The Infrared Emission and Vigorous Star Formation of Low-redshift Quasars

Abstract

The star formation activity of the host galaxies of active galactic nuclei (AGNs) provides valuable insights into the complex interconnections between black hole growth and galaxy evolution. A major obstacle arises from the difficulty of estimating accurate star formation rates in the presence of a strong AGN. Analyzing the $1-500\, \mu m$ spectral energy distributions and high-resolution mid-infrared spectra of low-redshift ($z < 0.5$) Palomar-Green quasars with bolometric luminosity $\sim 10^{44.5}-10^{47.5}\rm\,erg\,s^{-1}$, we find, from comparison with an independent star formation rate indicator based on [Ne II] 12.81$\, \mu m$ and [Ne III] 15.56$\, \mu m$, that the torus-subtracted, total infrared ($8-1000\, \mu m$) emission yields robust star formation rates in the range $\sim 1-250\,M_\odot\,{\rm yr^{-1}}$. Combined with available stellar mass estimates, the vast majority ($\sim 75\%-90\%$) of the quasars lie on or above the main sequence of local star-forming galaxies, including a significant fraction ($\sim 50\%-70\%$) that would qualify as starburst systems. This is further supported by the high star formation efficiencies derived from the gas content inferred from the dust masses. Inspection of high-resolution Hubble Space Telescope images reveals a wide diversity of morphological types, including a number of starbursting hosts that have not experienced significant recent dynamical perturbations. The origin of the high star formation efficiency is unknown.