We re-examine the effect of Lyman continuum ( \lambda \leq 912 Å ) extinction ( LCE ) by dust in H ii regions in detail and discuss how it affects the estimation of the global star formation rate ( SFR ) of galaxies .
To clarify the first issue , we establish two independent methods for estimating a parameter of LCE ( f ) , which is defined as the fraction of Lyman continuum photons contributing to hydrogen ionization in an H ii region .
One of those methods determines f from the set of Lyman continuum flux , electron density and metallicity .
In the framework of this method , as the metallicity and/or the Lyman photon flux increase , f is found to decrease .
The other method determines f from the ratio of infrared flux to Lyman continuum flux .
Importantly , we show that f \lesssim 0.5 via both methods in many H ii regions of the Galaxy .
Thus , it establishes that dust in such H ii regions absorbs significant amount of Lyman continuum photons directly .
To examine the second issue , we approximate f to a function of only the dust-to-gas mass ratio ( i.e. , metallicity ) , assuming a parameter fit for the Galactic H ii regions .
We find that a characteristic \hat { f } , which is defined as f averaged over a galaxy-wide scale , is 0.3 for the nearby spiral galaxies .
This relatively small \hat { f } indicates that a typical increment factor due to LCE for estimating the global SFR ( 1 / \hat { f } ) is large ( \sim 3 ) for the nearby spiral galaxies .
Therefore , we conclude that the effect of LCE is not negligible relative to other uncertainties of estimating the SFR of galaxies .