SALT long-slit spectroscopy of LBQS 2113-4538: variability of the Mg II and Fe II component

TL;DR

This study analyzes the variability of Mg II and Fe II emission lines in a quasar using SALT spectroscopy, providing insights into systematic effects affecting black hole mass measurements and the structure of the emission regions.

Contribution

It offers detailed spectral analysis of LBQS 2113-4538, demonstrating the stability of line shapes and variability in amplitudes, and evaluates Fe II models for accurate line fitting.

Findings

Mg II line is well fit by a single Lorentzian shape.

Fe II emission amplitudes vary over time, but shapes remain stable.

NLR contribution to Mg II is less than 3%, indicating possible BLR-NLR merging.

Abstract

The Mg II line is of extreme importance in intermediate redshift quasars since it allows us to measure the black hole mass in these sources and to use these sources as probes of the distribution of dark energy in the Universe, as a complementary tool to SN Ia. Reliable use of Mg II requires a good understanding of all the systematic effects involved in the measurement of the line properties, including the contamination by Fe II UV emission. We performed three spectroscopic observations of a quasar LBQS 2113-4538 (z = 0.956) with the SALT telescope separated in time by several months and we analyze in detail the mean spectrum and the variability in the spectral shape. We show that even in our good-quality spectra the Mg II doublet is well fit by a single Lorentzian shape. We tested several models of the Fe II pseudo-continuum and showed that one of them well represents all the data. The amplitudes of both components vary in time, but the shapes do not change significantly. The measured line width of LBQS 2113-4538 identifies this object as a class A quasar. The upper limit of $3\%$ for the contribution of the Narrow Line Region (NLR) to Mg II may suggest that the separation of the Broad Line Region (BLR) and NLR disappears in this class of objects.