Stars in the halo of the Milky Way often travel in groups

January 31, 2017, Netherlands Research School for Astronomy
The Milky Way disk is embedded in a roundish halo of stars. The stars (in purple) are from a computer simulation of the remains from a merger with a small galaxy. The arrows indicate the motion of these stars that are now part of the halo. Larger arrows indicate faster motion. The astronomers suspect that tens to hundreds of such flows of stars are crisscrossing the Milky Way. Credit: Amina Helmi/Jovan Veljanoski/Maarten Breddels/University of Groningen

Many stars in the halo that surrounds the Milky Way travel in groups. This is the outcome of a recent analysis of data for millions of stars from the Gaia space mission. Astronomers report their discovery today in the international journal Astronomy & Astrophysics.

The Milky Way, our own Galaxy, has likely formed in part from the merging of many smaller systems. How exactly that happened, is still a puzzle. To learn more about the history of formation of the Milky Way, from the University of Groningen in the Netherlands and UC Riverside in the US, have inspected the motions of stars in the so-called Galactic halo. Stars in the halo are more pristine and spend most of their time outside of the disk-like structure that gives the Milky Way its name. It is thought that these halo stars are the stars that joined the Milky Way onboard of .

For this study, a team led by Amina Helmi (University of Groningen) combined the vast Gaia dataset with data from the RAVE survey.

The researchers discovered that a large fraction of the halo stars travel in groups. Helmi: "This indicates that the stars indeed originate from small galaxies that were cannibalised by the Milky Way a very long time ago". The astronomers describe these groups as large flows of stars like flocks of birds traveling together through the Milky Way. "We believe there might be tens or even hundreds such flocks. At the moment, we only see small groups with just a few stars, but that is probably because we do not yet have all the necessary data".

Credit: Astonomie.nl

The team of astronomers were bewildered of the behaviour of halo stars that spend most of the time in the outskirts of the Milky Way. Surprisingly more than 70% of those stars appear to be moving in the opposite sense than the vast majority of stars in the Milky Way. Such a high fraction is unexpected in current models. Helmi: "One may compare stars from the outer halo with commuters that drive the wrong way. We do not yet quite understand why."

These discoveries were made using halo stars that, in their journey through the Milky Way, are by chance currently close to the Sun. In the future, Gaia will provide us with data from from all over the Milky Way. Helmi: "With such data we will get many new insights on how the Milky Way formed and be able to reconstruct its genealogy tree."

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More information: Amina Helmi et al. A box full of chocolates: The rich structure of the nearby stellar halo revealed byand RAVE, Astronomy & Astrophysics (2017). DOI: 10.1051/0004-6361/201629990

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Benni
1 / 5 (8) Jan 31, 2017
The Milky Way disk is embedded in a roundish halo of stars. The stars (in purple) are from a computer simulation of the remains from a merger with a small galaxy. The arrows indicate the motion of these stars that are now part of the halo


OK, let's try to figure out why the inferred gravitational effects of DM Cosmic Fairy Dust was left out of the computer modeling for this vast halo that is embedded exactly where DM Enthusiasts, like Zwicky, have always predicted we'd find DM.

If there ever existed halos of DM surrounding every Spiral galaxy, none of these other detectable halos of stars & dust that extend over half the distance from our galaxy to Andromeda could not exist because the parameters of the immense gravitational field DM would hypothetically create would prevent the formations of these Visible Matter halos. Nevertheless, there are those who cannot make it from day to day unless they have a fantasy beyond reality.....right RNP?
RNP
4.3 / 5 (6) Jan 31, 2017
@Benni
As usual you are posting utter rubbish. I bet you can not provide a single piece of evidence to support your ravngs. In fact, if you had read the paper, you would have seen that the model used in this paper is chosen to be consistent with the Milky Way rotation curve and therefore it's dark matter content.
Benni
1 / 5 (8) Jan 31, 2017
@Benni
As usual you are posting utter rubbish. I bet you can not provide a single piece of evidence to support your ravngs. In fact, if you had read the paper, you would have seen that the model used in this paper is chosen to be consistent with the Milky Way rotation curve and therefore it's dark matter content.


Way to go Zwicky acolyte, Oh, and by the way, your rubbish claim that DM Cosmic Fairy Dust was included in their model is totally false. If you think it isn't FALSE, then prove it is TRUE. Cmon there old crickey mate, I challenge you tp prove it.

Ok, if you think the Zwicky envelopes of DM exist & is accounted for, then you know the angle of inclination to the plane of the radial arms of a Spiral Galaxy, so what is it? You in all likelihood don't even know what I'm talking about do you? Ok, I'll help you out, is 0 or 90 degrees & how do you know?

