The merging history of dark halos dominates galaxy formation .

1. Cosmological Galaxy Formation The merging history of dark halos dominates galaxy formation. • Spiral galaxies: merging of small, • gas-rich substructures. • Elliptical Galaxies: merging of • equal-mass substructures • (Toomre & Toomre 1972). Spiral Galaxies Elliptical Galaxies (Moore 2000)

2. Angular Momentum and Galaxy Formation (d‘Onghia, Hetznecker, Burkert) Angular momentum j plays an important role in disk galaxy formation Infall into the inner regions central starburst • Low j: formation of galactic bulges and central black holes self-regulated star formation Infall into galactic disk • High j: j determines Models of elliptical galaxy formation by major mergers require a reasonable scenario for the formation of disk galaxies.

3. Simple Model of Galactic Disk Formation Basic assumptions (Fall & Efstathiou 80, Mo, Mao & White 98, Navarro & Steinmetz 00): • Initially the gas has the same specific angular momentum as • the dark halo. • Dark halos have a universal mean spin parameter If angular momentum would be conserved, the size distribution of galactic disks would be in agreement with observations.

4. The Cosmological Angular Momentum Problem Prediction of the simple model Disks retained about half the available angular momentum. However: (Navarro et al. 2000) Simulated disk galaxies have scale radii that are a factor of 10 smaller than observed bulges instead of disks

5. The Angular Momentum Distribution of Bulgeless Spirals • The angular momentum distribution of 20 bulge-less spirals has been • investigated, taking into account beam smearing. (Burkert 2000; van den Bosch, Burkert & Swaters 2001) The specific angular momentum is conserved during gas infall.

6. Correlation between Angular momentum and Baryon Fraction There exists a surprisingly strong correlation between and the disk mass fraction Simple explanation: The disks either have a very small baryon fraction or a very large specific angular momentum. (Burkert 2003)

7. The Importance of Major Mergers • The rotation of galaxies has long been discussed to result from • gravitational torques (Hoyle 49; Peebles 69). • High-resolution simulations find no • steady increase of angular momentum • with time as predicted for tidal torques • (Vitvitska et al. 02). • Major mergers increase • Minor mergers don‘t change • or even decrease (Vitvitska et al.02)

8. The Major Merger Problem Major mergers are required to produce large galactic disks. (d‘Onghia & Burkert, astro-ph/0402504) bulgeless disks (van den Bosch, Burkert & Swaters 01) minor mergers

9. The Major Merger Problem Major mergers are required to produce large galactic disks. (d‘Onghia & Burkert, astro-ph/0402504)

10. The Physics of Major Mergers Spin parameter evolution during major mergers (Hetznecker & Burkert 04) Even for major mergers there exists an angular momentum problem!

11. The Universal Angular Momentum Distribution • Universal specific angular momentum distribution (Bullock et al. 00) If angular momentum is conserved, the disks‘s surface density profiles depend on

12. The Predicted Surface Density Distribution (Bullock 00, Burkert 04)

13. The Specific Angular Momentum Distribution of Bulgeless Disks The angular momentum distribution is not in agreement with cosmological predictions.

14. Conclusions • The average spin parameter of dark matter halos, corrected for • unrelaxed major mergers is • This is in good agreement with the TF-relation, if gas does not • lose specific angular momentum during infall. • Bulges form naturally from low-angular momentum gas, predicted • to exist in cosmological models. • If rotation curve fits indicate a small baryon fraction • in bulge-less disk galaxies. Substantial gas loss or inefficient cooling (Binney 04) Selective gas loss of preferentially low-angular momentum gas could help to explain the origin of exponential, bulgeless disks.