4 Image analysis

The data reduction of the CCD images follows a procedure identical to the one described in previous papers of this series (Gavazzi et al. 1998; Papers I and II), based on IRAF/ STSDAS data reduction packages. To remove the detector response each image is bias subtracted and divided by the mean of several flat field exposures obtained on the twilight sky. In some cases images were fitted with a 2-D polynomial function to remove second order structures in the image. A median combination of the realigned images allows removal of cosmic rays. Subtraction of contaminating objects, such as nearby stars and galaxies, is done by direct editing of the frames. The sky background is determined in each frame in concentric object-free annuli around the object. The typical uncertainty on the mean background is estimated 10% of the rms in the individual pixels. This represents the dominant source of error in low S/N regions.

Total counts in the two frames have been obtained by integrating the pixel counts over the area covered by each galaxy, as derived by the optical major and minor diameters. Fluxes and equivalent widths and errors are computed using Eqs. (1) and (2) and (6) and (7) of Paper I, assuming K=1.

We corrected for the contamination of the H$\alpha$+[NII] line emission in the broad band filter (OFF-band) using the following relations:

$${F({\rm H}\alpha+[{\rm NII}])_{\rm c}} {=F({\rm H}\alpha+[{\r... ...m d}\lambda}{\int R_{\rm OFF}(\lambda){\rm d}\lambda}}\right)}$$ (1)

and

$${{\rm H}\alpha+[{\rm NII}]EW_{\rm c}}$$

$${={\rm H}\alpha+[{\rm NII}]EW_{\rm o} \left(1+{\frac{{\rm H}\alpha+[{\rm NII}]{EW}_{\rm o}}{\int R_{\rm OFF}}}\right)}$$

$${\times \left(1+{\frac{\int R_{\rm ON}(\lambda){\rm d}\lambda}{\int R_{\rm OFF}(\lambda){\rm d}\lambda}}\right)}$$ (2)

where $F({\rm H}\alpha+[{\rm NII}])_{\rm o}$ and ${\rm H}\alpha+[{\rm NII}]EW_{\rm o}$ are the observed values (from Eqs. (1) and (2) in Paper I), $F({\rm H}\alpha+[{\rm NII}])_{\rm c}$ and ${\rm H}\alpha+[{\rm NII}]EW_{\rm c}$ the corrected ones and  $R_{\rm ON}$ and  $R_{\rm OFF}$ the trasmissivity of the filters given in Col. 4 of Table 3.

 

 

Table 3: The characteristics of the adopted filters

Filter	$\lambda$	$\Delta \lambda$	$R(\lambda)$	$\int R(\lambda){\rm d}\lambda$	Telescope
6568	6574	95	89	83.95	INT
R	6380	1520	83	1307.73	INT
6562	6562	33	66	24.68	NOT
6564	6564	46	74	39.18	NOT
6610	6610	50	76	43.74	NOT
6883	6883	65	77	50.50	NOT
r	6800	1020	86	1249.41	NOT