Tuesday, February 5, 2013

Rotation Curves

With the latest updates to Galacticus it's now possible to output rotation curves for all galaxies in a model, including the effects of adiabatic contraction on the dark matter profile. This will allow more realistic comparison with measures of the dynamical properties of observed galaxies - such as the Tully-Fisher relation - which are often measured at a specific radius (e.g. 2.2 times the disk radius).

To output rotation curve data, add the following to a Galacticus parameter file:

  <parameter>
    <name>outputRotationCurveData</name>
    <value>true</value>
  </parameter>
  <parameter>

    <name>outputRotationCurveRadii</name>
    <value>
      diskRadius:all:baryonic:loaded:2.2
      diskRadius:all:dark:loaded:2.2
      diskRadius:all:dark:unloaded:2.2
    </value>
  </parameter>

The first option tells Galacticus to output rotation curves. The second parameter specifies which contributions to the should be counted rotation curve and at what radii. These specifiers break down as follows:

 radiusType:componentType:massType:loading?:radius

radiusType specifies which radius in the galaxy we're referring to. In the above we specified that radii will be given in units of the disk radius. We could have also used, diskHalfMassRadius, spheroidRadius, spheroidHalfMassRadius, darkMatterScaleRadius, virialRadius, or just radius (which implies radii are given in units of Mpc). The actual radius at which the rotation curve will be output is given by the radius value.

componentType specifies which components of the galaxy+halo should be counted. Currently allowed values are all, disk, spheroid, hotHalo, darkHalo, and blackHole.

massType specifies which types of mass should be counted. Currently allowed values are all, dark, baryonic, galactic, gaseous, stellar, and blackHole.

The loading? option should be either loaded or unloaded, and specifies whether the effect of baryonic loading (i.e. adiabatic contraction) should be included in the calculation of the rotation curve.

Using these specifiers you can request rotation curves at any radius, and get the contribution from any subset of galaxy or halo components. In the above example, we requested the rotation curve at 2.2 times the disk radius, and asked for the contribution from all components. In the first entry we requested the baryonic contribution to the rotation curve, including adiabatic contraction. In the second and third entries we requested the dark matter contribution to the rotation curve, first with and then without the effects of adiabatic contraction.

By outputting at many different radii, you can build up a full rotation curve. For example, here's the result for a roughly L* galaxy at z=0, plotted using scripts/plotting/plotRotationCurve.pl


It shows quite nicely how the rotation curve stays relatively flat out to large radii, and how this results from the interplay of disk and (adiabatically contracted) dark matter contributions.

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