ESA: Gaia Uncovers Major Event in Milky Way Formation

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ESA: Gaia Uncovers Major Event in Milky Way Formation

Post by bystander » Fri Nov 02, 2018 5:57 pm

Galactic Ghosts: Gaia Uncovers Major Event in the Formation of the Milky Way Galaxy
ESA | Space Science | Science & Technology | Gaia | 2018 Oct 31
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ESA’s Gaia mission has made a major breakthrough in unravelling the formation history of the Milky Way.

Instead of forming alone, our Galaxy merged with another large galaxy early in its life, around 10 billion years ago. The evidence is littered across the sky all around us, but it has taken Gaia and its extraordinary precision to show us what has been hiding in plain sight all along.

Gaia measures the position, movement and brightness of stars to unprecedented levels of accuracy.

Using the first 22 months of observations, a team of astronomers led by Amina Helmi, University of Groningen, The Netherlands, looked at seven million stars – those for which the full 3D positions and velocities are available – and found that some 30,000 of them were part of an ‘odd collection’ moving through the Milky Way. The observed stars in particular are currently passing by our solar neighbourhood.

We are so deeply embedded in this collection that its stars surround us almost completely, and so can be seen across most of the sky.

Even though they are interspersed with other stars, the stars in the collection stood out in the Gaia data because they all move along elongated trajectories in the opposite direction to the majority of the Galaxy’s other hundred billion stars, including the Sun.

They also stood out in the so-called Hertzprung-Russell diagram – which is used to compare the colour and brightness of stars – indicating that they belong to a clearly distinct stellar population. ...

The merger that led to the formation of the Milky Way's inner stellar halo and thick disk ~ Amina Helmi et al
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Re: ESA: Gaia Uncovers Major Event in Milky Way Formation

Post by MarkBour » Sat Nov 03, 2018 3:30 am

bystander wrote:
Fri Nov 02, 2018 5:57 pm
Galactic Ghosts: Gaia Uncovers Major Event in the Formation of the Milky Way Galaxy
Pretty cool! It would have been wild to find out that our Sun was one of the rare population. So we're surrounded by a group of immigrants, but not enough to be a large fraction of the neighborhood (I guess about 0.4% overall, perhaps a higher concentration near us).

It's also interesting to consider that we're in the middle of the splatter of an old galactic merger, but it is actually very hard to detect its effects and occurrence at all.
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Re: ESA: Gaia Uncovers Major Event in Milky Way Formation

Post by Ann » Sat Nov 03, 2018 5:00 am

MarkBour wrote:
Sat Nov 03, 2018 3:30 am
bystander wrote:
Fri Nov 02, 2018 5:57 pm
Galactic Ghosts: Gaia Uncovers Major Event in the Formation of the Milky Way Galaxy
Pretty cool! It would have been wild to find out that our Sun was one of the rare population. So we're surrounded by a group of immigrants, but not enough to be a large fraction of the neighborhood (I guess about 0.4% overall, perhaps a higher concentration near us).

It's also interesting to consider that we're in the middle of the splatter of an old galactic merger, but it is actually very hard to detect its effects and occurrence at all.
ESA wrote:
Instead of forming alone, our Galaxy merged with another large galaxy early in its life, around 10 billion years ago.
If the Sun had been a member of the galaxy that the young Milky Way collided with, or if it formed from the gas that the other galaxy probably brought along with it, the Sun would likely have been a lot more metal-poor than our own Sun is. If the Sun had been metal-poor, its planets would probably have formed differently, and - well, chances are that we would not be here. In any case, if our Sun had been born some 10 billion years ago, it would probably start turning into a red giant about now (although it is possible that metal-poor stars have longer main sequence life spans than metal-rich ones).

But like you said, this is a cool discovery indeed! :D I listened to a speech about Gaia about a month ago by a Swedish professor who has been one of the great movers and shakers behind Gaia, Lennart Lindegren. Both Lindegren and his speech were cool, too!

