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Dave Lee
2018-Mar-12, 12:27 PM
In the following article it is stated:

https://newatlas.com/milky-way-galactic-eviction-halo-stars/53614/

"It had previously been thought that the halo was comprised of stars stolen from smaller dwarf galaxies as they passed close to, or even merged with the Milky Way. However, the recently published study supports a different theory, which asserts that some of the stars that make up the halo were in fact created in the outer regions of our galaxy, and subsequently cast out."

In one hand we consider that the Milky Way gets its mass by "eating" dwarf galaxies, but on the other hand, based on updated verification, it was found that the milky way is actually ejecting stars.

So, could it be that most/all of the stars in distant halos had been ejected from the Milky way?

tusenfem
2018-Mar-12, 12:51 PM
Here is the original paper: https://www.nature.com/articles/nature25490

Apparently, for 14 stars they measured the composition was like the outer reaches of the galactic disk.
So it would be a giant leap to assume most/all stars in the halo are ejected from the MW.

Dave Lee
2018-Mar-12, 04:34 PM
Thanks

In the articale it is staed:
"The new research focussed on two relatively dense structures of stars orbiting in the Galactic halo, known as Triangulum-Andromeda (Tri-And) and A-13. The stellar groupings lie either side of the disk, positioned roughly 14,000 light-years above and below the Milky Way's galactic plane"

Why only two?
Do we know if they have verified also other dense structures of stars orbiting in the Galactic halo?

Dave Lee
2018-Mar-12, 05:05 PM
Just a simple statistical calculation.

Let's assume that for every "Milky Way" dense structures of stars (which had been ejected from the galaxy), there are 1000 dense structures that had been swallowed from outside.
In this case, do you agree that the chance to select randomly those two "Milky way" dense structures is one to one million?

Strange
2018-Mar-12, 05:12 PM
Just a simple statistical calculation.

Let's assume that for every "Milky Way" dense structures of stars (which had been ejected from the galaxy), there are 1000 dense structures that had been swallowed from outside.
In this case, do you agree that the chance to select randomly those two "Milky way" dense structures is one to one million?

Hilarious. Why do you think you can reach a meaningful conclusion based on made up numbers?

Dave Lee
2018-Mar-12, 05:24 PM
Hilarious. Why do you think you can reach a meaningful conclusion based on made up numbers?

So what is the estimated ratio between the "Milky Way" Dense structures to the "outsider" Dense structures?

kzb
2018-Mar-12, 05:49 PM
I don't like how the article calls halo stars "evictees". They are still gravitationally bound to the MW after all.

Yes there are a small number of hypervelocity stars and intergalactic stars, which you could fairly describe as evictees. But halo stars are not. I think what they mean is evictees from the disk.

The illustration also shows what I posted in the last thread, i.e the warped and flared outer disk.

Reality Check
2018-Mar-12, 07:31 PM
The letter makes it clear what is going on: Two chemically similar stellar overdensities on opposite sides of the plane of the Galactic disk (https://www.nature.com/articles/nature25490)

Here we report a spectroscopic analysis of 14 stars from two stellar overdensities, each lying about five kiloparsecs above or below the Galactic plane—locations suggestive of an association with the stellar halo. We find that the chemical compositions of these two groups of stars are almost identical, both within and between these overdensities, and closely match the abundance patterns of stars in the Galactic disk. We conclude that these stars came from the disk, and that the overdensities that they are part of were created by tidal interactions of the disk with passing or merging dwarf galaxies11.
The authors are researching the effects on the Milky Way of interactions between the Milky Way and dwarf galaxies. They find that 14 stars split between 2 overdensities below and above the Galactic plane have similar chemical compositions with each other and disk stars. This suggests that these stars were pulled from the disk via tidal interactions. Ditto for these two overdensities.

