= Hierarchical galaxy formation
{title2=$\sigma(M)\text{ increases as }M\text{ decreases}$}
In <hierarchical galaxy formation>, low-mass dark-matter structures typically collapse before larger structures because the smoothed linear density variance is larger on smaller mass scales. Accretion and mergers assemble increasingly massive haloes. Cooling, feedback and angular momentum determine how baryons form galaxies within that halo hierarchy; the ordering is statistical rather than an absolute rule for every object.
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