This article concerns the formation and structure of dark matter halos , including ( 1 ) their radial density profiles , ( 2 ) their abundance , and ( 3 ) their merger rates .
The last topic may be relevant to the nature of the small , bright , high-redshift galaxies discovered by the Lyman break technique .
( 1 ) Study of a statistical sample of galaxy-mass dark halos in high-resolution Adaptive Refinement Tree simulations shows that they have a central density profile \rho ( r ) \propto r ^ { - \gamma } with \gamma \approx 0.2 , in agreement with data on dark-matter-dominated disk galaxies .
We present recent , higher resolution results on this .
( 2 ) Another important new result is that the Press-Schechter approximation predicts about twice as many galaxy-mass halos at z = 0 as are present in large dissipationless N-body simulations ; more generally , PS overpredicts the abundance of M \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } 10 % ^ { -1 } M _ { \ast } halos at all redshifts .
( 3 ) Finally , we discuss the assembly of these halos , in particular the merger rate of ( sub- ) halos at high redshift and the distribution of the starbursts that these mergers are likely to trigger .
If most of the Lyman-break galaxies are such starbursts , this perhaps resolves the apparent paradox that these galaxies appear to cluster like massive halos ( \sim 10 ^ { 12 } M _ { \odot } ) , while their relatively low linewidths and their spectral energy distributions suggest that they have relatively low mass ( few \times 10 ^ { 10 } M _ { \odot } ) and young ages ( few \times 10 ^ { 8 } yr ) .
It also predicts much more star formation at high redshift in CDM-type hierarchical models for structure formation than if only quiescent star formation is included .