The halo stars are part of the MW, so no stars have been ejected from the MW. These overdensities are part of the MW.

kzb
2018-Mar-13, 12:26 PM
The letter makes it clear what is going on: Two chemically similar stellar overdensities on opposite sides of the plane of the Galactic disk (https://www.nature.com/articles/nature25490)

The authors are researching the effects on the Milky Way of interactions between the Milky Way and dwarf galaxies. They find that 14 stars split between 2 overdensities below and above the Galactic plane have similar chemical compositions with each other and disk stars. This suggests that these stars were pulled from the disk via tidal interactions. Ditto for these two overdensities.

The halo stars are part of the MW, so no stars have been ejected from the MW. These overdensities are part of the MW.

So I guess the debate has moved on somewhat, as this new work supports the Milky Way origin theory of these stars and not merging dwarf galaxies. (No mention of Canis Major which was the main focus of the previous debate however.)

But this article introduces a new idea. Yes these overdensities are part of the milky way, but the reason the outer disk is not flat is because of dwarf galaxies passing through it.

So do we accept that dwarf galaxies could pass through the disk and not leave behind any of their stars? This is the next question.

Dave Lee
2018-Mar-13, 01:24 PM
The letter makes it clear what is going on: Two chemically similar stellar overdensities on opposite sides of the plane of the Galactic disk (https://www.nature.com/articles/nature25490)

The authors are researching the effects on the Milky Way of interactions between the Milky Way and dwarf galaxies. They find that 14 stars split between 2 overdensities below and above the Galactic plane have similar chemical compositions with each other and disk stars. This suggests that these stars were pulled from the disk via tidal interactions. Ditto for these two overdensities.

The halo stars are part of the MW, so no stars have been ejected from the MW. These overdensities are part of the MW.


Thanks

Great article!.

It is stated:

"We find that the chemical compositions of these two groups of stars are almost identical, both within and between these overdensities, and closely match the abundance patterns of stars in the Galactic disk."

Can you please elaborate about those chemical compositions?
If it match with the abundance patterns of stars in the Galactic disk, does it mean that it also match with the Sun pattern and actually with all stars pattern in the galactic disc?
So, is it some kind of star "DNA"? Each galaxy carries its unique star DMA?

However, if all the stars in the galactic disc carry this kind of "DNA", does it mean that all of them had been created in the Milky Way disc?

Strange
2018-Mar-13, 02:05 PM
You can find the full paper here: https://arxiv.org/abs/1803.00563

I had a quick look and it seems they are comparing the relative amounts of various elements (for example the ratio of iron to hydrogen, or oxygen to iron, etc) and compare these to the stars in some dwarf spheroidal galaxies. The ratio for stars in the milky way are all around the same, whereas the ratios in dwarf galaxies vary over a much greater range. See Figure 2 on page 10, where you can see the how the stars in these over densities match the rest of the galaxy, while stars in dwarf galaxies are distinctly different.

Dave Lee
2018-Mar-14, 04:20 AM
You can find the full paper here: https://arxiv.org/abs/1803.00563

I had a quick look and it seems they are comparing the relative amounts of various elements (for example the ratio of iron to hydrogen, or oxygen to iron, etc) and compare these to the stars in some dwarf spheroidal galaxies. The ratio for stars in the milky way are all around the same, whereas the ratios in dwarf galaxies vary over a much greater range. See Figure 2 on page 10, where you can see the how the stars in these over densities match the rest of the galaxy, while stars in dwarf galaxies are distinctly different.

Thanks

Based on Figure 2 do you agree that?
1. "The ratio for stars in the milky way are all around the same" means that all of them share the same "DNA". Therefore, all the stars in the Milky Way disc have been formed in the disc?
2. "stars in dwarf galaxies are distinctly different" means that they have different "DNA". Therefore, the idea that dwarf galaxies merge with the Milky Way is not feasible?

AGN Fuel
2018-Mar-14, 04:38 AM
You can find the full paper here: https://arxiv.org/abs/1803.00563

Interesting paper, Strange. Thanks for the link. :)

AGN Fuel
2018-Mar-14, 04:57 AM
Thanks

Based on Figure 2 do you agree that?
1. "The ratio for stars in the milky way are all around the same" means that all of them share the same "DNA". Therefore, all the stars in the Milky Way disc have been formed in the disc?

