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

Cosmological Galaxy Formation

(Moore 2000)

Angular momentum j plays an important role in disk galaxy formation

• Low j: Infall into the inner regions central starburst

formation of galactic bulges and central black holes

• High j: Infall into galactic disk self-regulated star formation

j determines disk and star formation timr escale

Models of elliptical galaxy formation by major mergers requirea reasonable scenario for the formation of disk galaxies.

Angular Momentum and Galaxy Formation

(d‘Onghia, Hetznecker, Burkert)

Simple Model of Galactic Disk Formation

• Initially the gas has the same specific angular momentum as the dark halo.

If angular momentum would be conserved, the size distribution ofgalactic disks would be in agreement with observations.

• Dark halos have a universal mean spin parameter

1 2

5 20.04 0.05 disk

halo

J E R

GM R

Basic assumptions (Fall & Efstathiou 80, Mo, Mao & White 98,

Navarro & Steinmetz 00):

The Cosmological Angular Momentum Problem

Simulated disk galaxies have scale radii that are a factor of 10 smaller than observed bulges instead of disks

(Navarro et al. 2000)

diskjdisk rot diskj v r

Prediction of the simplemodel

Disks retained about half the available angular momentum.

However:

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.

0.06

Correlation between Angular momentum and Baryon Fraction

There exists a surprisingly strong correlation between and the disk mass fraction

disk

Simple explanation:

disk disk DMf M M

3 3diskdisk DM vir vir

vir vir

jand M r v

r v

2 3 2 3disk DM diskM f

diskj const. diskM const.

disk

diskfThe disks either have a very small baryon fraction or a very large specific angular momentum. (Burkert 2003)

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).

(Vitvitska et al.02)

1 (1 z)

• Major mergers increase

• Minor mergers don‘t changeor even decrease

(d‘Onghia & Burkert, astro-ph/0402504)

Major mergers are required to produce large galactic disks.

minor mergers

bulgeless disks

(van den Bosch, Burkert & Swaters 01)

The Major Merger Problem

disk' 0.03 f 0.008

disk

diskf

(d‘Onghia & Burkert, astro-ph/0402504)

Major mergers are required to produce large galactic disks.

The Major Merger Problem

The Physics of Major Mergers

Spin parameter evolutionduring major mergers

(Hetznecker & Burkert 04)

Even for major mergers thereexists an angular momentum

problem!

The Universal Angular Momentum Distribution

• Universal specific angular momentum distribution (Bullock et al. 00)

If angular momentum is conserved, the disks‘s surface density profilesdepend on

1

j

Mj

The Predicted Surface Density Distribution

1.06 2.0

scale

drr

d ln

1.25

(Bullock 00, Burkert 04)

N( )

1.06

2.0

The Specific Angular Momentum Distribution of Bulgeless Disks

The angular momentum distribution is not in agreement with cosmological predictions.

Conclusions

• The average spin parameter of dark matter halos, corrected for unrelaxed major mergers is 0.03

• This is in good agreement with the TF-relation, if gas does notlose specific angular momentum during infall.

• If rotation curve fits indicate a small baryon fraction in bulge-less disk galaxies.0.03 bar B DMf 0.1 /

Substantial gas loss or inefficient cooling (Binney 04)

Selective gas loss of preferentially low-angular momentum gas couldhelp to explain the origin of exponential, bulgeless disks.

• Bulges form naturally from low-angular momentum gas, predictedto exist in cosmological models.