Cold accretion discs and lineless quasars

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 T-eff,T- max = f(max)((M) over dot/M-2)(1/4), where M is the black hole mass, (M) over dot the accretion rate and f(max) is set by the black hole spin a(*). For a low enough value of (M) over dot/M-2, 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 = 10(46) erg s(-1) for M > 3.6 x 10(9) M-circle dot, when a(*) = 0, and for M > 1.4 x 10(10) M-circle dot, when a(*) = 0.998. Using the AD-based (M) over dot, derived from M and L-opt, and the reverberation-based M, derived from L-opt and the H beta full width at half-maximum, v, gives T-eff,T- max proportional to L-opt(-0.13) v(-1.45). Thus, T-eff,T- max is mostly set by v. Quasars with a local BB AD become lineless for v > 8000 km s(-1), when a(*) = 0, and for v > 16 000 km s(-1), 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(-1) 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 lambda < 1000 angstrom. Such an SED was observed by Hryniewicz et al. in SDSS J094533.99+100950.1, a WLQ observed down to 570 angstrom, which is well modelled by a rather cold AD SED. UV spectroscopy of z similar to 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.