Low surface brightness (LSB) disk galaxies have all the characteristics of unevolved galaxies. Those discovered so far constitute a population of gas-rich, metal-poor galaxies with very low star formation rates (see the review by Bothun et al. 1997). Their surface brightnesses are a few magnitudes lower than the values commonly found for so-called normal galaxies. Most of them are rather late-type galaxies, with diffuse spiral arms.
A direct probe of the evolutionary state of these galaxies is the metal abundance in the interstellar medium (ISM). A low abundance generally indicates only limited enrichment of the ISM and therefore (in a closed system) a small amount of evolution.
Because of their low surface brightness, obtaining spectra of the stellar disks of LSB galaxies is difficult and requires large amounts of telescope time. Conclusions on metallicities must therefore be derived from spectra of HII regions. These usually are the brightest distinct objects in a LSB galaxy. Their bright emission lines make them more easily observable than the underlying continuum. The HII regions that are observed in LSB galaxies are usually giant HII regions, that are ionized by star clusters rather than by a few stars.
The first measurements of the oxygen abundances in HII regions in LSB galaxies were presented in McGaugh (1994). He found, using an empirical oxygen abundance indicator, that LSB galaxies are low metallicity galaxies with typical values for the metallicity . It shows that low metallicities can occur in galaxies that are comparable in size and mass to the bright galaxies that define the Hubble sequence. As LSB galaxies are found to be isolated (Mo et al. 1994), this suggest that surface mass density and environment are as important for the evolution of a galaxy as total mass.
In this paper we present a follow-up study of oxygen abundances in LSB galaxies. We confirm the results by McGaugh (1994) that LSB galaxies are metal-poor. We present two direct measurements of the oxygen abundance from measurements of the [O III ] line, supplemented with a large number of empirically determined oxygen abundances. In addition, for those galaxies where sufficient data are available, we investigate the change in abundance with radius, and show that the measurements are consistent with no gradient. The steeper gradients found in HSB galaxies (Vila-Costas & Edmunds 1992 [VCE], Zaritsky et al. 1994, Henry & Howard 1995, Kennicutt & Garnett 1996) are not present. It is worth noting that the exact form and magnitude of the Milky Way oxygen gradient has now been consistently reproduced in early-type stars, HII regions and planetary nebulae, which supports that the extra-galactic HII region gradients in HSB galaxies are real (Smartt & Rolleston 1997).
The lack of abundance gradients in LSB galaxies supports the picture of stochastic and sporadic evolution, where the evolutionary rate only depends on local conditions and not on the global properties of LSB galaxies as a whole.
Sect. 2 describes the sample selection and observations. Section 3 presents the data, while in Sect. 4 the analysis is described. Sect. 5 discusses the abundances found. Sect. 6 discusses reddening towards the HII regions, while Sect. 7 concludes with presenting the gradients. In Sect. 8 the results are summarized.
© European Southern Observatory (ESO) 1998
Online publication: June 18, 1998