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

Abstract

We investigate the relation between star formation rates (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 M_s remains approximately constant with redshift, at 300 ± 100 M⊙ yr^(−1). Conversely, 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 M_s and black hole accretion rate (M_(bh)) existing in only parts of the z−M_s–M_(bh) plane, dependent on the free gas fraction, the trigger for activity, and the processes that may quench star formation. The relations between 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; 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 M_s and M_(bh). Finally, there is no strong relation between 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.

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