Got to love Gaia! :D

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Re: ESA: Gaia Uncovers Major Event in Milky Way Formation

Post by MarkBour » Tue Nov 06, 2018 4:31 pm

Ann wrote:
Sat Nov 03, 2018 5:00 am
... merged ... around 10 billion years ago.
Good point to remind me of, Ann. So our Sun could never have been part of the minor galaxy. This is a flaw in my mental picture of galactic mergers, so it helps to try to correct it. If a galactic merger takes a long enough time (or if one watches it for long enough after the initial dance), I shouldn't just imagine the movements of stars like specks of sand in a swirling volume of matter and dark matter. Many stars are born in such a process, of course. And, over the time scale of tracking this population, then, many stars will have died by now.

I wonder how well the given simulation accounts for star lifetimes in the model. Or, I wonder how many of the 30,000 stars in the population are over 10 billion years old.

A really interesting possibility from this, then, is that this population is giving evidence of a merger which may have been the impetus for the birth of our Sun? Our Sun may well have been born thanks to some compression forces on its natal cloud that this merger produced?

I am sure I would have loved hearing the speech by Professor Lindegren. I wonder if my last conjecture was something he discussed as well.
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Re: ESA: Gaia Uncovers Major Event in Milky Way Formation

Post by Ann » Tue Nov 06, 2018 5:01 pm

Interesting, Mark! I'll try to give you some personal musings on galactic mergers and star formation tomorrow or the day after tomorrow, because unfortunately I have no time now.

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Re: ESA: Gaia Uncovers Major Event in Milky Way Formation

Post by Ann » Tue Nov 06, 2018 11:54 pm

MarkBour wrote:
Tue Nov 06, 2018 4:31 pm
Ann wrote:
Sat Nov 03, 2018 5:00 am
... merged ... around 10 billion years ago.
Good point to remind me of, Ann. So our Sun could never have been part of the minor galaxy. This is a flaw in my mental picture of galactic mergers, so it helps to try to correct it. If a galactic merger takes a long enough time (or if one watches it for long enough after the initial dance), I shouldn't just imagine the movements of stars like specks of sand in a swirling volume of matter and dark matter. Many stars are born in such a process, of course. And, over the time scale of tracking this population, then, many stars will have died by now.

I wonder how well the given simulation accounts for star lifetimes in the model. Or, I wonder how many of the 30,000 stars in the population are over 10 billion years old.

A really interesting possibility from this, then, is that this population is giving evidence of a merger which may have been the impetus for the birth of our Sun? Our Sun may well have been born thanks to some compression forces on its natal cloud that this merger produced?

I am sure I would have loved hearing the speech by Professor Lindegren. I wonder if my last conjecture was something he discussed as well.
Professor Lindegren didn't discuss mergers with other galaxies.

But like you said, our own Sun is too young to have been part of the interloper galaxy. But maybe possibly maybe its birth had something to do with the tidal forces that might just possibly still have been coursing through our galaxy as a result of that early merger.

In his book, The Local Group of Galaxies, Sidney van den Bergh has included a chart that argues that the Large Magellanic Cloud experienced very low levels of star formation until about three billion years ago. The level of star formation increased by several magnitudes some two billion years ago and reached a peak about one billion years ago. But the LMC still forms stars at a vigorous pace today, some three billion years after the onset of increased star formation.

The reason why the LMC suddenly started many more stars is probably because it started interacting with a smaller galaxy, the Small Magellanic Cloud.

So if the LMC has kept churning out new stars at a heightened pace for three billion years, probably as a result of interactions with the Small Magellanic Cloud, it does seem possible that a collision some ten billion years between a smallish gas-rich proto-Milky Way and an even smaller gas-rich galaxy would have produced "ripples in the pond" of the gas in our galaxy, causing it to keep forming stars at an increased pace for billions of years. And maybe even our Sun was born out of a gas cloud that had been compressed due to the "last ringings of the bell after it was hit by a galaxy-sized clapper".

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Re: ESA: Gaia Uncovers Major Event in Milky Way Formation

Post by neufer » Fri Nov 09, 2018 7:31 pm

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