The spectra of these three Seyfert 2 nuclei are very rich in absorption features formed in the interstellar medium. The most conspicuous lines are CIV 1#11550, SiII 1#11526, SiIV 1#11400, CII 1#11335, OI+SiII 1#11303 and SiII 1#11260. Some of them are detected at zero redshift (in the observed spectrum), and are thus associated with gas in the Milky Way. However, we also detect absorption features formed in the interstellar medium of the Seyfert galaxies. These are very similar to those observed in starburst galaxies (González Delgado et al 1998), and unlike Mrk 477, where no interstellar absorption line associated with the Seyfert galaxy itself were detected. Instead, in Mrk 477 the lines are observed in emission (Heckman et al 1997). The reason is that the gas clouds in Mrk 477 seem to be illuminated by a powerful anisotropic UV source that has the net effect of scattering photons into our line-of-sight, leading to emission. Some of the absorption lines that we observe in NGC 7130, NGC 5135 and IC 3639 are superposed on, or blended with emission lines. For example, absorption in CII 1#11335 (1334.5 Å, Morton 1991) is partially superposed on a blend of emission multiplets that include the excited fine-structure transitions of CII 1#11335.7. Some of the interstellar absorption lines are also blended with wind resonance or photospheric absorption stellar lines, for example SiIV 1#11400 and CIV 1#11550.
Where are the interstellar lines formed? To address this question we need to consider only pure interstellar lines and exclude lines such as CIV and SiIV which are strongly contaminated by the blueshifted stellar wind features, and CII 1#11335 because its radial velocity reflects the effect of blending with the emission line CII 1#11335.7. Thus, we only consider SiII 1#11260 and SiII 1#11526. We measure their radial velocity with respect to the adopted systemic velocity for each galaxy. If we assume that the Galactic CII 1#11335 is at a heliocentric velocity of zero, then the center of the SiII lines indicates that they are blueshifted by 720, 240, and 290 km s-1 with respect to the systemic velocity in NGC 7130, NGC 5135 and IC 3639, respectively. This blueshift indicates that the warm interstellar gas in these Seyfert galaxies is outflowing (consistent with our inferences based on the L9#9 emission-line). Outflows of similar velocity have been detected in starburst galaxies (González Delgado et al 1998; Kunth et al 1997) and in the post-starburst galaxy NGC 1705 (Heckman & Leitherer 1997; Sahu & Baldes 1997).
In NGC 7130 and NGC 5135 the interstellar absorption lines are not spectrally resolved: their width is similar to those of the Galactic absorption lines, indicating that the UV source in these two Seyferts is extended and fills much of the GHRS aperture. However, the FWHM of SiII 1#11260 in IC 3639 is 2..6519#190.5 Å, corresponding to an intrinsic FWHM of 44019#19120 km s-1. This broadening of the lines is more evidence of the large scale motions of the interstellar gas in IC 3639.
Further evidence of the large scale motions of the interstellar gas comes from the equivalent widths of the SiII lines, which have values between 1-2 Å. These equivalent width are similar to those observed in starbursts (Leitherer et al 1996; Conti et al 1996; González Delgado et al 1998; Heckman et al 1998) and larger than the interstellar lines observed in the spectra of nearby stars. They are optically thick, because SiII 1#11260 and SiII 1#11526 have similar equivalent widths. This means that they are in the flat part of the curve-of-growth, and their equivalent width is mainly related to the velocity dispersion of the gas instead of the column density. Therefore, their equivalent width implies a velocity dispersion of order of 150 km s-1 for NGC 7130 and NGC 5135, and 120 km s-1 for IC 3639.