The Australia Telescope Compact Array ( ATCA ) has been used for high angular resolution imaging of 71 southern class I methanol maser sources quasi-simultaneously at 36 and 44 GHz .
The data reveal a high level of morphological and kinematical complexity , and allow us to demonstrate associations , at arcsecond precision , of the class I maser emission with outflows , expanding H ii regions , dark clouds , shocks traced by the 4.5- \mu m emission and 8.0- \mu m filaments .
More than 700 maser component features were found at each of the two methanol transitions , but with only 23 per cent recognisable at both transitions ; the morphology of class I emission is much better revealed by our survey of both transitions , compared with either one alone .
We found that the number of masers falls exponentially with the projected linear distance from the associated class II 6.7-GHz methanol maser .
This distribution has a scale of 263 \pm 15 mpc , irrespective of the transition .
The class I masers associated with OH masers were found to have a tendency to be more spread out , both spatially and in the velocity domain .
This is consistent with the expectation that such sources are more evolved .
Apart from a small number of high-velocity components ( which are largely blue-shifted and predominantly seen at 36 GHz ) , the velocity distribution was found to be Gaussian , peaking near the systemic velocity of the region , which had been estimated as the middle of the velocity interval of the associated class II methanol maser at 6.7 GHz .
The mean indicated a small , but significant blue shift asymmetry of - 0.57 km s ^ { -1 } ( uncertainties are 0.06 and 0.07 km s ^ { -1 } for the 36- and 44-GHz masers , respectively ) with respect to the 6.7-GHz masers .
The standard deviation of the velocity distribution was found to be 3.65 \pm 0.05 and 3.32 \pm 0.07 km s ^ { -1 } for the 36- and 44-GHz masers , respectively .
We also suggest a refined rest frequency value of 36169.238 \pm 0.011 MHz for the 4 _ { -1 } - 3 _ { 0 } E methanol transition .