UV emission line studies of NGC-low redshift Seyfert Galaxies, LINERS and HII regions

Abstract

Active galaxies as a special class of galaxies are characterized by very strong and broad emission lines. The strong emission lines such as Lyα, NV, Si IV, C IV, and Mg II observed in the UV spectra of Seyfert galaxies and quasars can be used to probe the physical conditions of the gas in the BLR regions surrounding the central accretion discs of these most luminous and exotic objects. In the standard model of broad line emission regions for active galaxies it is assumed that the broad permitted lines are emitted by the photo-ionization of a large number of spherically distributed optically thick clouds which are in Keplerian motion surrounding a central continuum source. However, issues related to variability time-scales, delays in the light curves and BLR sizes etc., remain unexplained consistent with observations. In this paper, a study of emission line properties 9 objects satisfying good SNR (> 5.0) out of 98 NGC (catalogued) IUE observed low redshift active galaxies (z ≤ 0.017) is presented. The International Ultraviolet Explorer (IUE) satellite launched in 1978 by NASA has made low redshift UV spectroscopic observations of many different kinds of UV sources including active galaxies till 1996 and the flux calibrated spectral data of almost all observations have been hosted in NED-IUE database. In the present studies, IUE spectral data of a complete sample of NGC-catalogued active galaxies has been undertaken to understand the emission line properties of low luminosity and low z active galaxies. We find that the emission lines such as Lyα, N V, Si IV, O III], C III], C IV, and Mg II are observed as strong and broad lines in the spectra of only 9 objects owing to the criterion of S/N ≥ 5.0 adopted for the spectral analysis. The Lyα has not been found to be a strong line unlike in high z Seyfert galaxies and quasars observed by IUE satellite. C IV and Mg II lines are observed to be stronger lines in all the nine objects consistent with their usual presence in the remaining (~ 400) active galaxies observed by the IUE satellite. These observations are indicative of different physical and geometrical conditions in the BLR regions surrounding the central accretion disk compared to the intermediate and high redshift Seyfert galaxies and quasars.