Next: 5.2.3 Extragalactic velocity field Up: 5.2 Images Previous: 5.2.1 Making an image   Contents   Index

## 5.2.2 Galactic and Extragalactic objects

snapgrid has a specific choice of defaults which would make observers of extragalactic objects, i.e. external observers from the positive Z axis, happy (i.e. xvar=x yvar=y zvar=-vz). However with the help of a few other tools in NEMO one can also make galactic observers happy.

First the extragalactic objects: A total intensity map is generated with the defaults arguments of snapgrid. Channel maps at specific velocities can be generated using snapgrid zrange=zmin:zmax or zrange=zmean,zsig, depending on the required velocity beam. A velocity map is also easy to generate: The raw zeroth and first order moment maps are saved (snapgrid moment=0,1) and smoothed (ccdsmooth) after which they can be divided (ccdmath) resulting in a velocity map. Position-velocity diagrams can be directly generated using snapplot/grid xvar=x yvar=-vz. If position-velocity maps need to be smoothed, remember that it may have to be done in two steps (independently in 'position' and 'velocity'), because the current version of ccdsmooth can only do circular gridded beams if the beam is two dimensional. A more detailed example is given in the next subsection.

snaprotate, snapshift, snapscale are helpful tools to project a galaxy before all these operations are performed.

For galactic objects: one must choose an internal viewpoint (x,y,z) and (vx,vy,vz) somewhere inside'' the object, and make this the new origin using snapspin.

A rotation (snaprotate) may also be necessary. Then use snapplot/grid xvar=vr yvar='atan2(y,x)' evar='m/(x*x+y*y+z*z)', where the Y axis of the plot will be a longitude between and , and the X axis the radial velocity, to create the familiar position-velocity diagram. The evar= keyword is only needed in gridding the snapshot.

Pretty pictures can be obtained using ccdplot, which can combine a contour map, overlayed on a greyscale image.

Next: 5.2.3 Extragalactic velocity field Up: 5.2 Images Previous: 5.2.1 Making an image   Contents   Index
(c) Peter Teuben