Dark Halo and Disk Galaxy Scaling Laws

I highlight recent progress in our understanding of the origin of disk galaxy scaling laws in a hierarchically clustering universe. Numerical simulations of galaxy formation in Cold Dark Matter (CDM) dominated universes indicate that the slope and scatter of the I-band Tully-Fisher (TF) relation are well reproduced in this model, although not, as proposed in recent work, because of the cosmological equivalence between halo mass and circular velocity, but rather as a result of the dynamical response of the halo to the assembly of the luminous component of the galaxy. The zero-point of the TF relation is determined mainly by the stellar mass-to-light ratio ($\Upsilon_I$) as well as by the concentration ($c$) of the dark halo. For $c \sim 10$, as is typical of halos formed in the ``concordance'' $\Lambda$CDM model, we find that this requires $\Upsilon_I \sim 1.5$, in reasonable agreement with the mass-to-light ratios expected of stellar populations with colors similar to those of TF galaxies. This conclusion supersedes that of Navarro & Steinmetz (2000a,b), who claimed the $\Lambda$CDM halos were too concentrated to be consistent with the observed TF relation. The disagreement can be traced to inconsistencies in the normalization of the power spectrum used in that work. Our new results show that simulated disk galaxies in the $\Lambda$CDM scenario are not clearly inconsistent with the observed I-band Tully-Fisher relation. On the other hand,their angular momenta is much lower than observed. Accounting simultaneously for the spin, size and luminosity of disk galaxies remains a challenge for hierarchical models of galaxy formation.