with e = AN/AB. These equations are also valid for the case of two contiguous narrowband filters making e = 0 (Pascual et al. 2006).

Figure 8.9. A magnitude-color selection diagram. Objects with excess emission in the narrow band appear in the upper part of the diagram.

The effective widths of the filters can be calculated to first order (assuming square filters) as the normalized integral of the transmittance of the filter profile (including the quantum efficiency of the detector). It agrees with the equivalent width of the band-transmittance profile Wo of Fiorucci & Munari (2003).

Further refinement can be achieved by estimating the position of the emission line inside the filters. The computation of the effective width can be found in detail in Pascual et al. (2006).


S.P. would like to thank his collaborator Juán Carlos Muñoz-Mateos for providing the multicolor images of the galaxy UCM 2325+2318.

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