The z-structure of the disc of NGC 3115

We have analysed the photometric observations of the edge-on disc of NGC 3115 presented by Capaccioli, Held & Nieto. By deprojection of the observed major axis light profile, it is shown that, outside the central region where the disc fades (R<20″), the face-on surface brightness is accurately described by the usual exponential law, with a scale-length $$\alpha_\text c^{-1} = 1.2$$ kpc, and an extrapolated central value μ_0 = 21.7 B-mag arcsec⁻². The deprojected scale-height in the z-direction is not constant with the galactocentric distance R. Its linear flare-up at $$R\simeq75$$″ suggests that the disc ceases to be self-gravitating at this radius, which implies $$M/L_B=7$$ (same as for the bulge). The radial dependence of $$\langle V^2_\text Z\rangle^{1/2} $$ is also deduced; the mean value (∼25 km s⁻¹ for $$R\gt75$$″), rather high for a disc with no gas, suggests that, in the past, the disc of NGC 3115 possessed a fair amount of gas.