Distributions of quasar hosts on the galaxy main-sequence plane

TL;DR

This study examines the placement of quasar host galaxies on the galaxy main sequence, revealing differences between optically-selected and near-IR-selected quasars and implications for galaxy evolution and quasar triggering.

Contribution

It provides the first detailed comparison of quasar host galaxy distributions relative to the galaxy main sequence at low redshift, highlighting differences based on selection method and luminosity.

Findings

PG quasar hosts mostly follow the main sequence of star-forming galaxies.

2MASS quasar hosts are systematically above the main sequence.

Higher nuclear luminosity correlates with higher specific star formation rates.

Abstract

The relation between star formation rates and stellar masses, i.e. the galaxy main sequence, is a useful diagnostic of galaxy evolution. We present the distributions relative to the main sequence of 55 optically-selected PG and 12 near-IR-selected 2MASS quasars at z <= 0.5. We estimate the quasar host stellar masses from Hubble Space Telescope or ground-based AO photometry, and the star formation rates through the mid-infrared aromatic features and far-IR photometry. We find that PG quasar hosts more or less follow the main sequence defined by normal star-forming galaxies while 2MASS quasar hosts lie systematically above the main sequence. PG and 2MASS quasars with higher nuclear luminosities seem to have higher specific SFRs (sSFRs), although there is a large scatter. No trends are seen between sSFRs and SMBH masses, Eddington ratios or even morphology types (ellipticals, spirals and mergers). Our results could be placed in an evolutionary scenario with quasars emerging during the transition from ULIRGs/mergers to ellipticals. However, combined with results at higher redshift, they suggest that quasars can be widely triggered in normal galaxies as long as they contain abundant gas and have ongoing star formation.