P. M. Gondhalekar

The interstellar ionization balance due to ultraviolet radiation

A new estimate of the interstellar ultraviolet radiation field is used to analyse some well established observations of interstellar Na i, Ca n, Mg i, n in £, e and 8 Ori. Four models of the interstellar medium are used and chemical abundances derived relative to that of hydrogen, obtained from Lyman a data. The scatter of the results is discussed together with the apparent anomalous abundance of calcium.

Excitation of the Extended Gas in NGC 1068

We present photoionization models of high exitation gas in the Extended Narrow Line Region (ENLR) of NGC 1068. The ENLR line fluxes have been calculated allowing for attenuation of the nuclear ionizing continuum as a function of distance from the central source. The diffuse continuum emission from the low density gas is also included as an important secondary source of ionizing radiation. The observed emission from high excitation gas, in the ENLR of NGC 1068, is reproduced by assuming the central luminosity to be 3.5 × 1044 erg s-1 between 1014.6 and 1018.4 Hz. The column density attenuating the central continuum is Nh=1022 cm-2. The predicted reflected continuum from the gas located at a distance of 1.5 kpc is about 10 % of the observed extended soft X-ray continuum in NGC 1068.

The Broad emission line region in AGNS and quasars: The Impact of variability studies

Coordinated observations of variability of the continuum and the emission-line luminosities (reverberation mapping) in AGNs and quasars have fundamentally altered our understanding of the broad-line regions in active galaxies. The constraints these observations impose on the models of the BLRs have been demonstrated here by an attempt to model the BLR of NCC5548, the most intensively monitored AGN. Two models of a BLR, with one having a power law radial distribution of density and the other a gaussian radial distribution, are described and the modelled line luminosities and centroid of the line response functions are presented. A self-consistent model is presented for the change in the Civ/Lyα ratio as the continuum luminosity changes. It is shown that BLR gas must be composed of a mixture of optically thin and optically thick gas and the proportion of thick and thin gas alters with the luminosity of the ionizing continuum. The observed centroid or the lag of a line, can be a function of the continuum luminosity.