Enhanced Star Formation Efficiency in the Central Regions of Nearby Quasars Hosts

We combine Atacama Large Millimeter/submillimeter Array and Multi Unit Spectroscopic Explorer observations tracing the molecular gas, millimeter continuum, and ionized gas emission in six low-redshift ($z \lesssim 0.06$) Palomar-Green quasar host galaxies to investigate their ongoing star formation at $\sim$kpc-scale resolution. The AGN contribution to the cold dust emission and the optical emission-line flux is carefully removed to derive spatial distributions of the star formation rate (SFR), which, complemented with the molecular gas data, enables the mapping of the depletion time ($t_{\rm dep}$). We report ubiquitous star formation activity within the quasar host galaxies, with the majority of the ongoing star formation occurring in the galaxy center. The rise of the star formation rate surface density ($\Sigma_{\rm SFR}$) toward the nucleus is steeper than that observed for the cold molecular gas surface density, reaching values up to $\Sigma_{\rm SFR} \approx 0.15-0.80\,M_\odot\,$yr$^{-1}\,$kpc$^{-2}$. The gas in the nuclear regions is converted into stars at a shortened depletion time ($t_{\rm dep} \approx 0.2-2.0\,$Gyr), suggesting that those zones can be deemed as starbursts. At large galactocentric radius, we find that the ongoing star formation takes place within spiral arms or H$\,$II region complexes, with an efficiency comparable to that reported for nearby inactive spirals ($t_{\rm dep} \approx 1.8\,$Gyr). We find no evidence of star formation activity shutoff in the PG quasar host galaxies. On the contrary, these observations shed light on how the central environments of galaxies hosting actively accreting supermassive black holes builds up stellar mass.