  The growth of active galactic nuclei (AGN) occurs under some form of
obscuration in a large fraction of the population. The difficulty in
constraining this population leads to high uncertainties in cosmic X-ray
background and galaxy evolution models. Using an SDSS-WISE cross-match, we
target infrared luminous AGN ($W1-W2$ > 0.8, and monochromatic rest-frame
luminosity above $\lambda L_{\lambda}$(12$\mu$m) $\approx$ 3 $\times$ 10$^{44}$
erg s$^{-1}$), but with passive galaxy-like optical spectra (Optically
Quiescent Quasars; OQQs). We find 47 objects that show no significant [O
III]$\lambda$5007 emission, a typically strong AGN optical emission line. As a
comparison sample, we examine SDSS-selected Type 2 quasars (QSO2s), which show
a significant [O III]$\lambda$5007 line by definition. We find a 1:16 ratio of
OQQs compared to QSO2s, suggesting that the OQQ duty cycle is likely much
shorter than that of QSO2s (though selection biases are not fully quantified).
We consider observed properties in comparison with other galaxy types, and
examine them for consistency with theories on their intrinsic nature: chiefly
(a) a high covering factor for surrounding obscuring matter, preventing the
detection of high-ionisation emission lines - `cocooned AGN'; or (b) ionised
gas being absent on the kpc scales of the Narrow Line Region (NLR), perhaps due
to a `switching on' or `young' AGN. OQQs do not obviously fit the standard
paradigm for merger-driven AGN and host galaxy evolution, implying we may be
missing part of the flow of AGN evolution.
