= Aperture dependence of mass-to-light ratio
{title2=$\Upsilon(<R)=M(<R)/L(<R)$}
A dynamical <mass-to-light ratio> must specify its aperture and whether the <mass> includes <dark matter>. A growing dark halo can increase $\Upsilon(<R)$ outside the bright stellar body even when the stellar ratio is constant. Comparing a tidal-radius satellite <mass> with a large-aperture host <mass> is different from comparing both within their <half-light radii>. From $M\propto\sigma^2R$ and $L\propto\sigma^4$, the inferred ratio scales as $R/\sigma^2$, so equal ratios additionally require homologous <radius> scaling.
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