# Questions on mass-to-light ratio

• Niles
In summary, the conversation discusses the concept of dark matter and its relation to mass-to-light ratio in galaxies. The speaker questions the existence of dark matter and suggests that the additional mass could be from dead stars. The second question asks about the constancy of mass in a spiralling galaxy based on its intensity.

#### 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?

Niles said:
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 doesn't 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 doesn't 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.

## 1. What is the mass-to-light ratio?

The mass-to-light ratio is a measure of the amount of mass present in a celestial object compared to the amount of light it emits. It is commonly used in astronomy to study the distribution of matter in galaxies and other astronomical bodies.

## 2. How is the mass-to-light ratio calculated?

The mass-to-light ratio is calculated by dividing the total mass of an object by its total luminosity, or the amount of light it emits. The mass is typically measured using techniques such as gravitational lensing or the rotation curve method, while the luminosity is measured using telescopes and other instruments.

## 3. What units are used for the mass-to-light ratio?

The units used for the mass-to-light ratio vary depending on the type of measurement being used. In astronomy, the most commonly used unit is the solar mass to solar luminosity ratio, with 1 solar mass equaling the mass of our Sun and 1 solar luminosity equaling the amount of light it emits.

## 4. What does a high or low mass-to-light ratio indicate?

A high mass-to-light ratio indicates that an object has a large amount of mass compared to the amount of light it emits, suggesting that it is dominated by dark matter. A low mass-to-light ratio suggests that an object has a smaller mass and is primarily made up of luminous matter, such as stars.

## 5. Why is the mass-to-light ratio important in astronomy?

The mass-to-light ratio is important in astronomy because it provides information about the distribution of matter in celestial objects. It can help scientists understand the formation and evolution of galaxies, as well as the presence and behavior of dark matter. The mass-to-light ratio also plays a role in determining the total mass of a galaxy, which is essential for studying its dynamics and interactions with other objects.