The Milky Way has been divided into many components depending on the distribution of stars, dust, and gas. But the main feature that trumps the others and makes the most difference between each component is the different populations of stars found there.
This classification of stars is mainly based on the metal content in them after observing their spectra. And REMEMBER! In Astrophysics, we refer to every element that is heavier than Hydrogen and Helium as a metal i.e. every element in the periodic table except Hydrogen and Helium is a metal. That is how we have Population 1 type of star which is the most metal-rich star population with Z=0.02 (like the Sun!) meaning it is made of 0.02 parts metals and 0.98 parts Hydrogen and Helium. With the same logic, there are Population 2 stars which are considered metal-poor stars with Z=0.001. And then there is the hypothetical star type called Population 3 which is supposed to have 0 metal content.
Metallicity index for different part
Now, let’s take a look at the metallicity index for different components of our Milky Way. In this case, the reference metal used is Iron. Therefore, the metallicity index is [Fe/H].
For the thin disk, the metallicity is between -0.5 and 0.3 but for the thick disk it is between -1.0 and -0.4. The meaning behind these numbers is very clear – the thin disk is far more metal enriched than the thick disk. It directly implies that the stars in the thin disk are younger than the stars in the thick disk. This implication is of course due to the assumptions we made to get the Age-Metallicity relation.
Now that the facts are out of the way, we need to understand the reason(s) behind this large difference in metallicity. And how they tie up to the evolution of Milky Way.
Why is there a difference in metallicity?
There are a few ideas that we must explore and determine how probable each one is.
Idea 1:
The process of star formation began in the thick disk earlier and hence ceased earlier.
Do you think this idea makes sense? Honestly, it already falls short! Let me explain.
We know that stars are formed out of molecular clouds. This means that in the early days of the galaxy, the molecular cloud should have been diffuse enough that star formation began in the thick disk. Rather, they should have been denser in the thick disk for the formation to begin earlier there. Now the problem is that this molecular gas is concentrated in our galactic plane i.e. in the thin disk. So why would it have been any different during the early days? Why would the gas cloud be more concentrated in the thick disk during the galaxy's early days? That just doesn’t make sense!
So, we will move on to the next idea.
Idea 2:
The stars from the thin disk move onto the thick disk by the time they get old.
How does this be even possible?! The answer lies in the multiple bodies surrounding each star. These objects which can be other stars, gas clouds, etc. exert their own gravitational force on the star. Remember, there is already a gravitational force binding the star to the galactic center and keeping it attached to the disk. So the gravitational force from the other objects adds to this already existing gravitational force.
This additional force is seen in the form of fluctuations in the motion of the star. This fluctuation is the additional velocity component perpendicular to the disk that a star develops. This can explain how the stars in the thin disk slowly, with time, end up in the thick disk. But again, this reason is also not foolproof.
Idea 3:
The stars in the thick disk are alien to our galaxy!
The simplest explanation to this idea is that the stars in the thick disk might have come from other satellite galaxies when they merged with the Milky Way. What it means simply is that when our Milky Way was eating other smaller galaxies, their stars added onto what we now know as the thick disk.
But what proof do we have to support this possibility? For that, we must take a look at a very important factor – the rotational velocity of the thick disk. It doesn’t match with the thin disk and is rather ~50 km/h smaller than the thin disk! This is observed in not just our galaxy! Many spiral galaxies have stronger discrepancies and in one instance, the thick disk is rotating opposite the thin disk! The only reason such observations make sense is when the thick disk wasn’t originally part of the galaxy. In that case, the assumption that the thick disk might be a result of our galaxy’s cannibalism seems fair!
So, we have a few theories about why there is such a metallicity gap between the thin and thick disk of Milky Way. And out of them, the idea that the stars in the thick disk aren't native to our galaxy makes most sense. But these are still theories. There can be more of these to prove and disprove. Do you have a theory for this?
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