Subaru Telescope identifies the outermost edge of the Milky Way system

Subaru Telescope identifies the outermost edge of the Milky Way system
Figure 1: The Milky Way and Halo component. Credit: Tohoku University

A team of researchers identified the outermost edge of the Milky Way galaxy. Using the Subaru Telescope, the researchers examined the boundary of the stellar system that makes up the galaxy. The ultimate size of the galaxy is 520,000 light years in radius, 20 times larger than the distance between the galactic center and our solar system (26,000 light years) (Figure 1). Stars that reach these outermost regions of the galaxy during their orbital motions are ancient stellar populations with ages as old as 12 billion years. The spatial extent in which these ancient stars wander is, therefore, important for understanding the Milky Way's formation.

The galaxy holds a broadly extended halo , in addition to the bright Milky Way in the form of the stellar disk component. The halo contains about 1 billion ancient stars and 150 globular clusters with ages as old as 12 billion years (Figure 1). The halo thus contains the remnants of long-lived stars and star clusters that formed in the first stage of the galaxy. This suggests that the galaxy was quite large in its beginning before the later formation of the younger, disk component.

Investigating the extent of this halo component in the galaxy is similar to identifying the outer boundary of a forest from inside the forest and observing the trees. In other words, it is an arduous task. So-called blue horizontal branch (BHB) stars as well as RR Lyr variables are ideal indicators for tracing the halo component. This is because they are naturally bright enough to determine the distance to and from them. However, the galaxy is so large that it is impossible to identify the halo traces located at the outer boundary using 2.5 to four-meter telescopes.

The team of researchers led by Tohoku University graduate student Tetsuya Fukushima and his supervisor Masashi Chiba used the Hyper Suprime-Cam (HSC) on the 8.2 meter diameter Subaru Telescope. It enabled them to capture remote, very faint halo tracers at the outer edge of the galaxy. The team carefully selected the BHB stars from the on-going survey program (SSP: Subaru Strategic Program) data against other contaminants having similar colors such as so-called blue straggler stars, white dwarfs, quasars and distant .

Using the data from HSC-SSP, the team derived the spatial density of the BHB over the galaxy halo. While this density generally decreases the further you go from the , the team discovered a sharp drop in density at around 520,000 light years away from the galactic center. Thus, the team had finally observed the outermost edge of the galaxy. This is about 20 times larger than the distance between our solar system and the galaxy center.

Twelve billion years ago, successive merging of small galaxies confined by dark matter halos occurred. Key to understanding this is measuring the distribution of the halo component to ascertain the volume. This merging process differs from galaxy to galaxy. Our neighbor, the Andromeda galaxy, is reported to have an extended halo component as large as 538,000 (at the very least) in radius. It is, therefore, systematically larger when compared to the galaxy . The researchers are planning to further map out this ancient component of the galaxy after the final completion of the HSC-SSP.


Explore further

A massive collision in the Milky Way's past

More information: The stellar halo of the Milky Way traced by blue horizontal-branch stars in the Subaru Hyper Suprime-Cam Survey. Publications of the Astronomical Society of Japan, doi.org/10.1093/pasj/psz052.

Preprint available: arXiv:1904.04966v2 [astro-ph.GA]. arxiv.org/abs/1904.04966

Provided by Subaru Telescope
Citation: Subaru Telescope identifies the outermost edge of the Milky Way system (2019, June 24) retrieved 16 September 2019 from https://phys.org/news/2019-06-subaru-telescope-outermost-edge-milky.html
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Jun 24, 2019
So, with 2.5 million light years to Andromeda we have to subtract a half a million light years each leaving one and a half million light years between edges. Add the pure dark matter halos with over double the radius each then we are already directly interacting, Milky Way and Andromeda, and have been for some time.
No wonder the approach speed is seven hundred kilometers per second and accelerating. Tighten those seat belts.

Jun 24, 2019
I dont think the distance calculation is between galactic centers

Everything i've been able to find to read about the distance just takes distances of various stars in andromeda to earth and not a calculation of estimates on galactic centers. Are you sure the distance of 2.5 million is between galactic centers? Seems more likely it's 2.5 million to the closest observable stars within the andromeda galaxy to earth.

Also, the dark matter halo thing ....not sure you can really say much about that. Dark matter is not supposed to interact with itself or normal matter except thru gravity, but it seems that has to be fundamentally wrong or all the dark matter would collapse into any gravity well unimpeded ...and it doesn't seem to. So at best, we dont know hardly enough about it to predict. I'm not a fan of the dark matter is fake club, but it seems we're still pretty far away from any kind of useful description of what it is and how it behaves.

