Coupling local to global star formation in spiral galaxies: the effect of differential rotation

Star formation is one of the key factors that shapes galaxies. This process is relatively well understood from both simulations and observations on a small ‘local’ scale of individual giant molecular clouds (GMCs) and also on a ‘global’ galaxy-wide scale (e.g. the Kennicutt–Schmidt law). However, there is still no understanding on how to connect global to local star formation scales and whether this connection is at all possible. Here, we analyse spatially resolved kinematics and the star formation rate (SFR) density ΣSFR for a combined sample of 17 nearby spiral galaxies obtained using our own optical observations in Hα for nine galaxies and neutral hydrogen radio observations combined with a multiwavelength spectral energy distribution analysis for eight galaxies from the THINGS project. We show that the azimuthally averaged normalized SFR density in spiral galaxies on a scale of a few hundred parsecs is proportional to the kinetic energy of GMC collisions due to differential rotation of the galactic disc. This energy is calculated from the rotation curve using the two Oort parameters A and B as log (ΣSFR/SFRtot)∝log [2A2 + 5B2]. The total kinetic energy of collision is defined by the shear velocity that is proportional to A and the spin energy of a cloud proportional to the vorticity B. Hence, shear does not act as a stabilizing factor for the cloud collapse thus reducing star formation but rather increases it by boosting the kinetic energy of collisions.