  We present Spitzer IRS observations of a complete sample of 57 SDSS type-1
quasars at z~1. Aromatic features at 6.2 and/or 7.7 um are detected in about
half of the sample and show profiles similar to those seen in normal galaxies
at both low- and high-redshift, indicating a star-formation origin for the
features. Based on the ratio of aromatic to star-formation IR (SFIR)
luminosities for normal star-forming galaxies at z~1, we have constructed the
SFIR luminosity function (LF) of z~1 quasars. As we found earlier for
low-redshift PG quasars, these z~1 quasars show a flatter SFIR LF than do z~1
field galaxies, implying the quasar host galaxy population has on average a
higher SFR than the field galaxies do. As measured from their SFIR LF,
individual quasar hosts have on average LIRG-level SFRs, which mainly arise in
the circumnuclear regions. By comparing with similar measurements of
low-redshift PG quasars, we find that the comoving SFIR luminosity density in
quasar hosts shows a much larger increase with redshift than that in field
galaxies. The behavior is consistent with pure density evolution since the
average SFR and the average SFR/BH-accretion-rate in quasar hosts show little
evolution with redshift. For individual quasars, we have found a correlation
between the aromatic-based SFR and the luminosity of the nuclear radiation,
consistent with predictions of some theoretical models. We propose that type 1
quasars reside in a distinct galaxy population that shows elliptical morphology
but that harbors a significant fraction of intermediate-age stars and is
experiencing intense circumnuclear star formation.

Cosmology and Nongalactic Astrophysics