Self-interacting dark matter delays bar growth in an isolated gas-rich galactic disk: torque-resolved numerical evidence

We investigate how a dynamically responsive gas disk changes the influence of self-interacting dark matter (SIDM) on stellar-bar formation. In an isolated Milky-Way-mass galaxy with fixed total baryonic mass, the gas-rich collisionless (CDM) model develops a persistent bar at 1.96,, whereas the matched SIDM model with =1, cm squared,g-1 does so at 2.69,. The final stellar m=2 amplitude is 0.437 in CDM and 0.340 in SIDM, a 22 per cent suppression; the corresponding gas-free controls remain weak over the same interval. An independently sampled gas-rich pair preserves the direction of both differences. Direct component-force measurements show that the SIDM stellar disk loses 2.82 times10 11,,, less angular momentum than its CDM counterpart. The integrated torque contrast is dominated by the dark halo (approximately 74 per cent), with a substantial gas contribution (26 per cent), and is redistributed in radius and time rather than being a uniform reduction of halo coupling. Conservation, force-accuracy, aperture, threshold, and independent-sampling checks support the ordering as a dynamical effect within this model. The result demonstrates that a live gas component can change both the sign and timing of the early SIDM impact inferred from collisionless disks, providing a mechanism-based constraint for future hydrodynamic SIDM studies.