Are Low Surface Brightness Discs Young?

We reconsider the problem of the age of the stellar discs of late-type Low Surface Brightness (LSB) galaxies by making use of a new IMF recently derived from numerical fluid dynamical simulations (Padoan, Nordlund and Jones, 1997). While a Miller-Scalo IMF cannot adequately describe the photometric properties of LSBs, when we apply the new Padoan et al. (1997) IMF to a simple exponential disc model with parameters appropriate to LSBs, we get excellent fits of the colors and color gradients. We then conclude that: a) The star formation history of LSB disc galaxies can be described by an initial burst of a few times 10yr followed by a quiescent period with only sporadic star formation; b) LSBs’ discs are not young. The age of the LSB disc galaxies for which colors have been measured are all larger than about 10 Gyr.