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Paper: A Preliminary Budget for the Ionizing Photons in the HII Regions of M51
Volume: 390, Pathways Through an Eclectic Universe
Page: 300
Authors: Gutiérrez, L.; Beckman, J.E.
Abstract: Using HST/ACS broad- and narrow-band (Hα) images of M51 we have examined the ionizing radiation budget in a set of luminous HII regions in a spiral arm between 5 kpc and 11 kpc from the center. The extinctions within the regions were estimated, and a mean value for the differential extinction coefficient, RV , of 3.3, similar to the Galactic value, was derived. The measured AV values for the regions was in the region 0.82–3.24, with a mean value of 1.57 magnitudes. The Lyman continuum (Lyc) luminosities of the ionizing clusters were derived from their absolute magnitudes, and thence the corresponding predicted Hα luminosities from the HII regions; these exceeded the measured values by a factor of ≈ 7. Even if this discrepancy is due entirely to Lyc extinction, the range of values for this would be from 0.8 to 3.7, too small to be explained except in a highly inhomogeneous model of gas and dust distribution. Assuming that in fact 50% of the Lyc escapes from a region, the need for a high degree of inhomogeneity is even greater.
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