Distributions of quasar hosts on the galaxy main-sequence plane
  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.
