Effect of Dust Extinction on Estimating the Star Formation Rate of Galaxies: Lyman Continuum Extinction

We re-examine the effect of Lyman continuum ($\lambda \leq 912$ \AA)extinction (LCE) by dust in H {\sc ii} regions in detail and discuss how itaffects the estimation of the global star formation rate (SFR) of galaxies. Toclarify the first issue, we establish two independent methods for estimating aparameter of LCE ($f$), which is defined as the fraction of Lyman continuumphotons contributing to hydrogen ionization in an H {\sc ii} region. One ofthose methods determines $f$ from the set of Lyman continuum flux, electrondensity and metallicity. In the framework of this method, as the metallicityand/or the Lyman photon flux increase, $f$ is found to decrease. The othermethod determines $f$ from the ratio of infrared flux to Lyman continuum flux.Importantly, we show that $f \la 0.5$ via both methods in many H {\sc ii}regions of the Galaxy. Thus, it establishes that dust in such H {\sc ii}regions absorbs significant amount of Lyman continuum photons directly. Toexamine the second issue, we approximate $f$ to a function of only thedust-to-gas mass ratio (i.e., metallicity), assuming a parameter fit for theGalactic H {\sc ii} regions. We find that a characteristic $\hat{f}$, which isdefined as $f$ averaged over a galaxy-wide scale, is 0.3 for the nearby spiralgalaxies. This relatively small $\hat{f}$ indicates that a typical incrementfactor due to LCE for estimating the global SFR ($1/\hat{f}$) is large ($\sim3$) for the nearby spiral galaxies. Therefore, we conclude that the effect ofLCE is not negligible relative to other uncertainties of estimating the SFR ofgalaxies.Comment: 18 papges, 11 figures, accepted by Ap