GALWORKS Results for the COMA Core: Algorithm 1


    Tom Jarrett
    IPAC

    (970223)



This work represents our first efforts to verify the functionality and design of the 3-channel version of GALWORKS. Since the GALWORKS package was ready for full scale tests sans real 3-channel data (as of late 1996), we 'created' a 3-channel dataset using a set of scans from the prototype camera 1995 observing run. This memo describes some of the key results from these "3-channel" tests. In particular, we focus on the GALWORKS algorithm 1 results -- normal galaxies and extended sources. A later memo will describe results from algorithm 2, the low central surface brightness galaxies. In the first section we describe how the experiment was constructed, the second section describes the "seeing" conditions of the data set, the third section provides an image gallery of various objects in the study, the fourth describes the various scoring parameters used to distinguish galaxies from stars, and the final section presents the overall reliability of the GALWORKS output.

  • Construction of 3-Channel Data Set


    • SEEING

    The following figure shows the seeing for scans 056 to 058, night of 95-05-03. The seeing is represented by the "shape" parameter, which is a combination of two parameters describing the stellar profile:
    The figure represents shape versus scan coordinate. Two versions are included: The first is the "seeman" version, or the seeing values computed by the SEEMAN module (for additional details see the SEEMAN SDS); The second is the GALWORKS version, "seeridge" which uses the same information as the SEEMAN version but computes the seeing with a slightly more sources per seeing value (and thus is a more accurate measure of the seeing, assuming that the seeing is tracked during the duration of time comprising the set of sources per seeing value).

    For good seeing, (FWHM < 2"), the shape value is less than 1.1 or so. Very bad seeing corresponds to shape values > 1.2 or so. Notice that the seeing is reasonably good at all three bands.



    Postage Stamp Images

    Below are the 39 brightest galaxies in the COMA core (ed: yes, there are many more galaxies, but I'm tired of making gif images). The first column is the J band image, second column the H band image, third column the K band image and the fourth column is the DSS optical image. The dark blue elliptical contour represents the 20 mag per sq. arcsec isophotal area, and the light blue contour the "flux growth" elliptical area. Sources that had been "subtracted" from the object fields are circled in red with the size of the circle given by the subtraction radius. Sources circled with a green circle/ellipse represent sources that were previously processed and subsequently blanked from the object field (blanked pixels are then substituted with corresponding isophotal values given by the object of interest, thereby recovering pixel information).



    Score Parameters

    A number of parameters are used to distinguish stars (point sources) from galaxies (extended sources). There are three basic kinds of parameters to this end: (1) the peak flux versus integrated flux, (2) radial profile, and (3) emission located beyond the central peak and wings of a stellar PSF, and various combinations thereof. The central surface brightness to total brightness parameter is referred to as "mxdn". The radial profile parameters derive primarily from the radial profile function:

    We assign a "score" to each value based on its N-sigma relationship to the nominal value displayed by stars (the stellar "ridge"); e.g., for the "sh" score
    Thus the parameter score represents the n-sigma value measured for the source of interest. Typical threholds are set a 2*sigma, or a score = 2.0.

    The following plots show the scoring results for the COMA GALWORKS candidate galaxies. Every candidate was visually inspected using the JHK images as well as the DSS images and POSS prints (when necessary) in order to classify/verify the objects as follows:

    SCMXDN vs Mag : "mxdn" score

    The "mxdn" score is derived from the peak pixel brightness versus the integrated flux (using an adaptive circular aperture). Galaxies are denoted by the filled white circles, false galaxies (stars) by small red triangles, double stars by large red triangles, and unknown sources by yellow crosses. The score threshold was set at 2-sigma, or scmxdn = 2.


    SCSH vs Mag : "sh" score

    The "sh" score is derived from the radial profile, as discussed earlier in this memo. It is effective for most kinds of stars, except double stars and other multiple point source objects.


    SCWSH vs Mag : "wsh" score

    The "wsh" score (a.k.a. "wedge shape") is derived similar to the "sh" score, except that it employs a rotating wedge-shaped mask designed to exclude sections of the radial profile from the modified exponential function fit. It's primary purpose is to minimize contamination from double stars.


    The remaining score parameters were designed specifically to counter contamination from multiple stars. Since the COMA field has few doubles and even fewer triple stars (if any), this data set is not useful for testing these "triple killer" parameters. We will have to wait for real 3-channel data of high stellar density fields.

    It is still instructive to see how the triple killer parameters handle low stellar density fields (i.e., single stars), as well as the galaxies themselves. It can bee seen from the plots that the triple star killers will also kill galaxies at the faint end if the thresholds are not chosen carefully (i.e., use triple killers for only high stellar density fields).




    Reliability

    The reliability is measured as follows:

    From the last figure below, it can be seen that the reliablity is for the most between 90% and 100% for K< 13.5, J&H < 14.5, which is about what the level-1 specifications are for high galactic latitude fields. Given the relatively few sources (only 1 square degree was covered) these results should not be overinterpreted.