2011MNRAS.417..681L

The optical–UV continuum of quasars is broadly consistent with the emission from a geometrically thin optically thick accretion disc (AD). The AD produces the ionizing continuum which powers the broad and narrow emission lines. The maximum AD effective temperature is given by {img}, where M is the black hole mass, {img} the accretion rate and f_max is set by the black hole spin a_*. For a low enough value of {img}, the AD may become too cold to produce ionizing photons. Such an object will form a lineless quasar. This occurs for a local blackbody (BB) AD with a luminosity L_opt= 10⁴⁶ erg/s for M > 3.6 {x} 10⁹ M_☉, when a_*= 0, and for M > 1.4 {x} 10¹⁰ M_☉, when a_*= 0.998. Using the AD-based {img}, derived from M and L_opt, and the reverberation-based M, derived from L_opt and the Hβ full width at half-maximum, v, gives T_eff, max_∝L^–0.13_optv^–1.45. Thus, T_eff, max_ is mostly set by v. Quasars with a local BB AD become lineless for v > 8000 km/s, when a_*= 0, and for v > 16 000 km/s, when a_*= 0.998. Higher values of v are required if the AD is hotter than a local BB. The AD becoming non-ionizing may explain why line-emitting quasars with v > 10 000 km/s are rare. Weak low-ionization lines may still be present if the X-ray continuum is luminous enough, and such objects may form a population of weak line quasars (WLQ). If correct, such WLQ should show a steeply falling spectral energy distribution (SED) at λ < 1000 Å. Such an SED was observed by Hryniewicz et al. in SDSS J094533.99+100950.1, a WLQ observed down to 570 Å, which is well modelled by a rather cold AD SED. UV spectroscopy of z∼ 1–2 quasars is required to eliminate potential intervening Lyman limit absorption by the intergalactic medium and to explore if the SEDs of lineless quasars and some additional WLQ are also well fitted by a cold AD SED.