Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 419, Issue 2, Pages 1402-1412Publisher
WILEY-BLACKWELL
DOI: 10.1111/j.1365-2966.2011.19796.x
Keywords
dust; extinction; galaxies: active; galaxies: starburst; galaxies: statistics
Categories
Funding
- Alfred P. Sloan Foundation
- National Science Foundation
- US Department of Energy
- National Aeronautics and Space Administration
- Japanese Monbukagakusho
- Max Planck Society
- Higher Education Funding Council for England
- American Museum of Natural History
- Astrophysical Institute Potsdam
- University of Basel
- University of Cambridge
- Case Western Reserve University
- University of Chicago
- Drexel University
- Fermilab
- Institute for Advanced Study
- Japan Participation Group
- Johns Hopkins University
- Joint Institute for Nuclear Astrophysics
- Kavli Institute for Particle Astrophysics and Cosmology
- Korean Scientist Group
- Chinese Academy of Sciences (LAMOST)
- Los Alamos National Laboratory
- Max-Planck-Institute for Astronomy (MPIA)
- Max-Planck-Institute for Astrophysics (MPA)
- New Mexico State University
- Ohio State University
- University of Pittsburgh
- University of Portsmouth
- Princeton University
- United States Naval Observatory
- University of Washington
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High-resolution spectra are necessary to distinguish and correctly measure the Balmer emission lines due to the presence of strong metal and Balmer absorption features in the stellar continuum. This accurate measurement is necessary for use in emission-line diagnostics, such as the Balmer decrement (i.e. Ha/H beta), used to determine the attenuation of galaxies. Yet at high redshifts, obtaining such spectra becomes costly. Balmer emission-line equivalent widths are much easier to measure, requiring only low-resolution spectra or even simple narrow-band filters and therefore shorter observation times. However, a correction for the stellar continuum is still needed for this equivalent width Balmer decrement. We present here a statistical analysis of the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) emission-line galaxy sample, using the spectrally determined Balmer emission-line fluxes and equivalent widths. Using the large numbers of galaxies available in the SDSS catalogue, we determined an equivalent width Balmer decrement including a statistically based correction for the stellar continuum. Based on this formula, the attenuation of galaxies can now be obtained from low spectral resolution observations. In addition, this investigation also revealed an error in the H beta line fluxes, within the SDSS DR7 MPA/JHU catalogue, with the equivalent widths underestimated by average similar to 0.35 angstrom in the emission-line galaxy sample. This error means that Balmer decrement determined attenuations are overestimated by a systematic 0.1 mag in AV, and future analyses of this sample need to include this correction.
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