The Origin of Exponential Star-forming Disks

TL;DR

This paper proposes a physical model where magnetic stresses in galactic gas disks, driven by magneto-rotational instability, naturally produce stable exponential profiles in star-forming disks, linking magnetic fields, star formation, and disk structure.

Contribution

It introduces a model connecting magnetic stresses and star formation feedback to explain the origin of exponential galactic disks, supported by observationally consistent parameters.

Findings

Magnetic stresses can produce stable exponential disks.

The disk scale-length depends on gas inflow rate and halo velocity.

Magnetic field strength correlates with star-formation surface density.

Abstract

The disk components of galaxies generally show an exponential profile extending over several scale lengths, both in mass and star-formation rate, but the physical origin is not well understood. We explore a physical model in which the galactic gas disk is viewed as a "modified accretion disk" in which coplanar gas inflow, driven by viscous stresses in the disk, provides the fuel for star formation, which progressively removes gas as it flows inwards. We show that magnetic stresses from magneto-rotational instability are the most plausible source of the required viscosity, and construct a simple physical model to explore this. A key feature is to link the magnetic field strength to the local star-formation surface density, $B_{\rm tot} \propto \Sigma_{\rm SFR}^\alpha$. This provides a feed-back loop between star-formation and the flow of gas. We find that the model naturally produces stable steady-state exponential disks, as long as $\alpha \sim$ 0.15, the value indicated from spatially-resolved observations of nearby galaxies. The disk scale-length $h_{\rm R}$ is set by the rate at which the disk is fed, by the normalization of the $B_{\rm tot}-\Sigma_{\rm SFR}$ relation and by the circular velocity of the halo. The angular momentum distribution of the gas and stars within the disk is a consequence of the transfer of angular momentum that is inherent to the operation of an accretion disk, rather than the initial angular momentum of the inflowing material. We suggest that magnetic stresses likely play a major role in establishing the stable exponential form of galactic disks.