No.


2. "stars in dwarf galaxies are distinctly different" means that they have different "DNA". Therefore, the idea that dwarf galaxies merge with the Milky Way is not feasible?

No.

What I would say is that figure 2 provides a method to support an argument for the probable origin of the 14 stars examined.

Dave Lee
2018-Mar-14, 05:41 AM
No.
No.
What I would say is that figure 2 provides a method to support an argument for the probable origin of the 14 stars examined.

Why No?
Do you agree that: ""The ratio for stars in the milky way are all around the same" ?
Do you also agree that: "stars in dwarf galaxies are distinctly different"?

Do you agree that: If there was a merger between dwarf galaxies and the disc, it is expected to verify their distinctly different ratio also in the disc?
So, why we can't verify that distinctly different ratio also in the disc?

Shaula
2018-Mar-14, 12:19 PM
Why No?
Do you agree that: ""The ratio for stars in the milky way are all around the same" ?
Not even close to true. There are several different populations with different abundances. Which is how, for example, they rule out globular clusters as an origin.


Do you also agree that: "stars in dwarf galaxies are distinctly different"?
That is correct.


Do you agree that: If there was a merger between dwarf galaxies and the disc, it is expected to verify their distinctly different ratio also in the disc?
So, why we can't verify that distinctly different ratio also in the disc?
Dwarf galaxies typically have a few hundred million stars. Our galaxy has more than 200 billion. If enough effort was put into a detailed analysis of stellar spectra we might well find a very few candidates for being from a merger. But it is a needle in a haystack problem

kzb
2018-Mar-14, 01:46 PM
They are attempting to explain how these stars ended up where they did. They have come up with an idea which seems to explain it, that is, dwarf galaxies merging with the MW in the past.

The paper clearly identifies several sub-populations of stars. Dwarf galaxy stars seem chemically distinct from MW disk stars and all but one the stars studied map onto the MW disk star position on the composition plot.

Apart from this one star, which may be an outlier in any case, there is no direct evidence of dwarf galaxies being involved. They are presenting a credible theory which may or may not stand the test of time. That is all.

Dave Lee
2018-Mar-14, 01:47 PM
Not even close to true. There are several different populations with different abundances. Which is how, for example, they rule out globular clusters as an origin.


Do you mean that there are several different populations with different abundances in the Milky Way disc?
Please see again the message from Strange:


You can find the full paper here: https://arxiv.org/abs/1803.00563
The ratio for stars in the milky way are all around the same, whereas the ratios in dwarf galaxies vary over a much greater range. See Figure 2 on page 10, where you can see the how the stars in these over densities match the rest of the galaxy, while stars in dwarf galaxies are distinctly different.



Dwarf galaxies typically have a few hundred million stars. Our galaxy has more than 200 billion. If enough effort was put into a detailed analysis of stellar spectra we might well find a very few candidates for being from a merger. But it is a needle in a haystack problem

Well, the whole idea is that the Milky Way galaxy had increased its size due to the merger with Dwarf galaxies.

They are attempting to explain how these stars ended up where they did. They have come up with an idea which seems to explain it, that is, dwarf galaxies merging with the MW in the past.

Therefore, it is not just one or two or even 50 dwarf galaxies which had been merged with the Milky Way disc.
In order to set the real impact and increase the size of the Milky Way as it is today, hundreds or even thousands of Dwarf galaxies are needed.
Therefore, we had to see their distinctly different ratio everywhere.
If an effort is needed to find them, then how could they have any real impact on the size of the Milky Way galaxy?
You claim that "we might find". but on the other hand - we might not find...
So, how can we get any conclusion on something that we don't see?

