Hypermassive black holes have faint broad and narrow emission lines

Harshitha K. Bhat, Susmita Chakravorty, Dhrubojyoti Sengupta, Martin Elvis, Sudeb Ranjan Datta, Nirupam Roy, Caroline Bertemes, Gary Ferland, Savithri H. Ezhikode

Research output: Contribution to journalArticlepeer-review

Abstract

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; MBH ≥ 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 ≤ Lbol/LEdd ≤ 1.0 were input to the cloudy photoionization code. BELs become ∼100 times weaker in EW from MBH ∼108 M⊙ to MBH ∼1010 M⊙. The high-ionization BELs (O vi 1034 Å, C iv 1549 Å, and He ii 1640 Å) decline in EW from MBH ≥ 106 M⊙, reproducing the Baldwin effect, but regain EW for MBH ≥ 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 MBH.

Original languageEnglish
Pages (from-to)2992-3010
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Volume497
Issue number3
Early online date10 Jul 2020
DOIs
Publication statusPublished - 1 Sep 2020

Keywords

  • accretion, accretion discs
  • black hole physics
  • galaxies: Active
  • line: Formation
  • quasars: Emission lines
  • quasars: Supermassive black holes

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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