RNP
4.5 / 5 (8) Jan 31, 2017
@Benni
... your rubbish claim that DM Cosmic Fairy Dust was included in their model is totally false.

I said: "the model used in this paper is chosen to be consistent with the Milky Way rotation curve ".

To quote the paper: "... the relevant parameters in these models are chosen to provide a reasonable fit to the rotation curve of the Milky Way."

This rotation curve is one of the principle examples of why something such as dark matter is needed. If you have a better way of describing the motion of the Sun and all the other stars in the Milky Way, then let's hear it. Whatever the case, you said the effect was left out when it is clear that it is BUILT INTO their model.

You then ask about the "the angle of inclination to the plane of the radial arms of a Spiral Galaxy", and suggest that I do not know what you are talking about. You're right, I don't! This is because your comment makes NO sense. Apart from anything else, spiral galaxies do not have RADIAL arms.
SlartiBartfast
1 / 5 (4) Jan 31, 2017
@Benni
As usual you are posting utter rubbish. I bet you can not provide a single piece of evidence to support your ravngs. In fact, if you had read the paper, you would have seen that the model used in this paper is chosen to be consistent with the Milky Way rotation curve and therefore it's dark matter content.


Don't feed the trolls.
NeilC
1.6 / 5 (7) Feb 01, 2017
"Surprisingly more than 70% of those stars appear to be moving in the opposite sense than the vast majority of stars in the Milky Way. Such a high fraction is unexpected in current models."

But this is exactly what would be predicted in a Birkeland current comprising counter-rotating sheaths.
Benni
1 / 5 (6) Feb 02, 2017
your comment makes NO sense. Apart from anything else, spiral galaxies do not have RADIAL arms.
........showing how little you know about Spiral Galaxy Classification. Radial Arms are found in 2/3 - 3/4 of all Spirals including the Milky Way, these are "barred galaxies" and many contain multiple bars.

It is believed by many astronomers that spiral arms are formed from radial arms called "bars", but of course you wouldn't know this because you know so little about Astronomy that your comprehension of such subject matter is beyond your Copy & Paste skills.

Rotation Curves? You don't even know what they are. You just read something about them for the first time in your life a few days ago but are unable to explain what they are.

Cricky mate, come back when you've actually learned something about Astronomy & DM Spectroscopy & cease it with the trolling.
jonesdave
4.3 / 5 (6) Feb 02, 2017
"Surprisingly more than 70% of those stars appear to be moving in the opposite sense than the vast majority of stars in the Milky Way. Such a high fraction is unexpected in current models."

But this is exactly what would be predicted in a Birkeland current comprising counter-rotating sheaths.


Really? Please point us to the paper that predicted this. And please show us the observational evidence that such currents have been detected. And that the models are consistent with the known morphology of various types of galaxies. You'll find that such evidence doesn't exist. And don't bother quoting the failed Peratt galaxy model. It is known as a 'failed model' for a reason. It failed.
RNP
4 / 5 (4) Feb 02, 2017
@Benni
....showing how little you know about Spiral Galaxy Classification. Radial Arms are found in 2/3 - 3/4 of all Spirals including the Milky Way, these are "barred galaxies" and many contain multiple bars.


You are inventing your own language again. I defy you to find any reference that calls bars "radial arms". In fact, they are generally highly eccentric, oblate spheroids and nothing like the things normally called "arms".

Trying to sound knowledgeable when you do not even understand the language you are using is a futile exercise.

Benni
1 / 5 (5) Feb 02, 2017
@Benni

.......showing how little you know about Spiral Galaxy Classification. Radial Arms are found in 2/3 - 3/4 of all Spirals including the Milky Way, these are "barred galaxies" and many contain multiple bars.


You are inventing your own language again. I defy you to find any reference that calls bars "radial arms". In fact, they are generally highly eccentric, oblate spheroids and nothing like the things normally called "arms".

Trying to sound knowledgeable when you do not even understand the language you are using is a futile exercise.
.........typical semantics of one troll mimicking another on this site. You know so little about Astronomy it's the reason you've never known that the BARS of BARRED SPIRAL GALAXIES are RADIAL ARMS.

You know Rguy, your fantasyland of the depraved depths of creating Social Media on a science website continually points out your greatest personal problem, just how incomprehensible science is to you.

IMP-9
3.7 / 5 (6) Feb 02, 2017
OK, let's try to figure out why the inferred gravitational effects of DM Cosmic Fairy Dust was left out of the computer modeling


From the paper:

"Because we are not concerned with a specific sense of rotation with respect to a major Galactic component, we may use the stellar halos that result from coupling a semi-analytic galaxy formation model to the Aquarius dark-matter-only simulations (Cooper et al. 2010), after applying a suitable tagging and resampling scheme to the dark-matter particles (Lowing et al. 2015)."