Shaula
2018-Mar-14, 02:01 PM
Do you mean that there are several different populations with different abundances in the Milky Way disc?
I know what Strange said. I also know that you are either being sloppy with your terminology or you are reading more into what is said. You keep talking about a galactic DNA and dropping the word disc. Even within the disc there is variation in composition. As the paper says the composition of the stars is more similar to disc stars than dwarf galaxy stars.


Well, the whole idea is that the Milky Way galaxy had increased its size due to the merger with Dwarf galaxies.
Therefore, it is not just one or two or even 50 dwarf galaxies which had been merged with the Milky Way disc.
In order to set the real impact and increase the size of the Milky Way as it is today, hundreds or even thousands of Dwarf galaxies are needed.
Therefore, we had to see their distinctly different ratio everywhere.
If an effort is needed to find them, then how could they have any real impact on the size of the Milky Way galaxy?
You claim that "we might find". but on the other hand - we might not find...
So, how can we get any conclusion on something that we don't see?
It won't be everywhere because the signature doesn't last. Dwarf galaxies tend to be metal poor. Over time the stars die, their material is recycled and changed. The whole situation is vastly more complex than the sketch you seem to be working from where these stars last forever, chemically unchanged..

What I said was that we might be able to find a star we could point at as being a remnant from a merger. Because that seems to be what you want to see. Or instead of looking for an outlier that might actually just be an anomaly we could look at the bigger picture and consider indirect lines of evidence.

Dave Lee
2018-Mar-14, 03:03 PM
Thanks


It won't be everywhere because the signature doesn't last. Dwarf galaxies tend to be metal poor. Over time the stars die, their material is recycled and changed. The whole situation is vastly more complex than the sketch you seem to be working from where these stars last forever, chemically unchanged..

This is very interesting answer.

So, you claim that many dwarf galaxies have merged with the Milky Way in the past.
However, over time the stars die, their material is recycled and changed.
Therefore, we can't see today anymore their "old" distinctly different ratio.

However, in order to achieve it, we must kill all of those stars from the dwarf galaxies and then recycled their matter. What is the chance for that?

Never the less, do you agree that all/most of the stars in the milky way disc, have been formed in the disc (from any sort of matter...)?

Strange
2018-Mar-14, 04:56 PM
Do you mean that there are several different populations with different abundances in the Milky Way disc?
Please see again the message from Strange:
[QUOTE=Strange;2442988The ratio for stars in the milky way are all around the same[/QUOTE]

My wording wasn't very good there. They are not "all the same". As you can see, there is quite a wide range of compositions for the stars in the disk. The important point is that other sources of stars (dwarf and globular clusters) have a noticeably different distribution of ratios. The stars under consideration fit within the disk range better than in the other ranges.

Reality Check
2018-Mar-14, 11:25 PM
Can you please elaborate about those chemical compositions?
Stars have different chemicals in them, probably reflecting where they formed. Other than that you will have to do your own research.
Obviously "closely match the abundance patterns of stars in the Galactic disk" means closely matching the chemical abundance of stars in the Galactic disc. For more details, the letter has a preprint that you can read. (https://arxiv.org/abs/1803.00563)

We compare these abundance ratios with literature measurements of stars from the Galactic disk and halo, dwarf spheroidal (dSph) galaxies, and globular cluster...
It is a letter with no explicit reference to those measurements. You will have to look up the Galactic disk literature measurements for yourself. However Figure 2 is quite clear - the Na/H and Ba/Ge versus Fe/H abundances of the Galactic disk and halo (grey circles) just about overlaps the black crosses and circles of their 14 stars, other abundances do not.

AGN Fuel
2018-Mar-15, 12:39 AM
Why No?

Because your conclusions are a hasty generalisation fallacy.


Do you agree that: ""The ratio for stars in the milky way [disc] are all around the same" ?

As a very broad generalisation, yes. Meaning that if you were to randomly select a star from the disc, you would expect to see a chemical composition in that star within a range of ratios. Does that mean that every one of the 200 billion or so stars currently in the disc will have that range of abundances? No.

However, your conclusion expects that they will.


