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In five nights, sharing two observing runs (see Table 2 for details), we imaged eight clusters in the U-, B-, and V-band.
To fill the large gaps between the CCDs, we shifted the telescope to five different positions, thus covering the entire field of view of the camera.
To fill in this gap, we started the WIde Field Nearby Galaxy-cluster Survey (WINGS, Fasano et al. This survey has focused on clusters located in the redshift range 0.04−0.07 and has collected wide-field optical (B, V) imaging (Varela et al. To complement the WINGS database with U-band imaging, we gathered observations for a subsample of 17 WINGS clusters using three different telescopes equipped with different wide-field cameras and a few archival data.
The assembly of clusters, by itself, seems to be able to suppress the star formation, as suggested by the detection of post-starburst galaxies at the interface of cluster infalling substructures (Poggianti et al. In the past years many high-quality (HST) observations have been devoted to the study of clusters at intermediate and high redshift, while for the local volume, Virgo, Fornax and Coma clusters constituted the main reference sample until a few years ago.
During each night, dome and twilight sky flats were obtained and several photometric standard fields were imaged in each photometric band and at different zenithal distances.
Unfortunately, in both observing runs the weather conditions were inclement.
In particular, the average seeing was about 2′′ and the sky transparency was not good.
Owing to the very large angular view provided by the 90prime camera, we were forced to adopt a more laborious procedure than for the INT data reduction, to obtain a good enough astrometry over the whole field.
Bias subtraction and flat-field corrections were separately performed on each of the four CCDs of the WFC, while the mosaic-image of each exposure was produced using the IRAF tool wfcmosaic.
Moreover, the spatial distribution of the U-band emission within galaxies greatly helps in distinguishing between the various physical processes, by revealing whether star formation is preferentially suppressed and/or enhanced in the central and outskirt regions of galaxies. In Table 2 we report the observing log, one row per run (per night in the case of the LBC observations), each row including the number of imaged clusters.
Our U-band estimates of the current star formation are truly integrated values (though dustaffected), and can be compared with the estimates based on our optical spectroscopy, which samples only the very central regions of each galaxy (1.6″/2.6″). For one cluster (Abell 970) we used imaging data from the MPG/WFI (ESO 2.2 archive). Many clusters have also been imaged in the optical (B, V) bands.
Indeed, observations probing the star formation, Hubble types, and gas content of galaxies in clusters have proved that the cluster outskirts are essential for understanding galaxy transformations (Abraham et al. The full widths at half maximum (FWHMs) for each cluster in each filter are given in the headers of the catalogs (see the first group of rows of the header in Fig. With the WFC, 90prime, and LBC cameras we imaged eight, six, and six clusters, respectively.
For the clusters imaged with LBT, we list in parenthesis the filters used for the imaging.
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Using our astrometric projection defined in the WCS standard, SWARP resampled and co-added the set of five dithered exposures for each cluster in each filter, thus producing the final, backgroud-subtracted image.