Jun 24, 2019
What Dark Matter is exactly is unknown. How dark Matter behaves is known quite well.
It has no electrical attraction to make it stick together in solid masses. Gravity is less than 1/1000000000000000000000000000000000 as strong as electrical attraction and whatever the Dark Matter particles are they bounce off each other. The average density of Dark Matter is 5 protons per cubic meter, none of it congealed into blobs.
This density is measured through the effects of gravity. The total mass of Dark Matter between our sun at 25,000 light years from the edge of the nuclear halo is greater than the total mass of everything inside the halo including the 4,000,000 solar mass black hole.
The behavior is measured quite accurately.
Where they are and how they act is solidly known.

The edge of Andromeda's DM halo and ours is not pinpointed exactly to the mile, but the gravitational effects causing the closing rate is documented.

Jun 24, 2019
Bad fingers, bad. More Dark Matter as stated but not all inside our orbit, it is everywhere.

The Dark Matter in the Milky Way is consistent, putting more gravitational mass outside the nuclear halo but between our sun and the nucleus by a factor of five to one over regular mass.
These are solid measurements and the gravitational mass of Dark Matter continues outward, affecting the orbits of our dozens of satellite dwarf galaxies, as well as Andromeda.

Jun 24, 2019
if dark matter repels itself (something that contradicts recent observations : https://www.scien...fter-all ), then the halo's of dark matter would either :
A. not be halos' but in fact be somewhat uniform fields of dark matter that permeate the universe and so can't be identified as belonging to any one galaxy or
B. inhibit the collision of the galaxies by repelling eachother (if they are uniformly wrapping the galaxy in a bubble)- reducing the speed of colliding galaxies by resisting the gravitational force between the two as the bubbles collide.

I dont think either option is correct (And i dont think most people do, since they dont seem to interact with eachother). I think we have only discovered 1 force that dark matter is interacting with, but not all of them and that something keeps dark matter from collapsing in on gravity.


Jun 24, 2019
I dont think either option is correct (And i dont think most people do, since they dont seem to interact with eachother). I think we have only discovered 1 force that dark matter is interacting with, but not all of them and that something keeps dark matter from collapsing in on gravity.

"Dark Matter" implies a particulate of some kind.
What if the explanation is that we just don't have a complete description of gravitational force characteristics?

Jun 24, 2019
no better theory that explains the observational discrepancies exists ...which is why dark matter is thought of as something ...rather than just a term that encompasses an incomplete theory of gravity.

I'm sure if someone came up with a theory as to why gravity doesn't work the way that all existing theories say it does while simultaneously predicting the things those theories do correctly model we'd be singing a different tune. But so far, it seems all attempts fail at some level.

I think science is more inclined to believe that the standard model is flawed since we know it is at least on certain levels (hence a new undiscovered particle of some kind), than to believe that gravity is behaving in a way that's inconsistent with the existing established theories.

Jun 24, 2019
Add the pure dark matter halos with over double the radius each
....how do you know it's double if ONLY the baryonic matter halo has been observed?

These are solid measurements and the gravitational mass of Dark Matter continues outward,
.....how do you have solid measurements without solid observations?

Jun 24, 2019
There are some very complicated issues of galaxy formation. Unfortunately, here is the same problem as with the stars. The origin of galaxies remains unclear, in spite of huge activity in the field. What the "formation" means? It means that we have the material that is assembling into galaxies.
https://www.acade...ome_From

Jun 25, 2019
I let the article soak in then re read it.
It seems they studied older stars that were ejected during mergers but not lost and used them to establish the extent of the Dark Matter halo.
No outlandish claims are being made, this was information gathering which is ongoing.
So far it does not affect my personal pet theories, which due to having to conform to established facts are pretty much mainstream.

Jun 28, 2019
So that is just a disk vs sphere radius ratio of 20, while our planetary system disk vs comet cloud radius ratio is 100,000 au (2 ly) / 100 au or 1000.

Meanwhile, Andromeda is insignificantly larger in volume, some 10 % (or a bit more).

Dark matter is not supposed to interact with itself or normal matter except thru gravity, but it seems that has to be fundamentally wrong or all the dark matter would collapse into any gravity well unimpeded ...and it doesn't seem to.


How would it collapse without having any other interaction so the particle cloud can radiate energy toward infinity and shrink as normal matter would do?

It does exactly what we expect it to do, stays moving on according to the virial theorem as applied to gravitationally interacting particles.

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