Galaxy Structural Parameters in Source Extractor

Over the last decade, the Concentration, Asymmetry and Smoothness (CAS), as well as the M20 and GINI parameters have become popular to automatically classify distant galaxies in images. Ellipticals, spirals and irregular galaxies all appear to occupy different regions of this parameter space. At the same time, the Source Extractor (SE) program has become the mainstay to produce object catalogs from large image surveys. A logical next step would be to incorporate the structural parameters into the Source Extractor software. There are however several problems that arise: 1) the CAS parameters are fits to the images and Source Extractor eschews fits in the interest of speed, 2) the definition of the structural parameters changed over time. Now that there is a clear and agreed-upon definition of the structural parameters, I am incorporating computed versions in the Source Extractor code (v2.5). The fitted CAS parameters are available for the GOODS-N/S fields and I compare the computed structural parameters to those found by the previous fits. My goal is to expand the source structure information in Source Extractor catalogs in order to improve automatic identification of sources, specifically of distant galaxies. The computed parameters perform reasonably close to the fitted versions but noise appears in faint objects due to a lack of information. For a subset of objects, the asymmetry signal is outside the SE boundaries and Smoothness still fails to compute for many objects. Type classification based on the SE parameters still lacks resolving power. 1. Galaxy Structural Parameters There are six parameters that are popular for galaxy classification: Concentration (C), Asymmetry (A), Smoothness/Clumpiness (S), Moment of the top 20 % pixels (M20), the GINI-parameter (G) and the ellipticity of the object (E): Concentration: Conselice (2003) defines Concentration as the 5× log10(r80/r20) with r80 and r20 the circular radii encompassing 80% and 20% of the flux are computed by Source Extractor (FLUX RADIUS with PHOT FLUXFRAC at 0.8 and 0.2). Different flux percentages are sometimes used, e.g., the SDSS uses r90/r50. Since the computation of radii is done in SE, one would expect an straightforward implementation of the Concentration parameter in SE. Asymmetry: Asymmetry is the absolute difference of the original object (I) with the same object, rotated by 180 (R), and divided by the total flux of the object: A = abs(I − R)/I. Asymmetry is normally fit to objects with the x and y values of the center of rotation as variables. The information – position and value of all the pixels belonging to an object– is available in the SE data-structure. Thus, implementation of a calculated –not fitted– version of asymmetry is possible.