As a first step of a program aimed to the detection of dark matter ( or radial variations of M / L ) in early–type galaxies , we report deep spectroscopic observations of the bulge–dominated edge–on S0 galaxy NGC 3115 , made at ESO , La Silla , using EFOSC at the 3.6 m telescope and EMMI at NTT .
Such observations allow measurements of the rotational velocity out to 1.8 a _ { e } ( effective radii ) from the galaxy center , where the surface brightness is \mu _ { B } \simeq 24 mag arcsec ^ { -2 } .
The rotation curve quickly reaches an asymptotic value , \mbox { $ \langle v _ { f } \rangle$ } \simeq 260 km s ^ { -1 } , with only marginal indication of systematic decline within the range of our observations .
The line–of–sight velocity dispersion has also been measured ; it decreases steeply from a rather high central value and flattens out ( \langle \sigma \rangle \simeq 100 km s ^ { -1 } ) within our observing range ( a \raise - 2.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } \raise 5.0 pt \hbox { $ < $ } } 1.3 % a _ { e } ) .

Models built on these data and simple dynamical arguments show that the M / L of NGC 3115 must thus be increasing from \mbox { $M / L$ } = 6 ( in solar units ) in the inner regions ( \sim 1 a _ { e } ) to at least \mbox { $M / L$ } \geq 10 in the outermost regions ( \sim 2 a _ { e } ) .