We present measurements of the oxygen abundance of the Milky Way ’ s ISM by observing the K-shell X-ray photoionization edge towards galaxy clusters .
This effect is most easily observed towards objects with galactic columns ( n _ { H } ) of a few times 10 ^ { 21 } cm ^ { -2 } .
We measure X-ray column densities towards 11 clusters and find that at high galactic columns above approximately 10 ^ { 21 } cm ^ { -2 } the X-ray columns are generally 1.5–3.0 times greater than the 21 cm H i columns , indicating that molecular clouds become an important contributor to n _ { H } at higher columns .
We find the average ISM oxygen abundance to be ( O/H ) = ( 4.85 \pm 0.06 ) \times 10 ^ { -4 } , or 0.99 solar when using the most recent solar photospheric values .
Since X-ray observations are sensitive to the total amount of oxygen present ( gas + dust ) , these results indicate a high gas to dust ratio .
Also , the oxygen abundances along lines of sight through high galactic columns ( n _ { H } ) are the same as abundances through low columns , suggesting that the composition of denser clouds is similar to that of the more diffuse ISM .