Kinematic-Distance Biases in the Inner Milky Way from a Stellar-Dynamically Constrained Bar

Abstract

We quantify how bar-driven non-circular motions bias Milky-Way gas maps inferred with the kinematic-distance (KD) method. KD reconstructions of H\,\textsc{i} and CO surveys assume circular rotation in an axisymmetric potential, an assumption that is strongly violated in the barred inner Milky Way. We use high-resolution hydrodynamical simulations of gas flow in an observationally constrained barred Milky Way potential. From a quasi-steady snapshot we generate synthetic longitude--velocity data and apply a standard axisymmetric KD inversion using the circular-speed curve derived from the $m=0$ component of the same potential. To isolate non-circular effects, we remove the near--far ambiguity by selecting, for each gas element, the KD branch closest to its true distance. We find that the KD method reproduces the gas distribution reasonably well outside the bar-dominated region ($R \gtrsim 5$~kpc), but fails systematically in the bar region ($R \sim 0.5$--3~kpc). There the KD-reconstructed face-on map exhibits anisotropic, quadrant-dependent artifacts, including arc-like overdensities and LOS-elongated low-density cavities. In azimuthally averaged profiles, these anisotropic misassignments translate into net radial mixing: the axisymmetric KD inversion substantially fills in the true bar-induced depression (hereafter, the ``bar gap'') and yields a flatter inner profile. Distance-error maps show coherent structures with $|\Delta d| \sim 1$--2~kpc and relative errors of several tens of percent along the bar and inner ring, coincident with zones where the KD mapping is intrinsically ill-conditioned, quantified by a large geometric sensitivity $S \equiv \left|\partial d/\partial V_{\rm LOS}^{\rm circ}\right|$. In these regions the error is well approximated to first order by $\Delta d \simeq S\,\Delta V_{\rm LOS}$, linking KD failures directly to bar-driven streaming velocities. ...