2020MNRAS.497.2992B

The extreme ultraviolet region (EUV) provides most of the ionization that creates the high equivalent width (EW) broad and narrow emission lines (BELs and NELs) of quasars. Spectra of hypermassive Schwarzschild black holes (HMBHs; M_BH >= 10¹⁰ M_☉) with α-discs, decline rapidly in the EUV suggesting much lower EWs. Model spectra for BHs of mass 10⁶-10¹² M_☉ and accretion rates 0.03 <= L_bol/L_Edd <= 1.0 were input to the CLOUDY photoionization code. BELs become ∼100 times weaker in EW from M_BH ∼ 10⁸ M_☉ to M_BH ∼ 10¹⁰ M_☉. The high-ionization BELs (O VI 1034 Å, C IV 1549 Å, and He II 1640 Å) decline in EW from M_BH >= 10⁶ M_☉, reproducing the Baldwin effect, but regain EW for M_BH >= 10¹⁰ M_☉. The low-ionization lines (Mg II 2798 Å, H β 4861 Å, and H α 6563 Å) remain weak. Lines for maximally spinning HMBHs behave similarly. Line ratio diagrams for the BELs show that high O VI/H β and low C IV/H α may pick out HMBH, although O VI is often hard to observe. In NEL BPT diagrams, HMBHs lie among star-forming regions, except for highly spinning, high accretion rate HMBHs. In summary, the BELs expected from HMBHs would be hard to detect using the current optical facilities. From 100 to 10¹² M_☉, the emission lines used to detect active galactic nuclei (AGNs) only have high EW in the 10⁶-10⁹ M_☉ window, where most AGNs are found. This selection effect may be distorting reported distributions of M_BH.