# Questions on mass-to-light ratio

Niles
Hi guys

I have some questions about this topic. Within 1 kpc of the sun, the mass-to-light ratio is 4 M/L - so for every photon emitted, we have 4 matter-particles.

Now the ratio for the Milky Way is 10 M/L, which is 2.5 times bigger. In my book they say that this means that there's 2.5 times more matter, which we do not see (i.e. dark matter). My question is: Why is that? Who says that the 2.5 times more matter isn't just dead stars?

Question #2: I have the following expression for the intensity ofa spiralling galaxy: I(R) = K*exp(-R/R_s), where R_s is a few parsecs. If I(R) is constant, then is the mass constant too?

Atakor
Hi guys

I have some questions about this topic. Within 1 kpc of the sun, the mass-to-light ratio is 4 M/L - so for every photon emitted, we have 4 matter-particles.

Now the ratio for the Milky Way is 10 M/L, which is 2.5 times bigger. In my book they say that this means that there's 2.5 times more matter, which we do not see (i.e. dark matter). My question is: Why is that? Who says that the 2.5 times more matter isn't just dead stars?

Question #2: I have the following expression for the intensity ofa spiralling galaxy: I(R) = K*exp(-R/R_s), where R_s is a few parsecs. If I(R) is constant, then is the mass constant too?
Hi Niles,

Indeed, you are right .. that mass2light ratio doesnt imply by its own the existence of Dark Matter (ie a new unknown form of matter) and in fact, we started by searching for
MACHOs (Massive Halo Compact Objects) first.
Machos are massive objects that do not shine (or at least not too much) like brown dwarfs or black holes..
such surveys concluded that the amount of hidden mass in such objects doesnt solve the missing matter problem. that's why (among other reasons) we're searching now for WIMPs.. candidates of Dark Matter, very well motivated from High Energy Physics.

Cheers.