Dust Emission in an Accretion-rate-limited Sample of z ≳ 6 Quasars

Abstract

We present Atacama Large Millimeter Array 1 mm observations of the rest-frame far-infrared (FIR) dust continuum in 27 quasars at redshifts 6.0 ≾ z < 6.7. We detect FIR emission at ≳3σ in all quasar host galaxies with flux densities at ~1900 GHz in the rest-frame of 0.12 < S_(rest,1900 GHz) < 5.9 mJy, with a median (mean) flux density of 0.88 mJy (1.59 mJy). The implied FIR luminosities range from L_(FIR) = (0.27–13) × 1012 L⊙, with 74% of our quasar hosts having L_(FIR) > 10^(12) L⊙. The estimated dust masses are M_(dust) = 10^7–10^9 M⊙. If the dust is heated only by star formation, then the star formation rates in the quasar host galaxies are between 50 and 2700 M⊙ yr^(-1). In the framework of the host galaxy–black hole coevolution model a correlation between ongoing black hole growth and star formation in the quasar host galaxy would be expected. However, combined with results from the literature to create a luminosity-limited quasar sample, we do not find a strong correlation between quasar UV luminosity (a proxy for ongoing black hole growth) and FIR luminosity (star formation in the host galaxy). The absence of such a correlation in our data does not necessarily rule out the coevolution model, and could be due to a variety of effects (including different timescales for black hole accretion and FIR emission).