The evolution of the broad-line region among SDSS quasars

By Tohru Nagao, A. Marconi, Roberto Maiolino Arcetri Observatory, National Space Development Agency of Japan. 2005

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Based on 5344 quasar spectra taken from the SDSS Data Release 2, the dependences of various emission-line flux ratios on redshift and quasar luminosity are investigated in the ranges $2.0 \leq z \leq 4.5$ and $-24.5 \geq M_B \geq -29.5$. We show that the emission lines in the composite spectra are fitted better with power-law profiles than with double Gaussian or modified Lorentzian profiles, and in particular we show that the power-law profiles are more appropriate to measure broad emission-line fluxes than other methods. The composite spectra show that there are statistically significant correlations between quasar luminosity and various emission-line flux ratios, such as Nv/Civ and Nv/Heii, while there are only marginal correlations between quasar redshift and emission-line flux ratios. We obtain detailed photoionization models to interpret the observed line ratios. The correlation of line ratios with luminosity is interpreted in terms of higher gas metallicity in more luminous quasars. For a given quasar luminosity, there is no metallicity evolution for the redshift range $2.0 \leq z \leq 4.5$. The typical metallicity of BLR gas clouds is estimated to be $Z \sim 5~ Z_\odot$, although the inferred metallicity depends on the assumed BLR cloud properties, such as their density distribution function and their radial distribution. The absence of a metallicity evolution up to $z \sim 4.5$ implies that the active star-formation epoch of quasar host galaxies occurred at $z \ga 7$.

Subfields: Astronomy and Astrophysics, Electrical and Electronic Engineering, Instrumentation

View Paper: https://www.semanticscholar.org/paper/5050d4280286fd139b5c74930cf8f5916c6f1ce8

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Citations in the corpus

Stellar sources of dust in the high-redshift Universe (2009, methodology): “In a recent analysis, Juarez et al. (2009) investigated the metallicity of the BLR of a sample of 30 QSOs in the range 4 < z< 6.4, including J1148+5251, using the (SiIV+OIV)/CIV ratio, which is a good metallicity tracer (Nagao et al. 2006).”
De re metallica: the cosmic chemical evolution of galaxies (2018, methodology): “This parameter shows some correlation with stellar mass and SFR (e.g., Dopita et al., 2006; Brinchmann et al., 2008; Nagao et al., 2006c).”
BLACK HOLE MASS ESTIMATES AND EMISSION-LINE PROPERTIES OF A SAMPLE OF REDSHIFT z > 6.5 QUASARS (2013, methodology): “…various emission-line flux ratios (e.g., N iii]/O iii], N v/(C iv+O iv), and N v/He ii) can be used to derive chemical abundances of the broad line region (BLR) gas (e.g., Hamann et al. 2002; Nagao et al. 2006) and, thus, to set constraints on the star formation history of the quasar host galaxy.”
Gemini Near-Infrared Spectroscopy of Luminous z ∼ 6 Quasars: Chemical Abundances, Black Hole Masses, and Mg II Absorption (2007, methodology): “For the purpose of comparison, we show in Figure 3 the line ratios of quasars (squares) in the luminosity range of 27:5 < MB < 26:5 from Nagao et al. (2006).”
Observational evidence of quasar feedback quenching star formation at high redshift (2011, methodology): “The emission lines are fitted with multiple Gaussians and, in the case of the broad lines, by also using power-law profiles, as in Nagao et al. (2006).”