Ever since the Gaia satellite began mapping the Milky Way in three dimensions with the very best precision yet, we’ve been learning tons about our home galaxy’s violent past.
The scrappy cannibal has collided with, and subsumed, multiple other galaxies over its 13.6-billion year lifespan. the most important of those , the Gaia-Enceladus dwarf galaxy (AKA the Gaia Sausage), happened around 10 billion years ago, and was thought to be liable for a curious feature of the Milky Way’s structure called the thick disk.
Now, however, it appears that this might not be the case. Astronomers have studied another galaxy with a thick disk, and determined that its evolution wasn’t a cataclysmic accident, but just a reasonably normal way for spiral galaxies to grow.
“Our observations indicate that the Milky Way’s thin and thick disks didn’t come about due to gigantic mash-up, but a sort-of ‘default’ path of galaxy formation and evolution,” said astronomer Nicholas Scott of the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) and therefore the University of Sydney in Australia.
“From these results we expect galaxies with the Milky Way’s particular structures and properties might be described as’ the ‘normal’ ones.”
It’s easiest to know the thick and thin disks if you think that of the Milky Way a little bit like a burger. the skinny disc, which is about 400 light-years thick and contains gas, dust and stars, is analogous to the burger patty; the thick disc, which extends to 1,000 light-years and contains only stars, is just like the burger bun.
The thin disk is where you’ll find all the younger stars, richer in metals, although it contains stars of all ages. The thick disk is more sparsely populated, and contains only stars older than about 10 billion years.
This feature is merely seen in some spiral galaxies, and astronomers don’t really know about how they got there, but supported Gaia data, a team of astronomers thought that they had figured it out. They traced the motions of clusters of stars within the Milky Way’s halo, and their chemical compositions, and located that they had originated outside the galaxy. Modelling then suggested that the galactic merger that delivered these stars to the Milky Way also heated the pre-existing thin disk, inflating it into a thicker one.
Although we’ve seen other spiral galaxies with thick disks, it had been impossible to inform if these structures had an equivalent star distribution because the Milky Way . Enter a spiral-galaxy called UGC 10738, located 320 million light-years away.
You can tell roughly how old a star is predicated on its chemical composition. Younger stars have more metals in them than older ones, since these elements didn’t exist within the galaxy until a generation or two of stars came along to create them via nuclear fusion of lighter elements.
The chemical compositions of stars also can be read in their light spectrum – some wavelengths are brighter or dimmer, based on which elements are present.
Individual stars can’t really be studied in distant galaxies; they’re too distant to resolve with our current technological capabilities. What we will do is study the light coming from different regions, and ascertain what sorts of stars are in those regions as a population.
This is what makes UGC 10738 such an excellent laboratory for studying a thick disk. Its edge is facing us, giving us a really clean, clear view of the skinny and thick disk structures – we will actually figure out the burger and therefore the bun, and separate the light from each section. this is often what Scott’s team did, using the EU Southern Observatory’s powerful Very Large Telescope in Chile.
“Using an instrument called the multi-unit spectroscopic explorer, or MUSE, we were ready to assess the metal ratios of the celebs in its thick and thin discs,” said astronomer Jesse van de Sande of ASTRO 3D and therefore the University of Sydney.
“They were just about an equivalent as those within the Milky Way – ancient stars within the thick disc, younger stars within the thin one. We’re watching another galaxies to make sure, but that’s pretty strong evidence that the 2 galaxies evolved within the same way.”
This doesn’t mean that the Milky Way hasn’t engaged during a little bit of a fight with other galaxies, and it certainly doesn’t suggest the Gaia Sausage merger never happened (there’s many other evidence for that encounter). But it does seem to suggest that the Gaia Sausage wasn’t liable for puffing the thick disk.
“It was thought that the Milky Way’s thin and thick disks formed after a rare violent merger, then probably wouldn’t be found in other spiral galaxies,” Scott said.
“Our research shows that’s probably wrong, and it evolved ‘naturally’ without catastrophic interventions. this suggests Milky Way-type galaxies are probably quite common . It also means we might use existing very detailed observations of the Milky Way as tools to better analysis far more distant galaxies which, for obvious reasons, we can not see as well.”
While that puts us back a small ways in determining what did puff the Milky Way’s thick disk, it brings us forward in our study of thick disks and therefore the evolution of spiral galaxies as an entire .
Now that the team has demonstrated that it’s possible to spatially resolve chemical distributions of other galaxies, they decide to apply their techniques to a statistically significant sample of same galaxies to see how well their findings hold-up’.
The research has been published in The Astrophysical Journal Letters.