The Balmer decrement of SDSS galaxies

TL;DR

This paper develops a statistical correction for the stellar continuum in low-resolution spectra to accurately measure the Balmer decrement in SDSS galaxies, enabling attenuation estimates without high-resolution spectra.

Contribution

It introduces a new formula for the equivalent width Balmer decrement with a correction for stellar continuum, based on SDSS data, improving attenuation measurements at low spectral resolution.

Findings

Identified an error in SDSS Hbeta fluxes leading to overestimated attenuation.

Derived a correction formula for the Balmer decrement using SDSS galaxy data.

Provided a systematic correction for future low-resolution spectral analyses.

Abstract

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.