The connection between shear and star formation in spiral galaxies

We present a sample of 33 galaxies for which we have calculated (i) the average rate of shear from published rotation curves, (ii) the far-infrared luminosity from IRAS fluxes, and (iii) the K-s-band luminosity from the Two Micron All Sky Survey (2MASS). We show that a correlation exists between the shear rate and the ratio of the far-infrared to K-s-band luminosity. This ratio is essentially a measure of the star formation rate per unit mass, or the specific star formation rate. From this correlation we show that a critical shear rate exists, above which star formation would turn off in the discs of spiral galaxies. Using the correlation between shear rate and spiral arm pitch angle, this shear rate corresponds to the lowest pitch angles typically measured in near-infrared images of spiral galaxies.