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Published online by Cambridge University Press: 19 July 2016
While it is widely held that the densities of QSO and Seyfert broad-line regions lie below 1011 cm−3, there is a growing body of evidence, indicating that at least some of the broad-line gas has densities equal to or exceeding 1011 cm−3. Such evidence includes (1) the presence of a 3μm “bump” in roughly half of all QSOs, (2) the depression of Brγ in several Seyfert galaxies, (3) the broad-line variablity in some objects, and (4) the ability to produce the hydrogen spectrum theoretically.