Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1979
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1979baas...11..622g&link_type=abstract
Bulletin of the American Astronomical Society, Vol. 11, p.622
Astronomy and Astrophysics
Astronomy
3
Scientific paper
It has been suggested by various authors that the UV line strengths of QSOs can largely be explained by external dust between us and the broad-line emitting clouds, corresponding to E(B-V) ~ 0.5. Were this the case, the true photoionizing continuum would be "harder" than the typical alpha = 1 usually assumed. The slopes of the continua of 26 flat radio spectrum QSOs from the survey of Baldwin et al. (1979), Nature, 273, 431, were measured in the range 1300 - 2100 Angs in the emission-line redshift systems. This range was chosen to avoid possible Lyman alpha absorption below 1216 Angs., and broad blends of Fe II lines to the red of 2200 Angs. Absorption lines and wings of strong emission lines were avoided in the measurements. After a correction (usually small) for Galactic reddening, the mean alpha was 0.88 with a scatter of 0.3. The distribution of alpha is actually somewhat asymmetrical with a sharp cutoff at 0.5, a peak at 0.7, and a tail out to 1.7, but with 60% of the QSOs having 0.5 < alpha < 0.9. The measuring error for a typical QSO is 0.26. The data are thus consistent with most flat radio spectrum QSOs having a unique UV power-law index of ~0.7, and a few having slightly steeper spectra. This rules out thermal emission from accretion disks as a significant contributor to the UV continuum here. The average UV to radio (5 GHz) spectral index for these QSOs is 0.61 +/- 0.07, and the similarity of these values suggests that the UV continuum has a common synchrotron-Compton origin with the compact radio source. The small range of UV continuum slopes rules out reddening of more than E(B-V) ~ 0.05 in all but one or two of the QSO. This implies that QSOs cannot be events in the nuclei of spiral galaxies, but is consistent with them being located in the nuclei of elliptical galaxies.
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