Starburst-driven galactic outflows

Aims. We investigate the role of cosmic ray (CR) halos in shaping the physical properties of starburst-driven galactic outflows. Methods. We constructed a model for galactic outflows driven by a continuous central injection of energy, gas, and CRs, where the treatment of CRs accounts for the effect of CR pressure gradients on the flow dynamics. The model parameters were set by the effective properties of a starburst. By analyzing the asymptotic behavior of our model, we derived the launching criteria for starburst-driven galactic outflows and determined their corresponding outflow velocities. Results. We find that in the absence of CRs, stellar feedback can only launch galactic outflows if the star formation rate (SFR) surface density exceeds a critical threshold proportional to the dynamical equilibrium pressure. In contrast, CRs can always drive slow outflows. Outflows driven by CRs dominate in systems with SFR surface densities below the critical threshold, but their influence diminishes in highly star-forming systems. However, in older systems with established CR halos, the CR contribution to outflows weakens once the outflow reaches the galactic scale height, making CRs ineffective in sustaining outflows in such environments. Conclusions. Over cosmic time, galaxies accumulate relic CRs in their halos, providing additional non-thermal pressure support that suppresses low-velocity CR-driven outflows. We predict that such low-velocity outflows are expected only in young systems that have yet to build significant CR halos. In contrast, fast outflows in starburst galaxies, where the SFR surface density exceeds the critical threshold, are primarily driven by thermal energy and remain largely unaffected by CR halos.