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 $1.96\text{Gyr}$, whereas the matched SIDM model with $\text{sigmam}=1\mathrm{c}{\mathrm{m}}^2{\mathrm{g}}^{-1}$ does so at $2.69\text{Gyr}$. 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 $2.82\times {10}^{11}\text{Msun}\text{kpc}\text{kms}$ 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.