass function with DM halo mass function M⊙ Forero et al 2009 Gao et al 2009

47. Halo gas, SR5n1e1ML Temperature Metallicity • Complex large scale structure in temperature and metallicity • Satellite stripping a la Magellanic Stream

49. A diffuse gaseous halo is robustly predicted with a density and temperature profile in good agreement with the Milky Way non-detection. It can explain the observed HVC head-tail features as well as a ram-pressure origin of the Magellanic Stream. Using the obtained values (n~10^-4 cm-3 and T~ few 10^6 K) we can explain e.g. Smith’s Cloud: The cloud of 10^6 Msun of HI gas is moving towards the disk of the Milky Way at 73 km/s. Smith's Cloud is expected to merge with the Milky Way in 27 million years at a point in the Perseus arm. Observation Simulation

50. The interacting Magellanic Clouds – MW system