Biases in galaxy spectral analysis from strong lensing differential magnification effect and correction methods

TL;DR

This study investigates how strong gravitational lensing causes biases in spectral analysis of high-redshift galaxies and evaluates correction methods to recover intrinsic properties, emphasizing the importance of accurate lens modeling.

Contribution

It provides a detailed analysis of lensing-induced biases in spectral properties and compares the effectiveness of two correction methods using realistic mock systems.

Findings

Lensing introduces significant biases in stellar age, metallicity, and emission line ratios.

Hα flux is consistently overestimated due to lensing effects.

Full ray-tracing correction outperforms average magnification correction in bias recovery.

Abstract

Strong gravitational lensing has significantly advanced the study of high-redshift galaxies, but the differential magnification effect inevitably introduces biases in the spectral analysis of source galaxies. This work investigates these biases using mock lensing systems from MaNGA survey data and IllustrisTNG simulations. We analyze the impact of lensing effect on several spectral properties, including stellar age, metallicity, H$\alpha$ flux, and optical emission line ratios. Our results show significant biases in all properties after lensing. The values of quantities can be either over- or under-estimated, except for the consistently enhanced H$\alpha$ flux. The bias varies with lensing configurations and always arises when part of the source galaxy falls into the strong lensing regime. We evaluate two correction methods to recover the intrinsic source properties: the average magnification factor ($\bar{\mu}$) and full ray-tracing. While both methods reduce the overestimated H$\alpha$ flux, the $\bar{\mu}$ method shows a much larger discrepancy. For stellar population properties and emission line ratios, the $\bar{\mu}$ method fails whereas the ray-tracing method proves effective. Applying these two methods to a statistical sample of mock systems further shows their strong dependence on lens modeling accuracy. As a demonstrative study, our results highlight the importance of spatially resolved spectroscopic observations and precise lens modeling for reconstructing spectra of strongly lensed galaxies. While our conclusions are based on a specific source and lens galaxy, further studies with a statistical sample of realistic mock lensing systems are needed for understanding any systematic differences between the two correction methods.