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

Abstract

We reexamine the effect of Lyman continuum (λ ≤ 912Å) 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 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-togas mass ratio (i.e., metallicity), assuming a parameter fit for the Galactic H ii regions. We find that a characteristicf , which is defined as f averaged over a galaxy-wide scale, is 0.3 for the nearby spiral galaxies. This relatively smallf indicates that a typical increment factor due to LCE for estimating the global SFR (1/f) is large (∼ 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.

The Spectral Evolution of the First Galaxies. Ii. Spectral Signatures of Lyman Continuum Leakage From Galaxies in the Reionization Epoch

“…Other ratios of stellar to nebular attenuation can certainly be considered, but this serves to exemplify the EW(Hβ) and β trends. The resulting LyC depletion factor is somewhat lower than suggested by Inoue (2001) and Inoue et al (2001) for local H ii regions, but similar to that derived by Hirashita et al (2003) for low-redshift starburst galaxies. Our procedure gives rise to the dashed attenuation arrows in Figure 9.…”

“…However, if the dust is spatially mixed with the H ii gas as in Figures 8(c) and (d), some fraction of the LyC photons will be absorbed by dust before they have a chance to ionize the gas (e.g., Inoue et al 2001;Inoue 2001) and this will not be revealed by the Balmer line ratios. While radiation pressure and dust sublimation act to produce a dust cavity around the ionizing stars, neither of these effects is sufficiently strong to prevent LyC photons from getting absorbed directly by dust in local H ii regions (Inoue 2002).…”

Characterizing Dust Attenuation in Local Star-forming Galaxies: Near-infrared Reddening and Normalization

“…There are two components to consider for the absorption of UV-to-NIR photons: the stellar continuum from l m < < 0.0912 2.85 m, which will be characterized by the attenuation curve, and ionizing photons (l m < 0.0912 m) absorbed by dust either directly or after they have been re-emitted by hydrogen through recombination lines. In practice, this comparison is quite difficult owing to the large uncertainty in calculating the fraction of ionizing photons directly absorbed by dust (e.g., Inoue et al 2001;Hirashita et al 2003).…”

High-Mass Star Formation Toward Southern Infrared Bubble S10

“…This estimate is with the assumption of optically thin emission and hence serves as a lower limit as the emission could be optically thick at 1280 MHz. Various studies in the literature have shown that dust absorption of Lyman continuum photons can be very high (Inoue et al 2001;Arthur et al 2004;Paron et al 2011). With limited knowledge of the dust properties, we have not accounted for the dust absorption here.…”