Star formation rates in luminous quasars at 2 &lt;<i>z</i>&lt; 3

Abstract

We investigate the relation between star formation rates ( s ) and AGN properties in optically selected type 1 quasars at 2 < z < 3 using data from Herschel and the SDSS. We find that s remains approximately constant with redshift, at 300 100 M yr -1 . Conversely, s increases with AGN luminosity, up to a maximum of 600 M yr -1 , and with C IV FWHM. In context with previous results, this is consistent with a relation between s and black hole accretion rate ( bh ) existing in only parts of the z -s -bh plane, dependent on the free gas fraction, the trigger for activity, and the processes that may quench star formation. The relations between s and both AGN luminosity and C IV FWHM are consistent with star formation rates in quasars scaling with black hole mass, though we cannot rule out a separate relation with black hole accretion rate. Star formation rates are observed to decline with increasing C IV equivalent width. This decline can be partially explained via the Baldwin effect, but may have an additional contribution from one or more of three factors; M i is not a linear tracer of L 2500 , the Baldwin effect changes form at high AGN luminosities, and high C IV EW values signpost a change in the relation between s and bh . Finally, there is no strong relation between s and Eddington ratio, or the asymmetry of the C IV line. The former suggests that star formation rates do not scale with how efficiently the black hole is accreting, while the latter is consistent with C IV asymmetries arising from orientation effects.