Distinguished Lecturer Series - Event Details
Our picture of galaxy formation via hierarchical clustering is very successful in explaining observations on the scales of galaxy clusters but has problems with observations on galaxy scales: (1) Why don't we see as many small satellite galaxies as we expect? (2) Why do we observe nearly-constant-density cores in dark matter (DM) halos when theory predicts cuspy profiles? (3) Why do we see no evidence for halo triaxiality when simulations show that halos should be very triaxial? (4) How can we understand the large numbers of giant galaxies that are pure disks when halos grow to giant size via violent mergers. The solutions to all of these problems probably involve the messy physics of galaxy baryons. But those solutions are no clearly established. The main part of this talk reviews the observed scaling relations between halo parameters. These are derived for high-luminosity galaxies by decomposing rotation curves into visible- and dark-matter contributions. We find that DM halos satisfy well defined scaling laws: Halos in less luminous galaxies have smaller core radii, higher central densities, and smaller central velocity dispersions. Scaling laws provide new constraints on the nature of DM and on galaxy formation. Conclusions: 1. The high DM densities in dwarf spherodial galaxies are normal for such tiny galaxies. Halo densities are proportional to the density of the Universe at the time each galaxy formed; the high densities of the smallest dwarfs show that they formed at redshifts z > 7. 2. The high DM densities in the dwarf companions of our Galaxy imply that they are real galaxies formed from primordial density fluctuations. They are not tidal fragments. Tidal dwarfs cannot retain even the low DM densities of their giant-galaxy progenitors. In contrast, dwarf spheroidal galaxies have higher DM densities than those of their giant-galaxy progenitors. 3. The fact that, as luminosity decreases, dwarf galaxies become much more numerous and much more nearly dominated by DM suggests that there exists a large population of objects that are completely dark. Such objects are a canonical prediction of cold DM theory (see question 1 above). We find a linear correlation between the circular-orbit rotation velocities attributable to baryons and the ones created by DM such that baryons become unimportant at halo rotation velocities of about 40 km/s. This is in good agreement with theoretical predictions of the supression of visible galaxies during cosmological reionization. 4. The slopes of the DM parameter correlations provide a measure on galactic mass scales of the slope of the power spectrum of primordial density fluctuations. The result is consistent with the theory of cold DM.