2024ApJ...977L..46B

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) × 10⁴² erg s^–1 [C II] 158 μm 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) × 10⁹M_⊙, implying a black hole mass to host a dynamical mass ratio of ${0.15}_{-0.05}^{+0.08}$. 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 > 5.7 quasar reported to date (∼10^–4). However, if only <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 < 1.