Milky Way may have formed 'inside-out': Gaia provides new insight into Galactic evolution

Jan 19, 2014
This is a figure illustrating latest Gaia-ESO research findings. Credit: Amanda Smith/Institute of Astronomy

A breakthrough using data from the Gaia-ESO project has provided evidence backing up theoretically predicted divisions in the chemical composition of the stars that make up the Milky Way's disc – the vast collection of giant gas clouds and billions of stars that give our Galaxy its 'flying saucer' shape.

By tracking the fast-produced elements, specifically magnesium in this study, astronomers can determine how rapidly different parts of the Milky Way were formed. The research suggests that stars in the inner regions of the Galactic disc were the first to form, supporting ideas that our Galaxy grew from the inside-out.

Using data from the 8-m VLT in Chile, one of the world's largest telescopes, an international team of astronomers took detailed observations of stars with a wide range of ages and locations in the Galactic disc to accurately determine their 'metallicity': the amount of chemical elements in a star other than hydrogen and helium, the two elements most stars are made from.

Immediately after the Big Bang, the Universe consisted almost entirely of hydrogen and helium, with levels of "contaminant metals" growing over time. Consequently, older stars have fewer elements in their make-up - so have lower metallicity.

"The different of which stars - and we - are made are created at different rates - some in which live fast and die young, and others in sun-like stars with more sedate multi-billion-year lifetimes," said Professor Gerry Gilmore, lead investigator on the Gaia-ESO Project.

Massive stars, which have short lives and die as 'core-collapse supernovae', produce huge amounts of magnesium during their explosive death throes. This catastrophic event can form a neutron star or a black hole, and even trigger the formation of new stars.

The team have shown that older, 'metal-poor' stars inside the Solar Circle – the orbit of our Sun around the centre of the Milky Way, which takes roughly 250 million years to complete – are far more likely to have high levels of magnesium. The higher level of the element inside the Solar Circle suggests this area contained more stars that "lived fast and die young" in the past.

The stars that lie in the outer regions of the Galactic disc - outside the Solar Circle - are predominantly younger, both 'metal-rich' and 'metal-poor', and have surprisingly low magnesium levels compared to their metallicity.

This discovery signifies important differences in stellar evolution across the Milky Way disc, with very efficient and short star formation timescales occurring inside the Solar Circle; whereas, outside the Sun's orbit, star formation took much longer.

"We have been able to shed new light on the timescale of chemical enrichment across the Milky Way disc, showing that outer regions of the disc take a much longer time to form," said Maria Bergemann from Cambridge's Institute of Astronomy, who led the study.

"This supports theoretical models for the formation of disc galaxies in the context of Cold Dark Matter cosmology, which predict that galaxy discs grow inside-out."

The findings offer new insights into the assembly history of our Galaxy, and are the part of the first wave of new observations from the Gaia-ESO survey, the ground-based extension to the Gaia space mission - launched by the European Space Agency at the end of last year - and the first large-scale survey conducted on one the world's largest telescopes: the 8-m VLT in Paranal, Chile.

The study is published online today through the astronomical database Astro-ph, and has been submitted to the journal Astronomy and Astrophysics.

The new research also sheds further light on another much debated "double structure" in the Milky Way's disc – the so-called 'thin' and 'thick' discs.

"The thin disc hosts spiral arms, young stars, giant molecular clouds – all objects which are young, at least in the context of the Galaxy," explains Aldo Serenelli from the Institute of Space Sciences (Barcelona), a co-author of the study. "But astronomers have long suspected there is another disc, which is thicker, shorter and older. This thick disc hosts many old stars that have low metallicity."

During the latest research, the team found that:

  • Stars in the young, 'thin' disc aged between 0 – 8 billion years all have a similar degree of metallicity, regardless of age in that range, with many of them considered 'metal-rich'.
  • There is a "steep decline" in metallicity for stars aged over 9 billion years, typical of the 'thick' disc, with no detectable 'metal-rich' stars found at all over this age.
  • But stars of different ages and metallicity can be found in both discs.

"From what we now know, the Galaxy is not an 'either-or' system. You can find stars of different ages and metal content everywhere!" said Bergemann. "There is no clear separation between the thin and thick disc. The proportion of stars with different properties is not the same in both discs - that's how we know these two discs probably exist – but they could have very different origins."

