This work quantifies the influence of the terrestrial atmosphere on the instrumental polarization (IP) of telescopes. Our numerical simulations show that seeing mostly induces a random crosstalk between Stokes parameters. The main contamination affects the linear polarization signals and is due to mixing with intensity. In general, circular

We present bidimensional spectroscopy of the circumnuclear region of the Seyfert galaxy NGC 3227. The physical and kinematical characteristics of the two gaseous components reported recently by Mediavilla & Arribas are discussed. One of the components seems to be directly related to activity. We propose that it represents gas streaming from the

We present a procedure for obtaining the mean density of clusters and their associated structures in a very general type of distribution. We first consider the one-dimensional case, and then use it to develop the procedure according to the d-dimensional case. These procedures require the probability that a randomly placed body should contain no

This work undertakes the study of the galaxy NGC 4861, using imaging centered at the strong lines H-alpha, H-beta, (O III) lambda 5007 and nearby continua. Integrated, absolute fluxes of 28 H II regions, and the region known as the bright knot, have been measured showing correlations between the equivalent width of H-beta and the excitation index

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We present the first results of a combined HST/IUE/ground-based spectroscopic monitoring campaign on the Seyfert 1 galaxy NGC 5548 that was undertaken in order to address questions that require both higher temporal resolution and higher signal-to-noise ratios than were obtained in our previous multiwavelength monitoring of this galaxy in 1988 -- 89