Galactic Velocity Fields

Here are some examples of how to generate some galactic velocity fields within NEMO, and analyze them. There are several ways to get a velocity field: from snapshots, from orbits (but turning them into a snapshot to make the velocity field) and directly from observations (usually via a FITS file). In addition, RA-DEC-VEL style datacubes can also be generated (or read) from which velocity fields can then be derived.

A simple model

The first step is create a rotating thin disk, and incline it a bit:

%  mkspiral - 200000 rmax=2 a=0 | snaprotate - snap1 60,60 xz

which is a snapshot. There are several ways to make a velocity field out of this:


%  snapmap snap1 map1 

%  ds9 & 

%  nds9 map1

Next method would be snapgrid, which has more options to simulate beam smearing.


%  snapgrid snap1 map2 moment=-1 

%  ccdmath map1,map2 map1-2 %1-%2

gives nearly the same result, as can be seen by viewing a difference map.

Third, create a cube:


%  snapgrid snap1 cube1  zrange=-0.6:0.6 nz=64 

%  ccdmom cube1 map3 axis=3 mom=1  

%  ccdmath map1,map3 map1-3 %1-%2 

%  ccdstat map3 bad=0 
Min=-0.515625  Max=0.516481
Number of points      : 1701
Mean and dispersion   : 0.000112 0.027680
Skewness and kurtosis : 3.350128 274.874773
Sum and Sum*Dx*Dy*Dz  : 0.190883 0.000746
2395/4096 out-of-range points discarded

Lastly, pvtrace can now also be used on cubes:


%  pvtrace cube1 out=map4 mode=3 
### Warning [pvtrace]: cube mode
### nemo Debug Info: Map [64 x 64 POS x 64 VEL] I_t=0.040192 vsys=0 (vrange: -0.590625 0.590625)
### nemo Debug Info: MinMax in map: -0.534375 0.534375

%  ccdmath map1,map4 map1-4 %1-%2 

%  ccdstat map4 bad=0            
Min=-0.515625  Max=0.516481
Number of points      : 1701
Mean and dispersion   : 0.000112 0.027680
Skewness and kurtosis : 3.350128 274.874773
Sum and Sum*Dx*Dy*Dz  : 0.190883 0.000746
2395/4096 out-of-range points discarded

This page was last modified on 17-Aug-2009 by PJT.