11 Profiling galaxies using intensity analysis

ELLPRO is intended to be the main ellipse profiling application within the ESP suite. It has been created as a robust application which attempts to generate ellipse profiles by placing a trial ellipse profile on a galaxy and then adjusting the ellipse characteristics to ensure the minimum possible variation in intensity around the ellipse. To do this it employs minimisation routines that are tuned for normal galaxies. With barred spiral galaxies, ELLFOU may be the better application to use.

For the examination of a few galaxies on an image it is best used in interactive mode. This requires that the cursor (a mouse or cursor control device) is used in conjunction with the keyboard and that, before starting, the image containing the galaxy to be profiled is displayed. The cursor is then used to indicate roughly where the galaxy centre is (a mistake of a few pixels is rarely important) and to indicate how far out from the centre the profiling action should cease. Thereafter information about the image (its background value, magnitude scale zero point, pixel size, etc.) is entered before profiling begins. Two additional parameters that must be set are AUTOL and FRZORI. AUTOL will (if set to TRUE) cause the software to look at your estimated position for the galaxy centre and will refine it using a distance weighted mean method or a centroid (via AUTOLT). FRZORI set to TRUE forces the application to accept your estimate of the galaxy centre as the value to be used throughout. In such a case it is usually best used in conjunction with AUTOL set to TRUE.

A number of other fine control ‘tuning’ parameters are available and are adjusted via the command line if the default value is not to be employed.

LIM1 may be used to adjust the threshold employed to stop profiling when the brightness results seem to have become scattered. Whenever the application finishes a new isophote it compares the brightness it found (B1) with the average found for the two previous isophotes (B2). If the value of B1/B2 is greater than LIM1 then the isophote is assumed to be low into image noise or possibly the ellipse has encountered contamination. These assumptions are for most galaxies perfectly reasonable and the main use of LIM1 is to force the profiling action to take place to lower isophotes than normally required. You may care to compare the results obtained when using the default value and (say) 5 or 10. LIM1 may prove particularly useful if you are examining the profiles of shell galaxies.

LIM2 provides another method of limiting the radius of the largest profile generated by ELLPRO. If it is found that the value of a newly generated isophote is below LIM2xSIGMA then the profiling action immediately ceases.

LIM3 is used to define an ellipse semi-major radius value at which the ‘fitting’ operation ceases to allow changes to the ellipticity or position angle. Consequently these remain fixed at their previous values while the mean value for the isophote drops for each succesive isophote until profiling ceases. This parameter can only sensibly be used when profiling very well behaved galaxies and is not generally recommended.

The FAST parameter is provided for those users particularly interested in the inner regions of galaxies. When normally estimating the brightness of pixels around the isophotal ellipses (FAST set to TRUE) a bi-linear interpolation method is used. At very small radii this will not be particularly accurate. To overcome this you may opt for an interpolated value based upon a 8x8 surface created using a bi-cubic spline. This involves considerable additional calculation. At present the 8x8 surface generation ceases at radii greater than 5 pixels, as computers become faster it may be possible to extend this to a greater radius.

The final additional tuning parameter is FINE. The default value is 1. If this is decreased then more profiles are generated (or at least attempted) while if it is increased the number of profiles generated drops. It is not recommended that values greater than 2 or less than .02 be used. The former will produce few ellipses and the latter might take some time.

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

% ellpro mode=true cursor=true in=img001 autol=true frzori=true

Allows you to use the keyboard and cursor for input. The initial cursor generated guess for the galaxy centre will be refined and not allowed to move from isophote to isophote.
% ellpro mode=true cursor=false in=img001 origin=’"96 92"’ rlim=10

Permits interactive use of ELLPRO but only via the keyboard. The origin to be used is at Current co-ordinates 96,92 and the profiling will extend out to a radius of 10 arc seconds. (A quirk of the ADAM parameter system means that the ORIGIN value has to be quoted like this).
% ellpro mode=false in=hh10 infile=objs out=profs autol=true autolt=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 using a centroid method.

An example output file for ELLPRO is shown in Appendix F and described in Section 15