Hβ luminosity of extragalactic objects and evolution of galaxies

Abstract

In order to examine the evolutionary sequence of extragalactic objects Hβ luminosity of 27 QSOs, 26 Seyferts 1,8 N galaxies, 11 Seyferts 2 and 2 normal galaxies has been calculated and compared with its observed value. For calculating Hβ luminosity photoionization model has been used. It has been assumed that theU-V continuum radiation is produced in the cores of the extragalactic objects by synchrotron mechanism and as such its flux is given by a power law. This radiation ionizes hydrogen atoms in the gas clouds surrounding the cores. It is found that the observed Hβ luminosity is less than its calculated value in case of 23 out of 27 QSOs and 14 out of 26 Seyferts 1 and hence it is concluded that the nebulosities surrounding these objects are optically thin for theU-V photons. On the other hand, the observed Hβ luminosity is found to be more than its calculated value in case of the remaining 4 QSOs, 12 Seyferts 1 and all the N, Seyfert 2 and normal galaxies. This discrepancy has been attributed to the presence of other sources-in addition to the synchrotron source-ofU-V photons in the object, in particular, to hot gases surrounding the stars. Consequently, we conclude that the greater the value of the observed Hβ luminosity as compared to its calculated value, the more evolved the extragalactic object. Using this as a guideline and the possibility that BL Lac objects may be ‘young’ QSOs which have not yet ejected much material from the core regions, we suggest that the extragalactic objects evolve along the following sequence: BL Lac objects→QSOs→Seyferts 1→N galaxies→Seyferts 2→normal galaxies. We further suggest that radio-galaxies may lie somewhere either between N galaxies and Seyferts 2 or between Syeferts 2 and normal galaxies, the former being more likely than the latter due to the fact that N galaxies are also strong radio-sources like radiogalaxies and as such radio-galaxies should not be far removed from N galaxies in their pedigree.