10 Obtaining galaxy cross-sections

The ESP application SECTOR may be used to examine, in a quick interactive manner, the intensity profile of a galaxy. Unlike the other profiling routines, ELLPRO and ELLFOU (see the following sections), SECTOR may only be used with the image required displayed on a graphics device, and a keyboard and cursor available to identify various input parameters. It is possible to use SECTOR with only a keyboard but in such a mode its ease of use is reduced.

In the mode allowing a mouse/cursor (parameter CURSOR=TRUE), you are asked to identify the centre of the galaxy on the image, the direction and distance outward from the centre to which the pieslice should extend and also its angular width. The application displays on the image the values entered, drawing the sector/slice defined. You are asked (via the keyboard in all modes) for various information about the image (magnitude scale zero point, background value, pixel size etc.) and a graph is displayed, either on the current graphics device (in one of its quadrants) or on another device, showing how its brightness varies as a function of distance outward from the galaxy centre. The graphs displayed can show the brightness in terms of brightness above sky (expressed in SIGMA) or magnitudes versus one of four transformations for the radius (see Appendix 0). Once this is done, you are prompted to indicate, from the graph, the radius range for the data points to be used to calculate the scale length of the galaxy. Finally, the scale length is calculated, and some information about the data derived is displayed.

Various parameters exist (see Appendix 0) which allow it to be used more efficiently, in particular, it can output the results to a text file for later examination (possibly using MONGO or some similar package) or, alternatively, the ESP application GRAPHS. Another, important parameter, AUTOL refines your estimate of the galaxy’s centre position, whilst parameter MIRROR assumes that the results for two diametrically opposite slices may be used and the mean pixel values for each radii considered, thereby reducing the influence of noise.

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