Do you also agree that: "stars in dwarf galaxies are distinctly different"?. Yes, in general you can say that with more confidence.


Do you agree that: If there was a merger between dwarf galaxies and the disc, it is expected to verify their distinctly different ratio also in the disc?

If the merger was recent and relatively 'nearby', then yes, you would expect to see areas of over abundance of metal-poor stars. This is an active area of research, as evidenced by the papers in this area you have been provided across several threads.


So, why we can't verify that distinctly different ratio also in the disc?

i think you underestimate the complexity of a dynamic and evolving system. You have a tendency to take simple diagrams & tables that are designed by help broadly explain an aspect of an issue and use them far beyond their purpose.

Here's a rough analogy to the problem - imagine you have a multi-nozzle overhead garden sprinkler system in your yard that will turn on one nozzle randomly for 10 minutes a day, and which uses a fluoridated water source. One day you have a heavy rain storm, but you also need to know if the sprinkler is on at that time.
You can tell if a drop has come from the sprinkler because of its velocity (speed and direction) and it's slightly different chemical composition (due to the fluoride). But the challenge is finding the right drops to analyse when you don't know if the sprinkler is even on....

Shaula
2018-Mar-15, 01:53 AM
However, in order to achieve it, we must kill all of those stars from the dwarf galaxies and then recycled their matter. What is the chance for that?
If you are asking what the odds of a star dying given enough time are ... about 100%.


Never the less, do you agree that all/most of the stars in the milky way disc, have been formed in the disc (from any sort of matter...)?
Most of the current population of the disc formed there, yes. The galactic merger hypothesis does not claim large fractions of stars came to be in the disc thanks to a merger. The hypothesis is that galaxies can gain mass via mergers, a process which leaves far more subtle signatures than you appear to think. That mass is added to the general mass of our galaxy and forms part of the cycle of star birth and death.

So the short form, and why does this seem familiar, is that you are arguing against your own interpretation of a mainstream idea using an over-simplified picture of what is happening and fixating on pieces of evidence you believe should be available because you don't really understand the observations and processing involved.

kzb
2018-Mar-15, 12:02 PM
I recall reading somewhere that a maximum of 4% of Milky Way stars have come from mergers.

Also, the interesting thing about the study under discussion is that it clearly identifies TriAnd as being part of the MW. Some previous studies have said it is a dwarf galaxy in the act of merging. But now we know this interpretation is improbable.

There's no mention of Canis Major "dwarf galaxy" in this study, but maybe that interpretation will go the same way. Time will tell.

Dave Lee
2018-Mar-15, 04:19 PM
I recall reading somewhere that a maximum of 4% of Milky Way stars have come from mergers.
Also, the interesting thing about the study under discussion is that it clearly identifies TriAnd as being part of the MW. Some previous studies have said it is a dwarf galaxy in the act of merging. But now we know this interpretation is improbable.
There's no mention of Canis Major "dwarf galaxy" in this study, but maybe that interpretation will go the same way. Time will tell.

Thanks

So, we have a clear evidence that the two over densities stellar: TriAnd and A-13, have beed identified as part of the MW.
Therefore, those over densities stellar have been ejected from the Milky way disc.

The question is quite simple:
Why our MW galaxy would In one hand ejects all the stars in those two over densities stellar, while on the other hand it merges with stars from Dwarf galaxies?
What is the benefit for our MW galaxy to do so?
Why the MW galaxy "eat" stars from dwarf galaxies, wait long enough to convert their chemically signature and then eject them?
There must be some logic in the Universe activity. What is the logic in this activity?
Why do we insist that the MW merges with Dwarf galaxies? Do we have any real evidence for that?



If the merger was recent and relatively 'nearby', then yes, you would expect to see areas of over abundance of metal-poor stars. This is an active area of research, as evidenced by the papers in this area you have bee.

