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We compare the CO(1-0) and H$\alpha$ kinematics in 34 nearby galaxies,selected from the ALMaQUEST and EDGE-CALIFA surveys. We use 3-D Barolo, a 3Dtilted ring model, to derive the CO and H$\alpha$ rotation curves. Beforecomparing rotation curves in the 34 nearby galaxies, we found systematicsbetween the MaNGA and the CALIFA data using eight MaNGA-CALIFA overlappinggalaxies. We assume the rotation curves based on the MaNGA data are accurateand made the corresponding correction to the CALIFA data. Our result shows that$\sim$56% (19/34) of our galaxies present slower H$\alpha$ rotation curvescompared to the CO rotation curves, with a median value of 6.5 km/s. Theremaining galaxies (15/34) show consistent CO-H$\alpha$ rotation velocitywithin uncertainties. As a result, the Ha rotation may underestimate the totaldynamical mass by ~6% for a circular velocity of 200 km/s (the median value inour sample). Furthermore, the velocity difference between the CO and H$\alpha$rotation velocity is found to correlate with the velocity dispersion differencebetween CO and H$\alpha$, suggesting that the gas pressure plays a role in thevelocity discrepancy. After incorporating the effect of pressure support due tothe turbulent gas motion to our sample, the median value of the velocitydifferences decreases to 1.9 km/s, which in turn reduces the underestimation ofdynamical mass to $\sim$2%. Finally, we also investigate the role that theextra-planar diffuse ionized gas (eDIG) plays in the CO-H$\alpha$ velocitydiscrepancy.

The ALMaQUEST survey. VIII. What causes the velocity discrepancy between CO and H$\alpha$ rotation curves in galaxies?