Added Gilmore: "This study provides exciting new evidence that the inner parts of the Milky Way's thick disc formed much more rapidly than did the thin disc , which dominate near our Solar neighbourhood."

In theory, say astronomers, the thick disc - first proposed by Gilmore 30 years ago - could have emerged in a variety of ways, from massive gravitational instabilities to consuming satellite galaxies in its formative years. "The Milky Way has cannibalised many small galaxies during its formation. Now, with the Gaia-ESO Survey, we can study the detailed tracers of these events, essentially dissecting the belly of the beast," said Greg Ruchti, a researcher at Lund Observatory in Sweden, who co-leads the project.

With upcoming releases of Gaia-ESO, an even better handle on the age-metallicity relation and the structure of the Galactic disc is expected, say the team. In a couple of years, these data will be complemented by positions and kinematics provided by the Gaia satellite and together will revolutionise the field of Galactic astronomy.

Explore further: Video: Guide to our Galaxy

More information: The study will be published in Astronomy & Astrophysics.

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Whydening Gyre
2.5 / 5 (2) Jan 19, 2014
With numerous different metals as part of our biological makeup, I wonder if this would preclude "life as we know it" among the inner stars(above and beyond the radiation and gravitational issues? Or do amino acids adapt to different chemical compositions?
Satalink
5 / 5 (3) Jan 19, 2014
I don't think we can look our galaxy to have uniformed aging as galaxies tend to collide and devour one another. In the process, stars are torn apart, captured, and born in the process.
Yelmurc
3.3 / 5 (3) Jan 19, 2014
Gaia data is already being published? That was quick Gaia has only been in operational orbit for what 7-10 days. Can't wait to find out what else it confirms or discovers.
grondilu
5 / 5 (6) Jan 19, 2014
Well, this title is quite misleading. This result has been obtained with the so-called "ground based extension of the Gaia mission".

IIRC, Gaia is currently being calibrated, verified and other boring stuff. It will keep doing so during the next three months I think. And after that no result will be released before an other twenty months.

Space astronomy really is a matter of patience.
mytwocts
5 / 5 (2) Jan 20, 2014
What about the halo of globular clusters? This text does not preclude that the thick disk stars are related to the globular clusters. The age difference between the halo and the disk has been known for a long time.
GSwift7
5 / 5 (2) Jan 20, 2014
lol, I thought they were talking about the Gaia spacecraft (which just launched in December, on a star-mapping mission). I said to myself "Holy _____! They have data already?!!"

Link to the Gaia mission wiki page:

http://en.wikiped...cecraft)

A very interesting mission, which will without a doubt lead to a truck-load of new finds.
Returners
5 / 5 (1) Jan 20, 2014
Because heavy elements are ejected at a lower velocity during a supernova, they have an increased chance of being captured by massive objects (or even clouds). The core has more stars in denser conditions, which means they are more likely to collide with the clouds of eject from other supernovas than would be stars in the outside of the disk.

Therefore I'd expect heavier elements to collect in the core stars more often, even if there were no other initial, fundamental difference between the stars.
GSwift7
5 / 5 (1) Jan 21, 2014
Therefore I'd expect heavier elements to collect in the core stars more often, even if there were no other initial, fundamental difference between the stars


Yeah, it made me do a double-take as well, but you're probably mis-reading the meaning of the article just like I did. The stars you're talking about, which would have higher percentages of heavy elements thanks to being formed from the remnants of previous stars, would be newly formed stars. What they are saying above is that the center of the galaxy still has first generation stars, composed of nearly pure hydrogen and helium. The outer regions of the galaxy don't have those extremely old, pure hydrogen stars, only younger stars with more heavy elements. They aren't saying that the center of the galaxy doesn't have heavy elements, they're saying that the outer rim of the galaxy doesn't have any pure ones. The wording was a little confusing, IMO.
SHREEKANT
1 / 5 (1) Jan 21, 2014
Knowing about Dark energy & Dark matter like Where they form? How they form? How they interact with White matter? solve the many mysteries of GR & Quantum like- Reason of Gravitation? Why Einstein mathematically correct but logically wrong? How planets are formed? How Galaxies are formed? Role of Black hole in Galaxy. What are the different stages or types of Galaxy? How stars move from Galactic Disk to Galactic Halo? etc.

