Star - Galaxy Discrimination

The following plots show the 2-D projection (in the flux plane) of the parametric scores, including the general star-galaxy discriminents, "mxdn" and "sh", as well as the double and triple star discriminents, "wsh", "msh", "vint" and "r23". The integrated flux is represented by the 20 mag per arcsec**2 isophotal mag.

For more information on the various star-galaxy discrimination parameters, see Star - Galaxy Discrimination Parameters .

Since double and triple stars are "extended" relative to isolated stars (with which the ridgefile is determined), their parametric value (i.e., score) is degenerate with galaxies. Other parametric measures, such as the FWHM or image moments, will also be inadequate discriminators of real galaxies from non-extended objects. Note the "artifact" detections -- meteor streaks appear very much like galaxies.

Now most of the double stars (red triangles) have relatively low scores, between 0 and 5 or 6. There are still a few double stars that have a large score, usually due to a further contaminent (bright star) local to the object. Note that most of the triple stars (blue crosses) have large "wsh" scores, ranging from a few to 20 or so. Most of the galaxies have scores tween 15 and 50, with one or two with scores less than 10.

The meteor streaks remain in the galaxy parameter space. It is evident that these artifact objects are not going away in any parametric space (see below).

Not only are the double stars (red triangles) suppressed relative to galaxies, but most of the triple stars (blue crosses) are also easily distinguished from real galaxies. A few blue crosses remain with high scores, due mostly to objects with more than three stars (i.e., quadruple stars or worse), as well as a few red trangles (double stars with additional comtamaination from nearby bright stars), both of which may be impossible to distinguish from galaxies. At first glance, it would appear that these sources, contaminated doubles and multiple-triple stars, will be the primary contaminent to the galaxy database (excepting artifacts and meteor streaks, which are variable in detection frequency).

N-Space Graphical Display Method

Recent discussion with Eric Feigelson (PSU) brought to light a new (but simple) way to visualize the n-dimensional parameter (score) space. The idea is to "connect the dots" for each score parameter per source, coloring the lines according to the class of object. The N-space cluster is then projected into the viewing plane: parameter vs score value. The plots below demonstrate this method. Six gif plots are given below, one for each mag band (to view only objects with approximately the same integrated flux).

Real galaxies are colored green, stars orange, double stars red and triple stars blue. The N-space scores include: mxdn, sh, wsh, msh, vint, r23, R(3sig) and normalized darea. The latter two parameters respectively refer to the 3-sigma isophotal radius (semi-major axis) and the differential area between the 5-sigma and 3-sigma isophotes (normalized by the 3-sigma isophotal radius). False sources start to show up at K > 12.5, J > 13.5.

It can be seen from the plots that faint galaxies and faint double/triple stars nicely "separate" with the triple-killer scores: vint and r23, whereas for brighter objects the additional double-killer scores: wsh and msh, also separate galaxies from non-extended sources. These results underscore the difficulty of star-galaxy separation at the sensitivity limit (or confusion limit, case being) of the survey.