The Balmer decrement of SDSS galaxies
  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. Halpha/Hbeta), 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 Data Release 7 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 Hbeta line fluxes,
within the SDSS DR7 MPA/JHU catalogue, with the equivalent widths
underestimated by average ~0.35A in the emission line galaxy sample. This error
means that Balmer decrement determined attenuations are overestimated by a
systematic 0.1 magnitudes in A_V, and future analyses of this sample need to
include this correction.
