Neutral versus ionized gas kinematics at z ≃ 2.6: the AGN-host starburst galaxy PKS 0529-549

We present a multiwavelength study of the AGN-host starburst galaxy PKS 0529-549 at z ≃ 2.6. We use (1) new ALMA observations of the dust continuum and of the [CI] 370 μm line, tracing molecular gas, (2) SINFONI spectroscopy of the [O III] 5007 Å line, tracing ionized gas, and (3) ATCA radio continuum images, tracing synchrotron emission. Both [C I] and [O III] show regular velocity gradients, but their systemic velocities and position angles differ by ∼300 km s^{-1} and ∼30°, respectively. The [C I] is consistent with a rotating disc, aligned with the dust and stellar continuum, while the [O III] likely traces an outflow, aligned with two AGN-driven radio lobes. We model the [C I] cube using 3D disc models, which give best-fitting rotation velocities V_rot ≃ 310 km s^{-1}and velocity dispersions σ_V ≲ 30 km s^{-1}. Hence, the [C I] disc has V_rot/σ_V ≳10 and is not particularly turbulent, similar to local galaxy discs. The dynamical mass (∼10¹¹ M_⊙) is comparable to the baryonic mass within the errors. This suggests that baryons dominate the inner galaxy dynamics, similar to massive galaxies at z ≃ 0. Remarkably, PKS 0529-549 lies on the local baryonic Tully-Fisher relation, indicating that at least some massive galaxies are already in place and kinematically relaxed at z ≃ 2.6. This work highlights the potential of the [C I] line to trace galaxy dynamics at high z, as well as the importance of multiwavelength data to interpret gas kinematics.