The age of blue LSB galaxies

Context. Low metallicities, large gas-to-star mass ratios, and blue colors of most low surface brightness (LSB) galaxies imply that these systems may be younger than their high surface brightness counterparts. Aims. We seek to find observational signatures that can help to constrain the age of blue LSB galaxies. Methods. We use numerical hydrodynamic modelling to study the long-term (∼ 13 Gyr) dynamical and chemical evolution of blue LSB galaxies adopting a sporadic scenario for star formation. Our model galaxy consists of a thin gas disk, stellar disk with a developed spiral structure and spherical dark matter halo. Our models utilize various rates of star formation and different shapes of the initial mass function (IMF). We complement hydrodynamic modelling with population synthesis modelling to produce the integrated B − V colors and Hα equivalent widths. Results. We find that the mean oxygen abundances, B − V colors, Hα equivalent widths, and the radial fluctuations in the oxygen abundance, when considered altogether, can be used to constrain the age of blue LSB galaxies if some independent knowledge of the IMF is available. Our modelling strongly suggests the existence of a minimum age for blue LSB galaxies. Model B − V colors and mean oxygen abundances set a tentative minimum age at 1.5–3.0 Gyr, whereas model Hα equivalent widths suggest a larger value of order 5–6 Gyr. The latter value may decrease somewhat, if blue LSB galaxies host IMFs with a truncated upper mass limit. We found no firm evidence that the age of blue LSB galaxies is significantly smaller than 13 Gyr. Conclusions. The age of blue LSB galaxies may vary between 1.5–6 Gyr and 13 Gyr, depending on the physical conditions in the disk that control the form of the IMF and the rate of star formation. A failure to observationally detect large radial fluctuations in the oxygen abundance of order 0.5–1.0 dex, which, according to our modelling, are characteristic of (1-2)-Gyr-old galaxies, will argue in favour of the more evolved nature of blue LSB galaxies.