SNUC: Surface Photometry of Multiple-Galaxy Systems

SNUC will extract the individual surface photometry profiles of overlapping or interacting galaxies comprising a multiple galaxy system. Each galaxy within the system is assumed to have concentric elliptical isophotes with otherwise arbitrary brightness, position angle, and ellipticity profiles. The isophotes parameters for all galaxies are solved for simultaneously by a nonlinear least-squares algorithm that decomposes the multiple system under the assumption that its morphology can be modeled as the simple superposition of normal elliptical galaxies.

The routine models the image displayed on the TV. The startup procedure for the decomposition is to mark the nuclei and extent of the individual galaxies with the cursor. Mark any nucleus with (1) to fit the specified galaxy to the image or [BOX=n] border, or (2) to then go on and mark an outer limiting isophote with (0). Mark with (0) once all galaxies have been selected to start the decomposition. At this time, five galaxies is the maximum that can be fit. If desired, of course, this routine will also fit single isolated galaxies.

The photometry solution is found iteratively. NPASS iterations are done for a given run (default is 3). Further iterations for a given setup can be run with the OLD keyword. The EX keyword is used with the OLD keyword to delete or add more isophotes to the outer extent of any galaxy used in a previous run. Isophotes are spaced geometrically by the ratio 1+DELT (default is 1.25). Isophote spacing is the same for all galaxies. Isophote centers are fixed by default and taken to be the nuclear centroid for each galaxy. Improved estimates of the centroids can be made during any pass with the RECEN keyword if the default centroiding is found to be inaccurate. Isophote centers can be solved for independently using the CENTER keyword; however, this option does not work well at the present. Since the decomposition solves for the isophotes by a least squares procedure, portions of the image contaminated by stars, defects, faint galaxies can be excluded from the fit. Pixels set identically to 0.0 are ignored by the algorithm. One handy use of this feature is to run a solution, reconstruct it (see RENUC), subtract it off, and clip any faint objects or features uncovered in the process to 0.0. The routine also generates error estimates for the isophote parameters assuming shot noise errors in the pixel values. For accurate error estimates the level of any sky level subtracted from the image must be given with the SKY keyword.

Photometry results can be saved and recalled with the SAVE and GET commands, or printed with the PRINT command, using the MULT option in either of these commands.

See Also: RENUC, PROFILE