To evaluate the impact of stellar feedback , it is critical to estimate the mass outflow rates of galaxies .
Past estimates have been plagued by uncertain assumptions about the outflow geometry , metallicity , and ionization fraction .
Here we use Hubble Space Telescope ultraviolet spectroscopic observations of the nearby starburst NGC 6090 to demonstrate that many of these quantities can be constrained by the data .
We use the Si IV absorption lines to calculate the scaling of velocity ( v ) , covering fraction ( C _ { f } ) , and density with distance from the starburst ( r ) , assuming the Sobolev optical depth and a velocity law of the form : v ~ { } \propto ( 1 - \mathrm { R } _ { \mathrm { i } } / \mathrm { r } ) ^ { \beta } ( where R _ { \mathrm { i } } is the inner outflow radius ) .
We find that the velocity ( \beta =0.43 ) is consistent with an outflow driven by an r ^ { -2 } force with the outflow radially accelerated , while the scaling of the covering fraction ( C _ { f } \propto \mathrm { r } ^ { -0.82 } ) suggests that cool clouds in the outflow are in pressure equilibrium with an adiabatically expanding medium .
We use the column densities of four weak metal lines and CLOUDY photoionization models to determine the outflow metallicity , the ionization correction , and the initial density of the outflow .
Combining these values with the profile fitting , we find R _ { \mathrm { i } } = 63 pc , with most of the mass within 300 pc of the starburst .
Finally , we find that the maximum mass outflow rate is 2.3 M _ { \odot } yr ^ { -1 } and the mass loading factor ( outflow divided by the star formation rate ) is 0.09 , a factor of 10 lower than the value calculated using common assumptions for the geometry , metallicity and ionization structure of the outflow .