The Balmer decrement of Sloan Digital Sky Survey 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. 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.