The H-alpha Galaxy Survey II. Extinction and NII corrections to H-alpha fluxes

Details The H-alpha Galaxy Survey II. Extinction and NII corrections to H-alpha fluxes The H-alpha Galaxy Survey II. Extinction and NII corrections to H-alpha fluxes

2004-10-15

arxiv.org/abs/astro-ph/0410386v1

Astronomy and Astrophysics

Astrophysics

18 pages, accepted for publication by A&A

Scientific paper

10.1051/0004-6361:20035892

We study the two main corrections generally applied to narrow-band H-alpha fluxes from galaxies in order to convert them to star formation rates, namely for NII contamination and for extinction internal to the galaxy. From an imaging study using narrow-band filters, we find the NII and H-alpha emission to be differently distributed. In most disk star formation regions the NII fraction is small, whereas some galaxies display a diffuse central component which can be dominated by NII emission. We consider explanations for variations in the NII/H-alpha ratio, including metallicity variations and different excitation mechanisms. We then estimate the extinction towards star formation regions in spiral galaxies, firstly using Br-gamma/H-alpha line ratios. We find that extinction values are larger in galaxy nuclei than in disks, and that there is no evidence for heavily dust-embedded regions emerging in the near-IR. We use optical emission line data from the literature to show only a moderate dependence of extinction on inclination, consistent with broad-band photometric studies; typical extinctions are smaller for late-type dwarfs than for spiral types. Finally, we show that the application of type-dependent extinction corrections significantly improves the agreement between star formation rates from H-alpha and IRAS fluxes. This agreement argues against star formation in normal galaxies being dominated by heavily dust-embedded components.

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