Abstract
The hypothesis that the optical continua of QSOs, BL Lac objects and the nuclei of Seyfert, N and normal galaxies arise due to incoherent synchrotron radiation from electrons and consequently the flux of radiation in the optical continua of these objects is given by the power law:F(v)∝v α is examined. Following Kinman, spectral indices αB−V and αU−B in(B-V) and(U-B) colours as well as their average 〈α〉 and difference Δα have been defined and calculated for samples of 227 QSOs, 32 BL Lac objects and nuclei of 62 Seyfert, 12 N and 7 normal galaxies. Here 〈α〉 has been assumed to be an estimate of the spectral index α. On the other hand, Δα has been regarded as a measure of departure from the power law. On the basis of this, the distributions of Δα and 〈α〉 in QSOs, BL Lac objects and the nuclei of Seyfert, N and normal galaxies have been studied. The value of α depends mainly on the balance of the energy loss due to synchrotron radiation and the rate of replenishment of energy by injection of high energy electrons in the radiating region. The increase in the value of α and therefore that of 〈α〉 indicates that the activity in the object is slowing down and the object is growing older. Assuming that the QSOs, BL Lac objects and the nuclei of Seyfert, N and normal galaxies essentially represent different phases in the evolutionary sequence of extragalactic objects, we suggest that they may be arranged in the sequence: QSOs→BL Lac objects → Seyferts 1→N galaxies → Seyferts 2 → Normal galaxies in the decreasing order of activity in the core or nuclei of these objects.
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On leave of absence from the Government Science College, Rajpur, M.P., India.
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Thakur, R.K., Sood, R.K. The spectral indices in the optical continuum of the QSOs, BL Lacertae objects and the nuclei of Seyfert, N and normal galaxies. Astrophys Space Sci 75, 473–499 (1981). https://doi.org/10.1007/BF00648657
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DOI: https://doi.org/10.1007/BF00648657