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

TL;DR

This study investigates star formation in low-redshift quasars, revealing that most host galaxies are actively forming stars, often at starburst levels, with infrared emission providing reliable star formation rates despite AGN interference.

Contribution

It demonstrates that infrared emission, after subtracting the torus contribution, accurately measures star formation rates in quasar hosts, showing many are on or above the star-forming main sequence.

Findings

Most quasars have star formation rates of 1-250 M_sun/yr.

A significant fraction are starburst systems.

High star formation efficiencies are observed despite diverse morphologies.

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.