Nebulosities around QSOs

Abstract

In this paper independent evidence in favour of the hypothesis proposed by Thakur and Sapre (1979) that a QSO consists of a bright central object embedded in an extended nebulosity has been presented. α _U−B and α _B−V , the spectral indices in (U-B) and (B-V) colours, have been calculated for a sample of 80 QSOs with redshiftz≧0.76. In Figure 1 Δα = (α _B−V − α _U−V ) has been plotted against 〈α〉 = (α _B−V + α _U−B )/2. In this figure the QSOs in which detectability of neubulosities has been predicted by Thakur and Sapre (1979) occupy a separate but adjacent part of the diagram as compared to those for which such a prediction has not been made. In Figure 2 (U-B)₀ has been plotted against (B-V)₀ for the same sample of 80 QSOs, where (U-B)₀ and (B-V)₀ are the (U-B) and (B-V) colours corrected for galactic extinction. In this plot also, a similar separation of the two classes of objects is discernible. Another empirical criterion—namely, (B-V)₀≧0.32 for the detectability of nebulosities around QSOs—has been suggested.