Automatic galaxy surface photometry
In the framework of the WINGS project, we have devised the tool GASPHOT, which is able to provide, in a single shot, the relevant photometric (total magnitude and effective surface brightness) and structural (effective radius, Sersic index, ellipticity) parameters for the several hundreds galaxies typically involved in our wide field cluster images. A flow-chart of GASPHOT is shown in Figure 1.
GASPHOT is heavily based on the use of SExtractor, which allows us to obtain the photometric and geometrical profiles of the galaxies in the field, altogether. The files of geometrical profiles can be used, outside the tool, for more sophisticated structural analyses, while the photometric profiles are used in the following steps of the tool. In particular, GASPHOT uses a Sersic law, convolved with a space-varying PSF (extracted by the tool itself using the stars present in the frame) to produce a simultaneous fitting of the light growth curves obtained along the major- and minor-axis of the galaxies. This approach exploits the robustness of the one-dimensional fitting technique, saving at the same time the capability, typical of 2D approaches, of dealing with PSF convolution of flattened galaxies.
We tested the results of GASPHOT against the widely used tools GALFIT and GIM2D. The comparison shows that the performances of GASPHOT are similar to those of GALFIT and GIM2D for large, regular (simulated) galaxies. Instead, the parameters retrieved by GASPHOT are much more stable for (real) galaxies having some kind of irregularity or blending, which is a crucial feature when dealing with blind surface photometry of huge galaxy samples.
GASPHOT runs on UNIX-based OS and makes use of the F90 compiler, as well as of the widely used software SExtractor and IRAF. In order to safely run GASPHOT, a Random Access Memory of at least 1Gb is strongly advisable.
GASPHOT is available here. However, in its present version, it is not user friendly enough to be used without a preliminary training. We plan to release an easy-to-use, web-bases version of the tool as soon as possible.
People: G. Fasano
Collaboration: E. Pignatelli (external), P. Cassata (INAF OA Milano)
Publications: Pignatelli et al. (2006), A&A 446,373