Is the Initial Mass Function of Low Surface Brightness Galaxies Dominated by Low Mass Stars?

The rotation curves of low surface brightness (LSB) galaxies suggest that they possess significantly higher mass-to-light (M/L) ratios than their high surface brightness counterparts, indicating that LSB galaxies may be dark matter dominated. This interpretation is hampered by the difficulty of disentangling the disc and dark halo contributions from the disc dynamics of LSB galaxies. Recently, Fuchs (2002) has attempted such a disentanglement using spiral arm density wave and swing amplification theory, allowing an independent measurement of the disc mass; this work suggests that LSB discs are significantly more massive than previously believed. This would considerably reduce the amount of matter required in the dark halos in fitting the rotation curves. Interestingly, the high mass-to-light ratios derived for the discs appear inconsistent with standard stellar population synthesis models. In this paper, we investigate whether the high M/L ratios for the Fuchs LSB discs might be understood by adopting a very “bottom heavy” initial mass function (IMF). We find that an IMF with a power law exponent of around α = 3.85 (compared to the standard Salpeter IMF, α = 2.35) is sufficient to explain the unusually high M/L ratios of the Fuchs sample. Within the context of the models, the blue colours ((B − R)0 < 1.0) of the sample galaxies result from being metal-poor ([Fe/H] = −1.5 ∼ −1.0) and having undergone recent (∼ 1 3 Gyr ago) star formation.