Do the rotation curves of spiral galaxies pose a problem for the cold dark matter model of galaxy formation?
http://antwrp.gsfc.nasa.gov/apod/image/0706/m65_wendel.jpghttp://antwrp.gsfc.nasa.gov/apod/image/0706/m65_wendel.jpg
George Rhee University of Nevada, Las Vegas
with Octavio Valenzuela, Anatoly Klypin & Fabio Governato
example of spiral galaxy data
NGC 891
Gas motions and masses of disk galaxies
the rotation curve
Galaxy formation and dark matter
Galaxies form in potential wells of dark matter halos
What form do the density profiles of halos have, and why?
Navarro Frenk and White (1997) find density profiles can be fit by one functional form
Key feature is the formation of a cusp in density at the halo center
Is this seen in galaxy and galaxy cluster density profiles?
NGC 6822 derived rotation curve and CDM fitWeldrake, de Blok and Walter (2002)
Simulations
Gasoline Code: SPH + gravity + star formation + feedback
Ndark 1.6 x 106
Ngas (initial) 2 x 105
Nstar (initial) 2 x 105
force resolution 60 pcMdisk/Mhalo = 0.025
Age = 1Gyr
simulation of a dwarf barred galaxy
face on view cold gas (T<15000 K) stellar componentValenzuela et al., Ap J. 657, 773, 2007
___ circular velocity
- - - stellar rotation
_ _ _ cold gas
Velocity curves for barred dwarf model
0 1 2 3 4 50
20
40
60
Recovering the circular velocity in the model with star formation and a bar__ vcirc...... vrot
--- circular velocity recovery using only the gas pressure gradient
x asymmetric drift correction assuming constant rms velocity of 6km/s
▲ circular velocity recovery with all the terms included:projection effectsrandom bulk velocitiespressure gradients
0 1 2 3 4 520
30
40
50
60
70
Case Studies
NGC 3109 NGC 6822
morphological type SBm IBm
distance 1.4 Mpc 0.5Mpc
inclination 75˚ 60˚
maximum vrot 67 km/s 55 km/s
NGC 6822 Weldrake et al 2003
Weldrake, de Blok and Walter (2002) tilted ring model fits, high resolution HI data
NGC 6822 rotation curve
... data
__ (upper)
circular velocity
__ (lower) rotation velocity measured by an artificial observer
NGC 6822 model
- - - halo contribution ...... disk contribution ___ total circular velocity
ngc 6822 residuals from circular rotation fit
ellipse fit 0.5 kpc semi-major axis
6822 semi major axis 0.5kpc
position angle
-50 0 50
0
0.02
0.04
radial velocity km/s
intensity weighted mean velocity -24.3km/s
maximum velocity -36.8 km/s
Conclusions
• rotation curves alone cannot distinguish between cores and cusps
• must quantify: projection effects, bulk motion, rms velocity and pressure support
• theory: need to understand effect of adiabatic contraction and effect of bar
