1. Alternative gravity vs.  CDM Jerry Sellwood

2. Settling the argument Requires clear predictions that distinguish one from the other –consistency with one or the other is not enough if both make similar predictions Alternative gravity is more easily falsifiable –e.g. Milgrom predicted TFR for LSBs not yet regarded as decisive by the  CDM folks –but predictions must be well-worked out!

3. WMAP 3-year data Rules out all no DM models? No!

4. Falsifiable predictions of AG Baryonic mass should be correlated with dynamical mass. Vulnerable to: –one rogue galaxy rotation curve –similar light distributions with very diff. M/L –etc. The shape of luminous matter should be reflected in the shape of the mass –no misalignments or offsets, etc.

6. Other concerns Galaxy clusters Dwarfs & globular clusters Dynamical friction and galaxy mergers ….

7. Challenging  CDM Gauntlet already thrown down: –TFR for LSBs –Why does MOND work? Issues involving gastrophysics are too murky Somewhat firm predictions of DM halos –cusp/core issue – still no surrender! –absolute density scale But target just moved! –baryon/dark mass fraction –tilted or running spectral index

8. The greatest challenge to  CDM Spherically averaged density of dark matter halos seems to approximate the form:  (r) =  s r s 3 / [r  (r+r s ) 3-  ] i.e. a broken power law, with 1 <  < 1.5  = 1  is “NFW”

9. Concentration  s is directly related to the concentration parameter c = r 200 /r s c correlates with mass – halos are predicted to be a 1-parameter family (e.g. Bullock et al.)

10. Halo density Dark matter halos are not as dense as predicted Plot from Alam et al.  v/2 is the mean density inside the radius at which the DM rotation curve reaches v max /2 Points are estimates from real galaxies Heavy curve is for NFW and standard  CDM

11. Tilted or running power spectrum Zentner & Bullock (2002): Lower values of  v/2 predicted –by about a factor 10 in their most extreme model (n.b.  8  0.65)

12. 1 practical difficulty How much mass should be assigned to the stars? Disk-halo degeneracy Low surface- brightness galaxies and dwarfs are more dominated by DM

13. Measure disk mass dynamically

15. Magnitude of discrepancy Weiner’s work gets around uncertainty in M/L Milky Way similar (Binney & Evans 2001) Better data are in worse agreement Halos are under-dense by factor > 30 for n=1 models > 5 for extreme tilted power spectra assumes  =1 and ignores compression!

16. Effect of halo compression Conservative values: –NFW halo –baryon fraction f b =0.05 –disk scale: r s /R d =5 Value of  v/2 increased by factor 4 In Weiner’s cases, it would be a factor > 30 (decompression is hard)

17. Bar-halo friction Consistent with Debattista’s work on dynamical friction R last is R c /a B when the simulation was stopped R c /a B > 1.4 quickly in high-concentration models Bars stay fast for 30 disk rots only if c < 6

18. Reduce DM density? Feedback – Gnedin & Zhao –points vs. dashed –maximum possible effect – factor  2 –for a disk of reasonable size

19. Reduce DM density? Feedback – Gnedin & Zhao Binary BHs – Milosavljevic & Merritt –DM particles ejected as the binary hardens –removes about as much mass as the BHs –but only to a radius of a few hundred pc

20. Reduce DM density? Feedback – Gnedin & Zhao Binary BHs – Milosavljevic & Merritt Bars – Weinberg & Katz

21. Bar-halo interaction Holley- Bockelmann, Weinberg & Katz (2005) Smaller changes reported by Weinberg & Katz (2006) –argue problem is very challenging numerically

22. Density reductions 5 skinny, massive bars of different lengths flatten the cusp to about 1/3 bar length interesting, but unreasonable bar required

23. Rapid convergence with N Use the shortest bar –10 4  N  10 7 –dotted curve for unequal mass particles Number of terms in expansion, fine grid, etc. all make no diff. No evidence to support WK05 worries

24. Weaker bars Flattening of the cusp occurs only for bars that are both –strong: axis ratio 4:1 or greater, and –massive: M b > 40% of enclosed halo mass Sudden change in density – a collective effect Smaller and more gradual density change for slightly weaker bars – but over a greater radial range

25. Maximum effect Rigid bar highly artificial –increase MoI by factor 5 –more significant density reduction Reduction in  v/2 is only by 39% in most extreme case –Angular momentum transferred:  0.01 –i.e. most of that in the baryons And this was for a huge bar (a = r s )

26. Conclusions Best data on halos in galaxies indicate densities lower than LCDM prediction by factor >10 –assumes  =1 and neglects compression No internal dynamical mechanism can reduce the density by much –maximum  40% for most extreme bars –results from careful simulations can be trusted Simply cannot unbind the halo –not enough energy can be extracted from the baryons –trying to make the tail wag the dog!