Star formation rates in luminous quasars at 2 <z< 3

Abstract

Listen

Translate article

We investigate the relation between star formation rates (⁠|$\dot{{M}}_s$|⁠) and AGN properties in optically selected type 1 quasars at 2 < z < 3 using data from Herschel and the SDSS. We find that |$\dot{{M}}_s$| remains approximately constant with redshift, at 300 ± 100 M⊙ yr−1. Conversely, |$\dot{{M}}_s$| 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 |$\dot{{M}}_s$| and black hole accretion rate (⁠|$\dot{{M}}_{{\rm bh}}$|⁠) existing in only parts of the |$z-\dot{{M}}_{s}-\dot{{M}}_{{\rm bh}}$| plane, dependent on the free gas fraction, the trigger for activity, and the processes that may quench star formation. The relations between |$\dot{{M}}_s$| 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; Mi is not a linear tracer of L2500, the Baldwin effect changes form at high AGN luminosities, and high C iv EW values signpost a change in the relation between |$\dot{{M}}_s$| and |$\dot{{M}}_{{\rm bh}}$|⁠. Finally, there is no strong relation between |$\dot{{M}}_s$| 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.