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T. A. Boroson (NOAO)
It is often assumed that the redshift of the narrow forbidden lines in QSOs indicate the systemic velocity of the object. The narrow-line region is thought to have a large physical extent, in part because the forbidden lines come from low density gas and in part because of the small velocity width of the lines, which is thought to predominantly reflect the stellar velocity dispersion of the host galaxy. While there are certainly objects that seem to have [O III] 4959,5007 emission lines (typically the strongest narrow lines) blueshifted relative to other indicators of systemic velocity, these are thought to be rare occurrences.
The Sloan Digital Sky Survey (SDSS) has now provided a sample of QSO spectra that is large and uniform enough to test this assertion. A sample of 800 QSOs from the SDSS DR1 has been analyzed with the intent of comparing the redshifts of the [O II], [O III], [N II], and [S II] lines. It is found that the low-ionization lines all indicate a consistent redshift with a dispersion of about 20 km/sec. The [O III] lines, however, are significantly blueshifted in about half of the objects, with velocity differences ranging up to 350 km/sec. This is most easily interpreted as emission from outflowing material in which the backside is obscured. The objects with the largest blueshifts have strong Fe II emission, though not all strong Fe II emitters show this effect. Correlations between the [O III] blueshift and other spectral characteristics are being explored.
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Bulletin of the American Astronomical Society, 36 #2
© YEAR. The American Astronomical Soceity.