A comparison between the rest-frame optical/UV continua of high ($z\ge3$) and low ($z\sim0.1$) redshift quasars

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Session 17 -- Radio Galaxies and High Luminosity AGNs
Oral presentation, Monday, 30, 1994, 10:00-11:30

[17.04] A comparison between the rest-frame optical/UV continua of high ($z\ge3$) and low ($z\sim0.1$) redshift quasars

O. Kuhn (Harvard University), M. Elvis (SAO), J. Bechtold, M. Rieke, R. Cutri (Steward Obsevatory), B. Wilkes (SAO)

The evolution of the optical quasar luminosity function is well described as `pure luminosity evolution' with the characteristic luminosity, $L_{*}$, $\propto (1+z)^{k}$ ($k\sim3$ for $z<\sim2$ and $k\sim0-1$ for $z>\sim2$; e.g. Boyle et al. 1991, in ``The Space Distribution of Quasars, ed D. Crampton; Hewett et al. 1993, ApJ, 406 L43). In an effort to relate the statistical to physical evolution for quasars and, thus, constrain models for the quasar energy source, we have collected IR and optical data for $z\ge3$ and $z\sim1$ quasars selected to represent equally well all those at a given epoch (i.e. $L/L_{*}$ within a fixed range). From these data, we have constructed optical/UV continua, and we compare these to the mean continuum for a set of 47 $z\sim0.1$ quasars (Elvis et al. 1994, ApJS, submitted).

For the bright, $z=3.02$ quasar, HS1946+7658 (Hagen et al. 1992, AA 253, L5), we have obtained 5GHz and 1415MHz data from the VLA, an upper limit at N (10.6$\mu$m), near-IR photometry and spectroscopy, optical spectrophotometry and high resolution (FWHM $\sim80$ km s$^{-1}$) spectra, and a ROSAT PSPC spectrum. The spectral energy distribution (SED) of HS1946+7658 appears generally similar to the mean $z\sim0.1$ continuum, however it differs within specific spectral regions. In the UV, after correcting for reddening and flux suppression due to the Ly$\alpha$ `forest', we find that the continuum turns down more rapidly than the mean $z\sim0.1$ SED. Absorption by numerous low column density line-of-sight clouds might produce this residual turnover (Press and Rybicki 1993, ApJ, 418, 585). At $\lambda_{rest} \sim 3000$\AA, the SED of HS1946+7658 shows little evidence for the blended FeII and Balmer continuum emission which is common to the spectra of low $z$ quasars, and thus suggests a lack of FeII. Finally, our upper limit at N ($\lambda_{rest} \approx 2.5\mu$m), indicates that HS1946+7658 is not redder than the average low $z$ quasar as would be expected if it were recently formed in a dusty galaxy merger (Sanders et al. 1988, ApJ, 325, 74).

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