Physical Sciences: Interpretation of the Hubble Diagram for Quasars

Abstract

It has been stated that the large scatter in the composite Hubble diagram for quasars may be greatly reduced if only quasars with steep ridio spectra and extended structures are considered (Astrophys. J. Lett. 181: L, 61 (197311. For the latter type of quasar there is a marked correlation between apparent optical magnitude and redshift, with a slope similar to that found for normal radio galaxies and the brightest galaxies in clusters. The absence of such a correlation for quasars with flatter spectra may be attributed either to a broader dispersion in luminosity or to redshifts that are intrinsic to the source rather than a measure of cosmological distance. In the present communication consideration is given in detail to the form of the radio spectra of a sample of 166 quasars of know angular size and redshift, taken from data in the published literature, supplemented by recent observations made at Jodrell Bank and NRAO, Greenbank. The three main categories considered are flat spectrum, concave spectrum, and steep spectrum (straight or convex). A striking correlation is found between spectral type and radio structure. The Hubble diagram for the steep spectrum quasars in the sample shows a significant correlation between apparent magnitudemore » and redshift, and this also applies to quasars with flat spectra considered on their own. The data for both steep and flat spectrum quasars are consistent with a Hubble slope of 5 mag per decade of redshift, and although such 5 slope is not conclusive evidence for cosmological redshifts, it is the value predicted on a Friedmann cosmology. Hubble diagrams are shown for quasars with flat and concave radio spectra. It is not clear why quasars with concave spectra should be a major class that fails to show a significant Hubble relationship. One possible explanation for the apparent steepness of the correlation between magnitude and redshift could be luminosity evolution in the sense that high redshift quasars with concave spectra have systematically greater luminosities than those at low redshift. An optical absorption peculiarity associated with concave spectrum quasars is attributed to the recent expulsion of material from the active nuclei. (UK)« less