What do you mean by "nearby"?
What is the expected average life of a star?
There are about 400 Billion stars in the Milky Way galaxy.
4% means - 16 Billion stars.
Some of them had been merged long time ago (over than 10BY) while others might merged just few year ago.
But in average, it is expected to find some significant numbers.
Why we don't see any? Could it be that even if we look long enough we still won't find any?



Here's a rough analogy to the problem - imagine you have a multi-nozzle overhead garden sprinkler system in your yard that will turn on one nozzle randomly for 10 minutes a day, and which uses a fluoridated water source. One day you have a heavy rain storm, but you also need to know if the sprinkler is on at that time.
You can tell if a drop has come from the sprinkler because of its velocity (speed and direction) and it's slightly different chemical composition (due to the fluoride). But the challenge is finding the right drops to analyse when you don't know if the sprinkler is even on....

I'm not sure that I fully understand your analogy.
Do you mean that we have to consider the Milky Way galaxy as some sort of a sprinkler?
However, instead of ejecting water it ejects stars and over densities stellar?
You can tell if star has come from the sprinkler because of its velocity (speed and direction) and it's slightly different chemical composition?

Never the less, if I understand it correctly, sprinkles work in one way. They only eject. There is no way to get water from outside.
So could it be that as those over densities stellar had been ejected from the Milky way sprinkler, no dwarf galaxy can merge in?

Shaula
2018-Mar-15, 05:04 PM
The question is quite simple:
Why our MW galaxy would In one hand ejects all the stars in those two over densities stellar, while on the other hand it merges with stars from Dwarf galaxies?
What is the benefit for our MW galaxy to do so?
Why the MW galaxy "eat" stars from dwarf galaxies, wait long enough to convert their chemically signature and then eject them?
There must be some logic in the Universe activity. What is the logic in this activity?
There need be no 'benefit' or 'logic' in what happens because this is not an example of a conscious decision making process. Things just happen, there needn't be some narrative about it. Galaxies can interact and may merge. When they do their material is pooled. While this going on and completely independently gravitational interactions can cause orbits to change, sometimes dramatically.


Why do we insist that the MW merges with Dwarf galaxies? Do we have any real evidence for that?
We see plenty of other galaxies very similar to ours doing it. We see disruptions likely associated with these events. Perhaps a better question is what do you think makes our galaxy so unique and special that it doesn't do what other galaxies are seen to do?


What is the expected average life of a star?
This is about where it looks like you stop asking questions to learn and start trying to badger. There are plenty of resources out there that address this question. Couldn't you spare 30s to look this up yourself?


Why we don't see any?
Already answered.


Never the less, if I understand it correctly, sprinkles work in one way. They only eject. There is no way to get water from outside.
So could it be that as those over densities stellar had been ejected from the Milky way sprinkler, no dwarf galaxy can merge in?
Umm, so you believe that garden sprinklers are eternal sources of water that need no external inputs? Might want to try some practical experiments here. I pretty much guarantee that unless you connect them to something that does supply water from outside they won't work.

Dave Lee
2018-Mar-15, 10:42 PM
We see plenty of other galaxies very similar to ours doing it. We see disruptions likely associated with these events. Perhaps a better question is what do you think makes our galaxy so unique and special that it doesn't do what other galaxies are seen to do?

I'm not sure that I fully understand your question. How do we know that other galaxies merge with each other? Do you mean colliding galaxies?



Perhaps a better question is what do you think makes our galaxy so unique and special that it doesn't do what other galaxies are seen to do?
Well, I do believe that our galaxy is unique and there must be a wisdom in the nature of this galaxy.
Our galaxy is spiral galaxy (2D - Disc). It should do whatever other spiral galaxy does. But it is totally different from Irregular galaxy (3D).
For example - I have seen many rotation curve of a spiral galaxies, but I have never seen a rotation curve for Irregular galaxy (3D).
Therefore, the question is: Does irregular galaxies also have a rotation curve problem?

As for the wisdom of the nature, let me use the following example:
Lets assume that we can see a cow from a distance of one million Km. We can see that it eats some kind of a matter and it also ejects some kind of a matter. The question is if both matters are the same. In a wisdom nature, I wouldn't expect that it should be the same matter.


