[C ii] Properties and Far-infrared Variability of a z = 7 Blazar

Abstract We present millimeter observations of the host galaxy of the most distant blazar known, VLASS J041009.05−013919.88 (hereafter J0410–0139) at z = 7, using Atacama Large Millimeter/submillimeter Array (ALMA) and NOrthern Extended Millimeter Array (NOEMA) observations. The ALMA data reveal a (2.02 ± 0.36) × 10 42 erg s −1 [C ii ] 158 μ m emission line at z = 6.9964 with a [C ii ]-inferred star formation rate (SFR) of 58 ± 9 M ⊙ yr −1 . We estimate a dynamical mass of M dyn,[C ii ] = (4.6 ± 2.0) × 10 9 M ⊙ , implying a black hole mass to host a dynamical mass ratio of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.15</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.05</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.08</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . The 238 GHz continuum (rest-frame IR) decreased by ∼33% from the NOEMA to the ALMA observations taken ∼10 months apart. The Very Large Array 3–10 GHz radio flux densities showed a ∼37% decrease in a similar time frame, suggesting a causal connection. At face value, J0410–0139 would have the lowest [C ii ]-to-IR luminosity ratio of a z &gt; 5.7 quasar reported to date (∼10 −4 ). However, if only &lt;20% of the measured IR luminosity was due to thermal emission from dust, the [C ii ]-to-IR luminosity ratio would be typical of (U)LIRGs, and the SFRs derived from [C ii ] and IR luminosities would be consistent. These results provide further evidence that synchrotron emission significantly contributes to the observed rest-frame IR emission of J0410–0139, similar to what has been reported in some radio-loud active galactic nuclei at z &lt; 1.