Class #03: Remove a galaxy to see the noise created by its photons

• Download the image gal.fits and the IRAF script run_residual.cl


• Modify the script where 'XXX' is given, so that an ellipse fit is performed, a galaxy model is created out of the fit, and the model is subtracted from the original image. To do this, use the help pages of the tasks geompar, ellipse, and bmodel. When you have successfully modified the script, you can either copy/paste it into the IRAF command line, or simply say
  cl < run_residual.cl
  
  Here is a possible solution, i.e. a complete and correct script,
  run_residual_solve.cl
  along with the resulting image:
  gal_residual.fits


• View the residual image with DS9, and set the scale parameters such that you can see the larger noise in the center.


• Measure the noise in three small regions at different distances from the galaxy center, using imstat.