Quadrupole signature as a kinematic diagnostic to constrain bar properties : implication for the Milky Way

Abstract: The presence of a 'butterfly' or a quadrupole structure in the stellar mean radial velocity field of the Milky Way is well known from the Gaia and the APOGEE surveys. Past studies indicated that a stellar bar can excite such a quadrupole feature in the $<V_R>$ distribution. However, a systematic study investigating the co-evolution of bar and quadrupole structure is largely missing. Furthermore, whether this quadrupole structure in $<V_R>$ can be used as a robust kinematic diagnostic to constrain bar properties, particularly for the Milky Way, is still beyond our grasp. Here, we investigate the bar-induced quadrupole feature using a suite of isolated N-body models forming prominent bars and a sample of Milky Way-like barred galaxies from the TNG50 cosmological simulation. We demonstrate that the properties of the quadrupole (strength, length, and orientation) are strongly correlated with the bar properties, regardless of the choice of the stellar tracer population; thereby making the quadrupole feature an excellent kinematic diagnostic for constraining the bar properties. In presence of spirals, the estimator which takes into account the phase-angle of m=4 Fourier moment, serves as a more appropriate estimator for measuring the length of the quadrupole. Further, we constructed a novel Gaia-like mock dataset from a simulated bar model while incorporating the dust extinction and the broad trends of observational errors of the Gaia survey. The quadrupole properties (strength and length) estimated from those Gaia-like mock data are larger (~35-45 percent) when compared with their true values. We determined that the majority of this effect is due to the uncertainty in parallax measurement. This demonstrates the potential caveat of inferring Milky Way's bar properties by using the stellar kinematic information from the Gaia DR3 without properly accounting for the observational uncertainties.