Umm, so you believe that garden sprinklers are eternal sources of water that need no external inputs? Might want to try some practical experiments here. I pretty much guarantee that unless you connect them to something that does supply water from outside they won't work.

Yes, this is the most important question. If spiral galaxy ejects stars like a sprinkler, then from where its supply source is coming? How it gets its food?

Swift
2018-Mar-15, 10:47 PM
<snip>
Well, I do believe that our galaxy is unique and there must be a wisdom in the nature of this galaxy.

Dave Lee

You are either using the word "wisdom" in a very different way, or you are advocating a metaphysical or religious viewpoint. If you continue you will be infracted.

Shaula
2018-Mar-15, 11:12 PM
I'm not sure that I fully understand your question. How do we know that other galaxies merge with each other? Do you mean colliding galaxies?
Colliding, tidally disrupted, infalling... we see pretty much all of the stages and aftermath of mergers. We see galaxies with companions, interacting galaxies, we see the intermediate stages, we see disrupted end stages. So unless you can provide a more plausible explantation for the hundreds of example out there then we see them.


Our galaxy is spiral galaxy (2D - Disc). It should do whatever other spiral galaxy does.
So you must agree it can merge with other galaxies. As we see other galaxies doing.


But it is totally different from Irregular galaxy (3D).
For example - I have seen many rotation curve of a spiral galaxies, but I have never seen a rotation curve for Irregular galaxy (3D).
Therefore, the question is: Does irregular galaxies also have a rotation curve problem?
Yes. And if you want to see rotation curves of irregular galaxies it is not hard. Google "irregular galaxy rotation curve". If you have never seen them it is because you have never looked.


Yes, this is the most important question. If spiral galaxy ejects stars like a sprinkler, then from where its supply source is coming? How it gets its food?
Ejected stars come from the vast number there are in our galaxy. Increases in mass come from mergers. You don't need many mergers to keep a galaxy going, ejections are rare. Mergers are more important in explaining how galaxies grow in the first place.

AGN Fuel
2018-Mar-16, 06:21 AM
What do you mean by "nearby"?

Such that the light from the stars on which you are doing a spectroscopic analysis is not obscured by dust or other intervening stars. The problems is that we are in the disc and you are looking for metal-poor stars in the disc. It's a bit like standing in a very large forest and trying to see a particular species of tree. Much easier if the tree is closer.


I'm not sure that I fully understand your analogy.
Do you mean that we have to consider the Milky Way galaxy as some sort of a sprinkler?
However, instead of ejecting water it ejects stars and over densities stellar?
Never the less, if I understand it correctly, sprinkles work in one way. They only eject. There is no way to get water from outside.

As charitable as I can be, this is so absurdly far from the intent of the analogy that I am struggling to believe that you are not being deliberately obtuse. No, that is NOT what I mean.

The (rough) analogy was to highlight the complexity of something you seem to expect is simple. It is comparing the problem of identifying scattered stars in the galactic disc as being possibly from a previous merger with a dwarf Galaxy based on their low metallicity, to the problem of identifying what drops (in mid-air) that might have come from a garden sprinkler whilst in the middle of a rain storm, based on the amount of fluoride in the drops.

Dave Lee
2018-Mar-16, 09:49 AM
Thanks for the suport.

kzb
2018-Mar-16, 03:53 PM
The MW stellar population is still growing, albeit at a slower pace than earlier in its existence.

There is gas in the disk already, and gas is falling into the disk from both sides.

The gas disk extends further out in radius than the main stellar disk, I've seen 30kpc radius. There are star forming regions observed at 19-20kpc radius.

Irrespective of any mergers the MW has not yet reached its peak stellar population. It does not actually require galaxy mergers to grow more stars.

Hardly any stars are ejected in the sense they exceed escape velocity. What is happening over time is the disk is getting thicker because of stellar interactions.

The merger with Andromeda will of course be a step change in this picture.