The different origins of high- and low-ionization broad emission lines revealed by gravitational microlensing in the Einstein cross

TL;DR

This study uses gravitational microlensing to reveal that high- and low-ionization broad emission lines in a quasar originate from regions with different kinematics, indicating distinct physical structures.

Contribution

It demonstrates that high- and low-ionization emission lines are governed by different kinematic regions, supported by microlensing effects in a gravitationally lensed quasar.

Findings

High-ionization CIV line is symmetrically magnified by microlensing.

Low-ionization Hα line shows asymmetric magnification, favoring a flattened structure.

Different microlensing effects imply distinct origins for high- and low-ionization regions.

Abstract

We investigate the kinematics and ionization structure of the broad emission line region of the gravitationally lensed quasar QSO2237+0305 (the Einstein cross) using differential microlensing in the high- and low-ionization broad emission lines. We combine visible and near-infrared spectra of the four images of the lensed quasar and detect a large-amplitude microlensing effect distorting the high-ionization CIV and low-ionization H$\alpha$ line profiles in image A. While microlensing only magnifies the red wing of the Balmer line, it symmetrically magnifies the wings of the CIV emission line. Given that the same microlensing pattern magnifies both the high- and low-ionization broad emission line regions, these dissimilar distortions of the line profiles suggest that the high- and low-ionization regions are governed by different kinematics. Since this quasar is likely viewed at intermediate inclination, we argue that the differential magnification of the blue and red wings of H$\alpha$ favors a flattened, virialized, low-ionization region whereas the symmetric microlensing effect measured in CIV can be reproduced by an emission line formed in a polar wind, without the need of fine-tuned caustic configurations.