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Alternative gravity vs. CDM

Alternative gravity vs. CDM. Jerry Sellwood. 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

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Alternative gravity vs. CDM

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  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.

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

  6. 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

  7. The greatest challenge to CDM • Spherically averaged density of dark matter halos seems to approximate the form: (r) = s rs3 / [r(r+rs)3-] • i.e. a broken power law, with 1 <  < 1.5 •  = 1  is “NFW”

  8. Concentration • s is directly related to the concentration parameter c = r200/rs • c correlates with mass – halos are predicted to be a 1-parameter family (e.g. Bullock et al.)

  9. 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 vmax/2 • Points are estimates from real galaxies • Heavy curve is for NFW and standard CDM

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

  11. 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

  12. Measure disk mass dynamically

  13. Measure disk mass dynamically

  14. 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!

  15. Effect of halo compression • Conservative values: • NFW halo • baryon fraction fb=0.05 • disk scale: rs/Rd=5 • Value of v/2 increased by factor 4 • In Weiner’s cases, it would be a factor > 30 (decompression is hard)

  16. Bar-halo friction • Consistent with Debattista’s work on dynamical friction • Rlast is Rc/aB when the simulation was stopped • Rc/aB > 1.4 quickly in high-concentration models • Bars stay fast for 30 disk rots only if c < 6

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

  18. 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

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

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

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

  22. Rapid convergence with N • Use the shortest bar • 104 N  107 • dotted curve for unequal mass particles • Number of terms in expansion, fine grid, etc. all make no diff. • No evidence to support WK05 worries

  23. Weaker bars • Flattening of the cusp occurs only for bars that are both • strong: axis ratio 4:1 or greater, and • massive: Mb > 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

  24. 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 = rs)

  25. 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!

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