We measure the star formation efficiency ( SFE ) , the star formation rate per unit gas , in 23 nearby galaxies and compare it to expectations from proposed star formation laws and thresholds .
We use H i maps from THINGS and derive H _ { 2 } maps from CO measured by HERACLES and BIMA SONG .
We estimate the star formation rate by combining GALEX FUV maps and SINGS 24 \mu m maps , infer stellar surface density profiles from SINGS 3.6 \mu m data , and use kinematics from THINGS .
We measure the SFE as a function of : the free–fall and orbital timescales ; midplane gas pressure ; stability of the gas disk to collapse ( including the effects of stars ) ; the ability of perturbations to grow despite shear ; and the ability of a cold phase to form .
In spirals , the SFE of H _ { 2 } alone is nearly constant at 5.25 \pm 2.5 \times 10 ^ { -10 } yr ^ { -1 } ( equivalent to an H _ { 2 } depletion time of 1.9 \times 10 ^ { 9 } yr ) as a function of all of these variables at our 800 pc resolution .
Where the ISM is mostly H i , on the other hand , the SFE decreases with increasing radius in both spiral and dwarf galaxies , a decline reasonably described by an exponential with scale length 0.2 – 0.25 ~ { } r _ { 25 } .
We interpret this decline as a strong dependence of GMC formation on environment .
The ratio of molecular to atomic gas appears to be a smooth function of radius , stellar surface density , and pressure spanning from the H _ { 2 } –dominated to H i–dominated ISM .
The radial decline in SFE is too steep to be reproduced only by increases in the free–fall time or orbital time .
Thresholds for large–scale instability suggest that our disks are stable or marginally stable and do not show a clear link to the declining SFE .
We suggest that ISM physics below the scales that we observe — phase balance in the H i , H _ { 2 } formation and destruction , and stellar feedback — governs the formation of GMCs from H i .