Bright [C II] and Dust Emission in Three z > 6.6 Quasar Host Galaxies Observed by ALMA

Abstract

We present ALMA detections of the [C II] 158 μm emission line and the underlying far-infrared (FIR) continuum of three quasars at 6.6 < z < 6.9 selected from the VIKING survey. The [C II] line fluxes range between 1.6 and 3.4 Jy km s<SUP>-1</SUP> ([C II] luminosities ∼(1.9-3.9) × 10<SUP>9</SUP> L<SUB>☉</SUB>). We measure continuum flux densities of 0.56-3.29 mJy around 158 μm (rest frame), with implied FIR luminosities of (0.6-7.5) × 10<SUP>12</SUP> L<SUB>☉</SUB> and dust masses M<SUB>d</SUB> = (0.7-24) × 10<SUP>8</SUP> M<SUB>☉</SUB>. In one quasar we derive a dust temperature of {30}<SUB>-9</SUB><SUP>+12</SUP> K from the continuum slope, below the canonical value of 47 K. Assuming that the [C II] and continuum emission are powered by star formation, we find star formation rates from 100 to 1600 M<SUB>☉</SUB> yr<SUP>-1</SUP> based on local scaling relations. The L<SUB>[C II]</SUB>/L<SUB>FIR</SUB> ratios in the quasar hosts span a wide range from (0.3-4.6) × 10<SUP>-3</SUP>, including one quasar with a ratio that is consistent with local star-forming galaxies. We find that the strength of the L<SUB>[C II]</SUB> and 158 μm continuum emission in z ≳ 6 quasar hosts correlates with the quasar’s bolometric luminosity. In one quasar, the [C II] line is significantly redshifted by ∼1700 km s<SUP>-1</SUP> with respect to the Mg II broad emission line. Comparing to values in the literature, we find that, on average, the Mg II is blueshifted by 480 km s<SUP>-1</SUP> (with a standard deviation of 630 km s<SUP>-1</SUP>) with respect to the host galaxy redshift, I.e., one of our quasars is an extreme outlier. Through modeling we can rule out a flat rotation curve for our brightest [C II] emitter. Finally, we find that the ratio of black hole mass to host galaxy (dynamical) mass is higher by a factor of 3-4 (with significant scatter) than local relations.