TOWARD AN UNDERSTANDING OF THE SEYFERT-GALAXY NGC-5252 - A SPECTROSCOPIC STUDY

We present long-slit spectra of the Seyfert 2 galaxy NGC 5252, coverin g the nucleus and part of the extranuclear emission-line region. All t he regions sampled by our spectra have emission-line ratios characteri stic of photoionization by an active galactic nucleus-like (AGN-like) continuum source. The spectrum of the nucleus differs from those measu red in extranuclear regions, showing higher values of the line ratios [O I] lambda 6300/H alpha, [S II] lambda lambda 6716, 6731/H alpha, an d [N II] lambda 6584/H alpha, and a lower value of He II lambda 4686/H alpha. These differences can be understood in terms of either a large r fraction of matter-bounded (optically thin) clouds in the extended g as than at the nucleus, or an additional excitation mechanism in the n ucleus, such as interaction with the radio-emitting plasma. The excita tion of the gas in the extended emission-line region, as measured by t he line ratios [O III] lambda 5007/H alpha, [O III] lambda 5007/[O II] lambda 3727, and [O III] lambda 5007/[N II] lambda 6584, is correlate d with the emission-line brightness. This result is another piece of e vidence favoring an important contribution from matter-bound clouds in the extranuclear gas. The electron temperatures in the extended emiss ion-line regions are in the range 20,000-24,000 K, higher than predict ed by the simplest photoionization models but similar to the temperatu res found in other extended emission-line regions associated with acti ve galactic nuclei. The higher spectral resolution observations are us ed to study the kinematics of the line-emitting gas. Very close to the nucleus, two different kinematical components are found: one extendin g toward the northwest of the nucleus and blueshifted with respect to the adopted systemic velocity and the other extending toward the south east and redshifted. Such kinematical components may be associated wit h the nuclear jet-like radio structure or with bright, compact knots r ecently detected in Hubble Space Telescope images. Outside the nucleus , the velocity field of the gas is complex, suggesting that radial mot ions, perhaps driven by the active nucleus, in addition to rotation, a re present. A broad H alpha component (FWHM similar to 2500 km s(-1)) is detected in the nucleus, but no similar one is found for H beta. Th e luminosity derived for the nucleus using recombination lines is rela tively high for a Seyfert 2 nucleus. Both results are consistent with the hypothesis that NGC 5252 contains a partially hidden broad-line re gion and a more luminous nucleus than is directly observed.