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Date and Time of the Query: 2019-06-20 T06:45:08 PDT
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For refcode 2005A&A...429..851J:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
2005A&A...429..851J The H{alpha} Galaxy Survey II. Extinction and [NII] corrections to H{alpha} fluxes P. A. James, N. S. Shane, J. H. Knapen, J. Etherton and S. M. Percival Received 17 December 2003 / Accepted 13 September 2004 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 carefully chosen narrow-band filters, we find the [NII] and H{alpha} emission to be differently distributed. Nuclear measurements are likely to overestimate the contribution of [NII] to total narrow-band fluxes. We find that in most star formation regions in galaxy disks the [NII] fraction is small or negligible, whereas some galaxies display a diffuse central component which can be dominated by [NII] emission. We compare these results with related studies in the literature, and consider astrophysical explanations for variations in the [NII]/H{alpha} ratio, including metallicity variations and different excitation mechanisms. We proceed to 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, that disk extinction values are similar to those derived from optical emission-line studies in the literature, and that there is no evidence for heavily dust-embedded regions emerging in the near-IR, which would be invisible at H{alpha}. The numbers of galaxies and individual regions detected in Br {gamma} are small, however, and we thus exploit optical emission line data from the literature to derive global H{alpha} extinction values as a function of galaxy type and inclination. In this part of our study we find only a moderate dependence on inclination, consistent with broad-band photometric studies, and a large scatter from galaxy to galaxy. Typical extinctions are smaller for late-type dwarfs than for spiral types. Finally, we show that the application of the type-dependent extinction corrections derived here significantly improves the agreement between star formation rates calculated using H{alpha} fluxes and those from far-infrared fluxes as measured by IRAS. This again supports the idea that heavily dust-embedded star formation, which would be underestimated using the H{alpha} technique, is not a dominant contributor to the total star formation rate of most galaxies in the local Universe. Keywords: galaxies: statistics, galaxies: spiral, galaxies: irregular, galaxies: fundamental parameters, galaxies: photometry, galaxies: stellar content
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