Don't accuse others of ignorance when the only thing you have demonstrated here is that you are a liar.
Benni
1 / 5 (5) Feb 02, 2017
Don't accuse others of ignorance when the only thing you have demonstrated here is that you are a liar.

]
...........coming from someone who believes infinite density of matter can exist inside a finite stellar mass. Impo, you're the same kind of social media troll Rguy is, you too know nothing about nuclear physics & the reasons for failure of DM Enthusiasts to come up with Spectroscopy that would prove 80-95% of the Universe is actually missing. Don't have anymore time today, bye, be happy living in your fantasyland of Perpetual Motion,
bschott
3 / 5 (2) Feb 02, 2017
"Because we are not concerned with a specific sense of rotation with respect to a major Galactic component, we may use the stellar halos that result from coupling a semi-analytic galaxy formation model to the Aquarius dark-matter-only simulations (Cooper et al. 2010), after applying a suitable tagging and resampling scheme to the dark-matter particles (Lowing et al. 2015)."

Translation: Because we are not modeling the galactic disk rotation curve we have altered the DM component of the Aquarius DM simulation to redistribute the DM content to account for this observation. In other words, the existing Aquarius simulation used on galactic rotation curves requires the DM to be distributed differently than for this simulation, despite the fact that both distributions cannot be true in the context of the observations.

If the quote means something different please correct me.....

RNP
5 / 5 (3) Feb 02, 2017
@bschott
If the quote means something different please correct me.....


Actually, the quote you reference in your post above is the second sentence in the section of the paper called "Comparison to cosmological simulations: granularity". In this section they compare a specific measure (granularity) in their data set to the results of a simulation/model. (I had to look up granularity, but see: https://en.wikipe...nularity
and and https://arxiv.org...7v1.pdf)

What the section is saying is that, when they compare the measure of granularity in their data to the results obtained from a DM+stelar-halo model, they find consistent results.
IMP-9
5 / 5 (4) Feb 02, 2017
In other words, the existing Aquarius simulation used on galactic rotation curves requires the DM to be distributed differently than for this simulation


No, that's not what it says at all. Tagging is how you add stars to a dark matter only simulation, it doesn't change the dark matter. Resampling means that they change the resolution of the simulation after it was run. Lowing et al. 2014 show that resampling doesn't affect the results.
bschott
not rated yet Feb 02, 2017
@RNP - I had thought IMP9 pulled that quote directly from the paper the article is referencing.I only read the abstract but it seemed to make sense that the DM values had to be altered to explain this from the abstract:
A strikingly high fraction (from 58% up to more than 73%) of the stars that are somewhat less bound than the Sun are on (highly) retrograde orbits. A simple comparison to Milky Way-mass galaxies in cosmological hydrodynamical simulations suggests that less than 1% have such prominently retrograde outer halos.

The reason I translated the way I did was that the quote was in direct reference to how the DM is modeled as it pertains to this observation, as IMP was responding to Bennis assertion that DM was left out of the simulation. The quote indicates DM was incorporated into the simulation using the Aquarius simulation for galactic formation, however different values for DM were applied to fit this observation.
IMP-9
5 / 5 (3) Feb 02, 2017
Again, it would be helpful if people read the words that are written rather than just making it up. They are not modifying the dark matter distribution for this simulation.
bschott
1 / 5 (1) Feb 02, 2017
hNo, that's not what it says at all. Tagging is how you add stars to a dark matter only simulation,

According to below ( the paper you pulled the quote from) Tagging is adding particles to a particle simulation, not stars to a DM simulation. However, as the Aquarius simulation is a DM particle simulation it would make sense a stellar body could only be entered as a particle.

ttps://arxiv.org/pdf/1611.03497.pdf

I was mistakenly commenting under the assumption that the Aquarius simulation modeled the galactic rotation curve as well as mergers. The discord I see is still the same one though, DM placed where it has to be and moving how it must to explain the galactic rotation curve is in direct opposition to how it moves during a merger and how it must be moving to explain the motion of the stars in the Halo.....otherwise, one model would fit both observations....
bschott
not rated yet Feb 02, 2017
Again, it would be helpful if people read the words that are written rather than just making it up. They are not modifying the dark matter distribution for this simulation.


My confusion arose from the fact that 2 different models of DM motion are required to explain observations centered around the motion of matter in one galaxy.... models which explain their individual observation nicely but cannot be applied simultaneously....despite the fact that the observations are happening simultaneously.....

