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

TL;DR

This study refines the conversion of H-alpha fluxes to star formation rates by analyzing NII contamination and internal extinction, revealing spatial emission differences and improving rate estimates with type-dependent corrections.

Contribution

It provides detailed analysis of NII and extinction corrections, including spatial distribution and variation, to enhance the accuracy of star formation rate calculations from H-alpha data.

Findings

NII emission is spatially distinct from H-alpha in galaxy regions.

Extinction is higher in galaxy nuclei than disks.

Type-dependent extinction corrections improve SFR estimates.

Abstract

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.