Discovery of the asymmetric effect in the response of photoionization gas

¹ Department of Astronomy, University of Science and Technology of China, Hefei 230026, China
² School of Astronomy and Space Science, University of Science and Technology of China, Hefei, Anhui 230026, China

^⋆ Corresponding author: mailto:zcho@ustc.edu.cn

Received: 1 July 2025
Accepted: 7 October 2025

Abstract

Context. Ionized gas is ubiquitous in the Universe and plays a central role in tracing the cosmic evolution and probing plasma physics under extreme conditions. Of the various ionizing sources, quasars (powered by supermassive black holes) are important contributors to the reionization of the universe. The variability of the quasar radiation provides a valuable opportunity to study the photoionization response of interstellar and intergalactic gas.

Aims. We investigate the physical origin of the asymmetric response of ionized gas to the variable quasar radiation, particularly as observed in broad absorption line (BAL) systems. We also place constraints on the gas density and spatial scale of the BAL outflows based on this asymmetry.

Methods. We conducted time-dependent photoionization simulations focusing on C IV to quantify the response timescales in different ionization states. Analytical estimates were also used to relate the response asymmetry to the gas density.

Results. We find that over 70% of BAL gas in quasar host galaxies exhibit a negative response to variations in the quasar radiation, indicating a strong asymmetry in the behavior of ionized gas. Our simulations show that this asymmetry arises from shorter response timescales at higher ionization states. For typical observational cadences (> 1 day), the observed asymmetry requires that at least 40% of the BAL gas has a density below n_H = 10⁶ cm⁻³, which is consistent with most measured BAL gas densities. This is in contrast to the typical density of accretion disk winds (n_H > 10⁸ cm⁻³), which suggests that BAL outflows either evolve significantly as they propagate outward or originate from larger-scale regions, such as the dusty torus.

Conclusions. We uncovered a fundamental asymmetry in the response of ionized gas: The response timescales of high-ionization states are shorter than those of low-ionization states. The role of the asymmetric response effects thus offers new constraints on the physical origin and structure of quasar outflows.