On how leakage can affect the star formation rate estimation using Hα luminosity

We present observational evidence that leakage of ionizing photons from star-forming regions can affect the quantification of the star formation rate (SFR) in galaxies. This effect could partially explain the differences between the SFR estimates using the far-ultraviolet (FUV) and the Hα emission. We find that leakage could decrease the SFR(Hα)/SFR(FUV) ratio by up to 25 per cent. The evidence is based on the observation that the SFR(Hα)/SFR(FUV) ratio is lower for objects showing a shell Hα structure than for regions exhibiting a much more compact morphology. The study has been performed on three object samples: low-luminosity dwarf galaxies from the Local Volume Legacy survey, star-forming regions in the Large Magellanic Cloud and the nearby Local Group galaxy M33. For the three samples we find differences (1.1σ–1.4σ) between the SFR(Hα)/SFR(FUV) for compact and shell objects. Although leakage cannot entirely explain the observed trend of SFR(Hα)/SFR(FUV) ratios for systems with low SFR, we show that the mechanism can lead to different SFR estimates when using Hα and FUV luminosities. Therefore, further study is needed to constrain the contribution of leakage to the low SFR(Hα)/SFR(FUV) ratios observed in dwarf galaxies and its impact on the Hα flux as an SFR indicator in such objects.

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