Molecular gas content of gravitationally lensed quasars at cosmic noon

¹ School of Astronomy and Space Science, Nanjing University, Nanjing 210093, People’s Republic of China
² Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, People’s Republic of China
³ Department of Astronomy, Westlake University, Hangzhou, 310030 Zhejiang Province, China
⁴ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, PR China
⁵ European Southern Observatory, Alonso de Córdova 3107, Casilla, 19001 Vitacura, Santiago 19, Chile

^⋆ Corresponding author: yshipku@gmail.com

Received: 4 April 2025
Accepted: 13 October 2025

Abstract

It is crucial to understand the star-forming activity in the host galaxies of high-redshift quasars for the connection between supermassive black hole activity and galaxy evolution. While most studies so far were biased toward luminous quasars, we conducted carbon monoxide (CO) observations of 17 gravitationally lensed quasars that have four images using the IRAM 30m telescope to investigate the molecular gas content of moderate- to low-luminosity quasars. CO emission is detected in 5 out of 17 quasars, which corresponds to a detection rate of about 30%. The analysis of their star formation activity revealed that these quasars live in gas-rich environments, but exhibit weaker starbursts and lower star formation efficiencies than other luminous high-redshift quasars. In addition, the CO spectral line energy distributions of two quasars (SDSS J0924+0219 and SDSS J1330+1810) are also consistent with mild star formation instead of extreme starbursts. These results suggest that these lensed quasars reside in weaker starburst environments.