Mg:Fe ratio- I am simply considering it as Mg- Star [having more Mg] & Fe -Star.

Most of the collision which effect the composition of the stars are undergoing in Disk only during the formation.

"radial migration" - it is a normal phenomenon [on the basis of DE & DM concept]. Mg-Star will move fast w.r.t Fe-Star in Halo.

The 5th force is governing the universe from GR to Quantum

My work is important because I am looking the Universe from another window of DM & DE while scientific window is different but on many topic we are drawing the same picture of universe
GSwift7
5 / 5 (1) Jan 22, 2014
shreekant:

The 5th force is governing the universe from GR to Quantum


That's still a fairly hot topic, though most evidence seems to point back to some variation of GR, with only 4 forces and 4 degrees of freedom (including temporal).

Which alternative are you talking about? String, MoND, Supergravity?

Those alternatives are running out of room to argue, especially String. The latest generation of instruments, such as the newest torsion balance, are sensitive enough that they almost eliminate the possibility of string theory (in its current version).

What I'm kinda expecting to see, perhaps not in my lifetime, is some modification of quantum theory that isn't based on statistics, averages and probability. Perhaps that is what leads to the mis-match between QFT and GR?
vidyunmaya
1 / 5 (1) Jan 22, 2014
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The Cosmological Index is a drive to Milky Way Sensitive Index and as such the Onset-mode must be viewed as a Curtain raiser to the present day concepts
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Scroofinator
5 / 5 (1) Jan 22, 2014
I watched a documentary a while ago that IMO cleared up galaxy formations. There were galaxy sized clouds of hydrogen floating around and they ultimately became so dense that they would implode on themselves creating a SMBH. The shockwave(or supernova?) that resulted from this blasted thru the rest of the cloud and is what started the process of creating planets and stars.

Anyone else heard of this theory?
Whydening Gyre
5 / 5 (1) Jan 23, 2014
I watched a documentary a while ago that IMO cleared up galaxy formations. There were galaxy sized clouds of hydrogen floating around and they ultimately became so dense that they would implode on themselves creating a SMBH. The shockwave(or supernova?) that resulted from this blasted thru the rest of the cloud and is what started the process of creating planets and stars.

Anyone else heard of this theory?

No. But it sounds plausible. Should give rise to a number of "look at the crackpot" claims..
GSwift7
5 / 5 (1) Jan 24, 2014
I watched a documentary a while ago that IMO cleared up galaxy formations


There are a few different variations right now, but no way to really know. Once the SKA and JWST get running, we may be able to narrow it down a bit. Right now, we just don't have anything that can really see well enough to answer that question. If you want to place bets and make guesses, that's fine, but be prepared to be proven wrong in the next 20 years.

Really, based on what we have seen so far, I think galaxy and SMBH formation had to have started before reionization. I think the seeds had to have already been in place by the time we had clouds of hydrogen floating around. Rather than gravity causing collapse, it could be more like pressure from shockwaves forcing things together, but who knows?
Scroofinator
5 / 5 (2) Jan 24, 2014
If you want to place bets and make guesses, that's fine, but be prepared to be proven wrong in the next 20 years.


I'm always ready to be proven wrong, I just make the best guess I can with the data I have.

Another reason I believe the theory is because it also explains why we see almost every galaxy with a SMBH in it since they all would've formed the same way.
GSwift7
5 / 5 (1) Jan 24, 2014
Another reason I believe the theory is because it also explains why we see almost every galaxy with a SMBH in it since they all would've formed the same way


yes, but your logic could just as easily support some of the conjectures like the one I mentioned, which suggest formation was well under way by the time you had clouds of hydrogen floating around and condensing by the force of gravity. You see, they still would have all formed at the same time.

btw, the conjecture I posited isn't mine. I read it somewhere, but no clue where. It's all barely more than BS for now anyway, so it doesn't really matter till we get better data. I also don't think they were thinking about kinetic shockwaves, but rather shockwaves in spacetime or some such (whatever that might mean). If we look farther back in time than ever before, right up to the edge of re-ionization, and we still see structures, then we'll know that it started before we had a Universe composed of normal matter.