Evidence of a diffuse, extended continuum source in quasars from the relative sizes of the broad line region and the UV-optical continuum source measured with microlensing

Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 19c, B5c, 4000 Liège, Belgium

^⋆ Corresponding author; d.hutsemekers@uliege.be

Received: 16 December 2024
Accepted: 5 February 2025

Abstract

Microlensing by stars in the lens galaxy of a gravitationally lensed quasar is a phenomenon that can selectively magnify quasar subregions, producing observable changes in the continuum brightness or distortions in the emission line profiles. Hence, microlensing allows us to probe the inner quasar regions. In this paper, we report measurements of the ratio of the broad emission line region (BLR) radius to the continuum source radius in eight lensed quasars, for the C IV, Mg II, and Hα emission lines and their respective underlying continua at λλ 1550 Å, 2800 Å, and 6563 Å. The microlensing-induced line profile distortions and continuum magnifications were observed in the same single-epoch datasets, and simultaneously compared with microlensing simulations. We found that, on average, the inner radius of the BLR starts at the end of the UV-optical continuum source, independently of the line ionization and the wavelength of the continuum. The half-light radius of the BLR is, on average, a factor of six larger than the half-light radius of the continuum source, independently of the quasar’s bolometric luminosity. We also found a correlation between the BLR radius and the continuum source radius, supporting the idea that the dominant contribution to the UV-optical continuum may come from the BLR itself. Our results independently confirm the results of reverberation mapping studies, and extend them to higher-redshift, higher-luminosity quasars.