Molecular gas content of gravitationally lensed quasars at cosmic noon

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.