12 Profiling galaxies using contour analysis

ELLFOU is a profiling application providing contour analysis generated profiles. This application takes longer to run and so is only really suitable if you really do not want an intensity analysis method, such as if you are examining barred spiral galaxies. It attempts to profile galaxies by identifying the pixels in a given contour and fitting an ellipse to that shape. This is done by minimising the pixel to ellipse distance sum. Care has been taken to try to make the application robust. As a consequence of this the algorithm used is not quick and for simple galaxies ELLPRO may be the better application.

It will be found that ELLFOU can be fine tuned in the same way as ELLPRO by use of additional parameters such as LIM1 and LIM2 (see above). However, due to the nature of the modelling technique employed, parameters LIM3 and FAST are no longer applicable.

Another consequence of the modelling technique utilised is that the profile radii values generated by ELLFOU may sometimes be clumped slightly. This arises as a direct result of a combination of the value for FINE employed and the actual physical distribution of the pixels contributing to a given contour. The clumping can usually be overcome by reducing FINE somewhat.

As for ELLPRO, the number of inputs required for ELLFOU can be quite large, but the following examples can get you started :

% ellfou mode=true cursor=true in=img001 autol=true device=xw zerop=29

Allows you to use the keyboard and cursor for input. The initial cursor generated guess for the galaxy centre will be refined and the profile displayed on device XW. The magnitude sacle zero point is at 29 magnitudes per arc second.
% ellfou mode=true cursor=false in=img001 origin=’"8:36:50.2 31:17:22"’

Permits interactive use of ELLPRO but only via the keyboard. The origin to be used is at Current co-ordinates (here in the SKY domain) RA=8:36:50.2, Dec=31:17:22.
% ellfou mode=false in=hh10 infile=objs out=profs autol=true

A list of galaxy co-ordinates on image HH10 will be read from text file OBJS. Each object will be profiled in turn and the results placed in text output file PROFS. All galaxy centre values will be refined.

A sample output file for ELLFOU is shown in Appendix F and described in Section 15.