Surface Photometry

The surface photometry of galaxies in the WINGS survey has been obtained by using the puposely devised, automatic tool GASPHOT by Pignatelli et al.2006. The followinf flow chart illustrates the working flow of the tool:

The primary goal of GASPHOT is to extract, in a fully automatic and blind mode, reliable estimates of the main photometric and structural parameters of large samples of galaxies from wide/deep-field images. The tool must be robust enough to provide reliable results even under critical conditions (small and/or low surface brightness and/or blended objects) and thereby to skip out the need of guessing the model parameters. GASPHOT favours the robustness of results with respect to the details of galaxy modeling. To this aim, a single Sersic-law for the models has been used and a hybrid 1D/2D approach for the best- fitting algorithm has been adopted.

Extraction of profiles

GASPHOT is heavily based on SExtractor and provides luminosity, position angle and ellipticity profiles of galaxies. In particular, the catalogs of photometric profiles are extracted running several times SExtractor (with some input reference catalogs used for association) on both the input and the segmentation image, by cycling on the isophotal threshold with a given step of surface brightness (usually a few hundredths of magnitude). Each run slices the input image and produces the isophotes corresponding to a given value of the surface brightness for the whole galaxy sample.

Extraction of the PSF

First, for each image, the appropriate, space-varying PSF, is evaluated by the tool itself and represented by a multi-gaussian function .

Global parameters of galaxies

Then, the global parameters of galaxies (total magnitudes, half-light radii and Sersic indices) are obtained by simultaneous fitting of the major and minor axis light growth curves of galaxies with a 2D flattened Sersic-law, convolved by the previously determined PSF (see Figure below).

This technique has the advantage of being less sensitive to the peculiar features of real galaxies, since the parameters defining each single isophote (coordinates of the center, area, flattening, and position angle) are averaged over a large number of pixels. In addition, fitting the light growth curves, instead of the profiles, has the advantage of avoiding the over-weighting of the very inner regions of galaxies, which are often just those more heavily affected by peculiarities (flat cores, dust lanes, double nuclei, etc..). Extensive simulations have shown that the global parameters derived in this way are reliable enough for galaxies with threshold area (at 2.5σ_bkg) greater than 200 pixels for both exponential (n=1) and deVaucouleurs (n=4) luminosity profiles.

Application to WINGS galaxies

We have run GASPHOT on the V-band images of 76 clusters (Abell 3562 has been excluded, due to the short exposure time). Just galaxies with threshold area (at 2.5σ_bkg) greater than 200 pixels have been included in the input reference catalogs . Globally, GASPHOT has provided the surface photometry of 43,027 galaxies (566 per cluster, on average). The Figures below illustrate how the Sersic index is distributed as a function of the morphology for galaxies with absolute magnitude M_V~-19.5.

Two more Figures illustrate, for different morphological types, the apparent ellipticity distribution and the relation between absolute magnitude and effective radius. The morphological type has been derived by means of the automatic tool MORPHOT.

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