Broad UV Emission Lines in Type-1 Active Galactic Nuclei: A Note on Spectral Diagnostics and the Excitation Mechanism

Abstract

This paper reviews several basic emission properties of the UV emission lines observed in the spectra of quasars and type-1 active galactic nuclei, mainly as a function of the ionization parameter, metallicity, and density of the emitting gas. The analysis exploits a general-purpose 4D array of the photoionization simulations computed using the code CLOUDY, covering ionization parameter in the range 10−4.5–10+1.0, hydrogen density nH∼107–1014 cm−3, metallicity Z between 0.01 and 100 Z⊙, and column density in the range 1021–1023 cm−2. The focus is on the most prominent UV emission lines observed in quasar spectra, namely Nvλ1240, Siivλ1397, Oiv]λ1402, Civλ1549, Heiiλ1640, Aliiiλ1860, Siiii]λ1892, and Ciii]λ1909, and on the physical conditions under which electron-ion impact excitation is predicted to be the dominant line producer. Photoionization simulations help constrain the physical interpretation and the domain of applicability of spectral diagnostics derived from measurements of emission line ratios, reputed to be important for estimating the ionization degree, density, and metallicity of the broad line emitting gas, as well as the relative intensity ratios of the doublet or multiplet components relevant for empirical spectral modeling.

The main sequence of quasars: The taming of the extremes

“…Emission line ratios in a photoionization context are dependent on the SED of the ionizing radiation that is known in turn to depend on Eddington ratio (Ferland et al 2020; Laor et al 1997), and hence on the location along the MS. Arrays of Cloudy 17.02 (Ferland et al 2017) photoionization simulations were computed covering the ‐density parameter plane with a step of 0.25 dex, for 12 values of metallicity covering the range 0.01 , with a SED appropriate for sources radiating at high Eddington ratio, including a prominent big blue bump. A detailed account of the simulation settings and a systematic presentation the physical basis of the method and of the results is provided by Marziani, del Olmo, Perea, D'Onofrio, & Panda (2020b) and Śniegowska et al (2021). The simulation results show that diagnostic line ratios CIV λ 1549/HeII λ 1640, AlIII λ 1860/HeII λ 1640, (Si iv + OIV]) λ 1400/HeII λ 1640 are monotonically increasing with over a wide range of ionization parameter values, for a fixed SED (Śniegowska et al 2021).…”

“…The simulation results show that diagnostic line ratios CIV λ 1549/HeII λ 1640, AlIII λ 1860/HeII λ 1640, (Si iv + OIV]) λ 1400/HeII λ 1640 are monotonically increasing with over a wide range of ionization parameter values, for a fixed SED (Śniegowska et al 2021). The HeII λ 1640 emission line is expedient because of unchanging He abundance and of simple HeII λ 1640 radiation transfer (collisional excitation is negligible, as the lower transition level is at a high energy above ground eV; Marziani, del Olmo, et al 2020b).…”

“…with a SED appropriate for sources radiating at high Eddington ratio, including a prominent big blue bump. A detailed account of the simulation settings and a systematic presentation the physical basis of the method and of the results is provided byMarziani, del Olmo, Perea, D'Onofrio, & Panda (2020b) and Śniegowska et al (2021). The simulation results show that diagnostic line ratios CIV𝜆1549/HeII𝜆1640, AlIII 𝜆1860/HeII𝜆1640, (Si iv + OIV])𝜆1400/HeII𝜆1640 are monotonically increasing with Z over a wide range of ionization parameter values, for a fixed SED ( Śniegowska et al 2021).…”

High Metal Content of Highly Accreting Quasars: Analysis of an Extended Sample

“…High n H and moderate or relatively high U contribute to make the line stronger (Marziani et al 2020). Figure 17 shows the distribution of the metallicity differences δ log Z = log Z(Aliii) − log Z(Civ) and log Z(Siiv) − log Z(Civ).…”

The Main Sequence of quasars: the taming of the extremes

“…Emission line ratios in a photoionization context are dependent on the SED of the ionising radiation that is known in turn to depend on Eddington ratio (Ferland, Done, Jin, Landt, & Ward, 2020;Laor, Fiore, Elvis, Wilkes, & McDowell, 1997), and hence on the location along the MS. Arrays of Cloudy 17.02 photoionization simulations were computed covering the -density parameter plane with a step of 0.25 dex, for 12 values of metallicity covering the range 0.01 ⊙ ≤ ≤ 1000 ⊙ , with a SED appropriate for sources radiating at high Eddington ratio, including a prominent big blue bump. A detailed account of the simulation settings and a systematic presentation the physical basis of the method and of the results is provided by Marziani, del Olmo, Perea, D'Onofrio, & Panda (2020) and Śniegowska et al (2021). The similation results show that diagnostic line ratios CIV 1549/HeII 1640, AlIII 1860/HeII 1640, (SiIV+OIV]) 1400/HeII 1640 are monotonically increasing with over a wide range of ionization parameter values, for a fixed SED (Śniegowska et al, 2021).…”