Magnetically Driven Outflows in a Starburst Environment

We here investigate the possibility that the observed collimated outflows in luminous infrared galaxies (LIGs) and some Seyfert galaxies can be produced in a starburst (SB) environment. In the former source class, in particular, there seems to be some observational evidence for the presence of nuclear SBs in some objects. A nuclear disk can be quickly produced by gas infall during star formation in a rotating, stellar cluster. We find that massive nuclear SBs with core disk masses M_d~10⁸-10⁹ M_☉ and supernova rates ν_SN ≃ 5 × 10^-3-2 yr^-1 (which are consistent with the ν_SN-values inferred from the observed nonthermal radio power in source candidates) may inject kinetic energies that are high enough to blow out directed flows from the accreting disk surface, within the SB lifetimes. In our models, the acceleration and collimation of the nuclear outflow are provided by magnetic fields anchored into the rotating SB disk. The emerging outflow carries a kinetic power that is only a small fraction (a few percent) of the supernovae energy rate produced in the SB. Based on conditions determined from observed outflows and disks, we find that moderate disk magnetic fields (≳ 8 × 10^-4 G) are able to accelerate the outflows up to the observed terminal velocities (≲few 100 km s^-1 in the case of the Seyfert galaxies and ~400-950 km s^-1 in the case of the LIGs). The outflow is produced within a wind zone in the disk of radius ≲ 100 pc in the LIGs and ≲ 10 pc in the Seyferts, with wind mass loss to disk accretion rate ratios _w/_d ≳ 0.1 (where _d~100 M_☉ yr^-1). The observation of rotating nuclear disks of gas within ~few 100 pc scales, like that in the prototype LIG, Arp 220 (for which _d~100 M_☉ yr^-1) and magnetized outflows in Seyfert galaxies and LIGs (with terminal B~10^-5 G at the kiloparsec scales), provides some observational support for the magnetocentrifugal disk picture drawn here.