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This is a question from Galaxies in the Universe: An Introduction(2nd edition),Linda S.Sparke&John S. Gallagher III. As shown in the pictureenter image description here

My question is why we can neglect the bulge of spiral galaxy and approximate the mass as $\mathcal{M} \propto V_{\max }^{2} h_{R}$

The equation 3.20 mentioned in this question is:$\frac{V^{2}(r)}{r}=-F_{\mathrm{r}}(r)=\frac{G \mathcal{M}(<r)}{r^{2}}$

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    $\begingroup$ The question is asking you to ignore the mass of the bulge for the sake of the calculation, not telling you that you can always safely do so. (Though there are plenty of spirals without bulges.) $\endgroup$
    – Peter Erwin
    May 28, 2021 at 12:42

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