Spectroscopy of Low Surface Brightness Galaxies with the Hobby-Eberly Telescope

Abstract

We have obtained low-resolution spectra of 19 red and blue low surface brightness (LSB) galaxies, using the Marcario Low-Resolution Spectrograph on the 9.2 m Hobby-Eberly Telescope. These galaxies form a very heterogeneous class, whose spectra qualitatively resemble those of high surface brightness galaxies covering the full range of spectra seen in galaxies of Hubble types from E to Irr. We use a combination of emission-line (EW(Hα), [N II] λ6584/Hα) and absorption-line (Mg b, Hβ, (Fe)) based diagnostics to investigate the star formation and chemical enrichment histories of these galaxies. These are diverse, with some galaxies having low metallicity and very young mean stellar ages and other galaxies showing old, supersolar-metallicity stellar populations. In contrast with some previous studies, which found a strong trend of decreasing metallicity with decreasing central surface brightness, we find a population of galaxies with low surface brightness and near-solar metallicity. Correlations between several of the gas-phase and stellar population age and metallicity indicators are used to place constraints on plausible evolutionary scenarios for LSB galaxies, The redshift range spanned by these galaxies is broad, with radial velocities from 3400 km s -1 to more than 65,000 km s -1 A subset of the sample galaxies have published H I redshifts and gas masses based on observations with the Arecibo 305 m single-dish radio telescope, which place these galaxies far off of the mean Tully-Fisher relation. Our new optical redshifts do not agree with the published H I redshifts for these galaxies. Most of the discrepancies can be explained by beam confusion in the Arecibo o bservations, causing erroneous H I detections for some of the galaxies.