Dark Halo Profiles: Dynamical Friction and Bars

1
Dark Halo Profiles Dynamical Friction and Bars

• J. A. Sellwood

2
Non-parametric estimator of inner halo
density(Alam, Bullock D. Weinberg)

• Weiners results for N4123 N3095 are well below
  any reasonable LCDM prediction
• Milky Way is unclear
• Halo compression neglected

3
Could halo density be reduced by a bar in the
disk?

• Large halo density changes require
• unrealistic bars
• more angular momentum than the disk can afford to
  lose
• Conclusions are numerically robust
• convergence test up to 160M particles
• resonances are adequately resolved
• why M. Weinbergs required N is wrong

4
Density reduction by a bar

• Rigid bar simulations
• Numerically easy case
• long, massive, skinny bar
• grows up to t10
• Strongly braked
• Inner halo mass distribution rearranged

5
Inner cusp is flattened

• Five separate simulations with different length
  bars
• All are
• skinny (51)
• massive
• Cusp flattened to ? 1/3 bar length

6
Bar strength

• Long bars of differing axis ratios
• Sharp transition cusp flattened if ab 31
• Weaker bars cause little change
• Similar result if bar mass is varied instead

7
Change to mean density

• ?10-fold reduction in ?v/2 possible if the cusp
  is flattened by a bar that is
• skinny ( 31)
• very long (15 kpc)
• massive
• Otherwise fractional change is very small

8
Change to mean density

• ?10-fold reduction in ?v/2 possible if the cusp
  is flattened
• Otherwise fractional change is very small
• Enhanced MoI for the bar little help
• Not enough L in the baryons to do more

9
Possible density reduction

• DM density can be reduced by interaction with a
  bar
• Bar needs to be extreme to reduce mean inner
  density by factor ? 10
• bar semi-major axis ? 12 20 kpc
• bar mass ? 30 of enclosed halo mass
• axis ratio 31
• Less extreme bars cause small changes
• Halo compression neglected

10
Numerical issues

• Weinberg Katz have argued that since resonances
  are important, experiments need huge N to get the
  correct behavior
• Two main arguments
• friction is 2nd order excess of gainers over
  losers requires large N to get balance right
• ?N fluctuations disturb orbits and destroy the
  resonance

11
Numerical convergence

• Short bar case

12
Weinbergs diagrams
13
First criticism

• All orbits along any one line precess relative to
  the bar at the same angular rate
• We need enough particles at each precession rate
  only, not at each point in phase space

14
Superior diagnostic

• Large bar
• over 10 of particles affected by the ILR
• Short bar
• still 0.7 affected by the resonance
• increased to 20 for unequal mass ptcls
• made no difference to the outcome

15
Main criticisms of Weinberg

• Unnecessary to ensure accurate changes in every
  corner of phase space
• in even the most delicate cases, 0.5 of
  particles participate in each important resonance
• Pattern speed changes fast in cases of real
  interest
• collisional relaxation has much longer timescale
• I find identical behavior with grid and field
  (SCF) methods
• Weinberg has not presented convincing evidence
  that meeting his criteria changes the outcome!

16
Conclusions

• DM halos of bright galaxies have mean inner
  densities 10 times lower than predicted by LCDM
• Halo density can be materially reduced
• only by large, massive and skinny bars
• not enough angular momentum in the baryons
• Weinberg grossly overstates the numerical
  difficulties
• Solution to the halo density issue?