To date at this site there have been at least 7 different DM models theorizing what the DM is doing to explain an observation....each one being a viable theory if left to stand on it's own...but crumbles when it has to integrate with the other DM models to explain all of the observations.
cantdrive85
1 / 5 (3) Feb 02, 2017
And don't bother quoting the failed Peratt galaxy model. It is known as a 'failed model' for a reason. It failed.

jonesdumb insists its a failed model due to pride. He doesn't want to admit his entire professional life was meaningless and nothing more than an exercise in pseudoscientific futility. No matter, the following quote sums up the soon to be fate of one jonesdumb.

"A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it." Max Planck
And that "failed" model which replicates well over a dozen phenomena/ morphologies of galactic observation using a simplified simulation, well far more accurate than any model the standard theory has to offer.
http://www.plasma...ormation
IMP-9
5 / 5 (3) Feb 02, 2017
According to below ( the paper you pulled the quote from) Tagging is adding particles to a particle simulation


Firstly that's not the right paper. Secondly tagging tags dark matter particles with stars, it doesn't add particles.

"These techniques attempt to model both stars and dark matter (DM) with a single collisionless particle species in a cosmological N-body simulation by 'painting' subsets of the particles with stellar mass, according to a weighting function, without changing the mass of the particle used in the gravitational calculation."

As I said. It adds stars to a dark matter only simulation. This a post-processing technique, it is not done while simulation is running in the case of Aquarius because it doesn't affect anything in a dark matter simulation because the only computation is gravity. It's only really effective in the halo where the galaxy is basically collisionless.

IMP-9
5 / 5 (3) Feb 02, 2017
DM placed where it has to be and moving how it must to explain the galactic rotation curve is in direct opposition to how it moves during a merger and how it must be moving to explain the motion of the stars in the Halo.


Again, completely made up. Aquarius is a dark matter only simulation, it shows how milky way like halos collapse from simple initial conditions. If they just put dark matter where they needed to it wouldn't be a simulation.

My confusion arose from the fact that 2 different models of DM motion


You aren't confused, you literally made something up. You know fine well they didn't say that.

bschott
1 / 5 (1) Feb 03, 2017
You aren't confused, you literally made something up. You know fine well they didn't say that

Correct, it is what they DID. Either you are confused, playing ignorant or you just are....which is it?
From the article
Surprisingly more than 70% of those stars appear to be moving in the opposite sense than the vast majority of stars in the Milky Way.

Now, why is this so surprising? The only answer I can come up with is that the DM model or simulation which accurately predicts/demonstrates the galactic disks rotation curve DOESN"T produce stars in the halo moving this way. But a merger simulation does....Aquarius.

Am I to understand it is your belief that the same model/simulation can be applied to both the galactic disk and the halo stars and will match both observations of motion? The DM moves the same way in both instances to produce our observations? If so, please...do tell. If not...well, stop arguing.
RealityCheck
1 / 5 (2) Feb 04, 2017
Hi bschott, IMP-9, everyone. :)

@IMP-9, you said:
Secondly tagging tags dark matter particles with stars, it doesn't add particles.

"These techniques attempt to model both stars and dark matter (DM) with a single collisionless particle species in a cosmological N-body simulation by 'painting' subsets of the particles with stellar mass, according to a weighting function, without changing the mass of the particle used in the gravitational calculation."

As I said. It adds stars to a dark matter only simulation.
From my reading of what you quoted from that paper, I understood it to be tagging subset 'chunks/volumes' OF the assumed DM distribution with STELLAR-equivalent MASSES OF DM 'chunks/volumes', but NOT 'normal matter' STARS as such. Also, I understand that the "collisionless" aspect is an ASSUMED characteristic of the assumed 'non-normal matter' DM (and any subset chunks/volumes of same). No actual collisionless 'ordinary-matter' stars per se involved.

Cheers.
IMP-9
5 / 5 (2) Feb 04, 2017
, I understood it to be tagging subset 'chunks/volumes' OF the assumed DM distribution with STELLAR-equivalent MASSES OF DM 'chunks/volumes', but NOT 'normal matter' STARS as such


I think you have the right idea. Essentially the simulation calulates all the dynamics just like a normal dark matter only simulation, just one type of particle and gravity. Each particle in the simulation however is assigned some amount of stars so the dark matter dynamics take the stars with them. Stars are essentially collisionless so simulating them the same way as dark matter isn't a bad approximation. How many stars each particle forms comes from a model. The benefit of this over using star particles is that you can have very small amounts of stars per particle much lower than you could simulate directly. This is ideal for the diffuse halo.

it is what they DID


You admit they don't say it but you claim to know this. It's a baseless claim, nothing more.

Nik_2213
not rated yet Feb 08, 2017
Besides, the stuff going the 'wrong way' seems to be from captured clusters...

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