The Maximum Flux of Star-Forming Galaxies

The importance of radiation pressure feedback in galaxy formation has been extensively debated over the last decade. The regime of greatest uncertainty is in the most actively star-forming galaxies, where large dust columns can potentially produce a dust-reprocessed infrared radiation field with enough pressure to drive turbulence or eject material. Here we derive the conditions under which a self-gravitating, mixed gas-star disc can remain hydrostatic despite trapped radiation pressure. Consistently taking into account the self-gravity of the medium, the star- and dust-to-gas ratios, and the effects of turbulent motions not driven by radiation, we show that galaxies can achieve a maximum Eddington-limited star formation rate per unit area $\dot{\Sigma}_{\rm *,crit} \sim 10^3 M_{\odot}$ pc$^{-2}$ Myr$^{-1}$, corresponding to a critical flux of $F_{\rm *,crit} \sim 10^{13} L_{\odot}$ kpc$^{-2}$ similar to previous estimates; higher fluxes eject mass in bulk, halting further star formation. Conversely, we show that in galaxies below this limit, our one-dimensional models imply simple vertical hydrostatic equilibrium and that radiation pressure is ineffective at driving turbulence or ejecting matter. Because the vast majority of star-forming galaxies lie below the maximum limit for typical dust-to-gas ratios, we conclude that infrared radiation pressure is likely unimportant for all but the most extreme systems on galaxy-wide scales. Thus, while radiation pressure does not explain the Kennicutt-Schmidt relation, it does impose an upper truncation on it. Our predicted truncation is in good agreement with the highest observed gas and star formation rate surface densities found both locally and at high redshift.Comment: Version accepted for publication in MNRAS. 12 pages, 8 figures. New appendix on photon tirin

Collapse, outflows and fragmentation of massive, turbulent and magnetized prestellar barotropic cores

Stars and more particularly massive stars, have a drastic impact on galaxy evolution. Yet the conditions in which they form and collapse are still not fully understood. In particular, the influence of the magnetic field on the collapse of massive clumps is relatively unexplored, it is thus of great relevance in the context of the formation of massive stars to investigate its impact. We perform high resolution, MHD simulations of the collapse of hundred solar masses, turbulent and magnetized clouds, using the adaptive mesh refinement code RAMSES. We compute various quantities such as mass distribution, magnetic field and angular momentum within the collapsing core and study the episodic outflows and the fragmentation that occurs during the collapse. The magnetic field has a drastic impact on the cloud evolution. We find that magnetic braking is able to substantially reduce the angular momentum in the inner part of the collapsing cloud. Fast and episodic outflows are being launched with typical velocities of the order of 3-5 km s$^{-1}$ although the highest velocities can be as high as 30-40 km s$^{-1}$. The fragmentation in several objects, is reduced in substantially magnetized clouds with respect to hydrodynamical ones by a factor of the order of 1.5-2. We conclude that magnetic fields have a significant impact on the evolution of massive clumps. In combination with radiation, magnetic fields largely determine the outcome of massive core collapse. We stress that numerical convergence of MHD collapse is a challenging issue. In particular, numerical diffusion appears to be important at high density therefore possibly leading to an over-estimation of the number of fragments.Comment: accepted for publication in A&

A fundamental test for stellar feedback recipes in galaxy simulations

Direct comparisons between galaxy simulations and observations that both reach scales < 100 pc are strong tools to investigate the cloud-scale physics of star formation and feedback in nearby galaxies. Here we carry out such a comparison for hydrodynamical simulations of a Milky Way-like galaxy, including stochastic star formation, HII region and supernova feedback, and chemical post-processing at 8 pc resolution. Our simulation shows excellent agreement with almost all kpc-scale and larger observables, including total star formation rates, radial profiles of CO, HI, and star formation through the galactic disc, mass ratios of the ISM components, both whole-galaxy and resolved Kennicutt-Schmidt relations, and giant molecular cloud properties. However, we find that our simulation does not reproduce the observed de-correlation between tracers of gas and star formation on < 100 pc scales, known as the star formation 'uncertainty principle', which indicates that observed clouds undergo rapid evolutionary lifecycles. We conclude that the discrepancy is driven by insufficiently-strong pre-supernova feedback in our simulation, which does not disperse the surrounding gas completely, leaving star formation tracer emission too strongly associated with molecular gas tracer emission, inconsistent with observations. This result implies that the cloud-scale de-correlation of gas and star formation is a fundamental test for feedback prescriptions in galaxy simulations, one that can fail even in simulations that reproduce all other macroscopic properties of star-forming galaxies.Comment: 13 pages, 10 figures, accepted for publication in MNRA