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[ intro ]
As a species, we humans can be a pretty self-centered bunch,
always imagining ourselves to be just a little more special than we maybe deserve.
This has tripped up astronomers enough times in history
that they’ve even developed a simple rule of thumb.
Delightfully named the principle of mediocrity, it says that we should simply assume
something is more or less average unless there is good evidence to the contrary.
And yet, astronomers this week find themselves once more admitting that
maybe our existence is a little less unique than they previously thought.
On the chopping block today is our Milky Way galaxy.
In a new paper published in The Astrophysical Journal Letters,
researchers describe another galaxy whose arrangement is strikingly similar to ours.
And that similarity could rewrite the Milky Way’s history from the very beginning.
The heart of the situation is the Milky Way’s multi-part structure.
Overall, our galaxy looks like a spiral disk.
But, in detail, that shape isn’t quite so simple.
For one thing, there’s a bulge at the center,
which surrounds our supermassive black hole.
But the Milky Way is also made up of not one,
but two disk-like pieces, sort of nested together.
And their names are extremely technical here, so listen closely:
They are called the thick disk and the thin disk,
and one is more than twice as tall as the other.
I’ll let you figure out which one is which.
But the two disks are different in more ways than just their size:
They rotate around the Galactic Center differently and,
critically, each has a distinct composition.
Stars in the thin disk, including our Sun,
are generally much younger than those in the thick disk.
And because stars convert hydrogen and helium into heavier elements,
each successive generation of stars contains a larger fraction of heavier stuff.
Astronomers call the amount of those heavier elements metallicity,
because for some reason in astronomy
everything that isn’t hydrogen or helium is considered a metal.
Thin-disk stars can have more than ten times higher metallicity
than those in the thick disk.
But why would stars with higher and lower metallicity
be so clearly separated in the galaxy?
Astronomers reasoned that maybe thin-disk stars formed
from material that came from a different, more enriched source.
Like maybe a whole separate galaxy.
This has all led astronomers to conclude that the Milky Way likely formed
from the collision of two different galaxies,
with each of the two disks representing the remnants of the colliding objects.
As far as we can tell from simulations, such large collisions early in a galaxy’s life
were probably pretty unusual, leading scientists
to imagine the Milky Way was, if not unique, at least very rare.
This new study suggests that that conclusion may have been a bit hasty.
The Australian-led team used Europe’s Very Large Telescope, located in Chile,
so it’s a global effort here, to make some of
the first detailed cross-sectional observations of another spiral galaxy.
The galaxy is called UGC 10738, and it’s located
around 320 million light-years from Earth.
Like the Milky Way, 10738 showed evidence of having both a thick and thin disk.
What’s more, the team was able to measure the metallicity in each component.
Remarkably, not only were they different, just like in our galaxy,
they were different in the same way,
with older stars in the thick disk and younger ones in the thin part.
If the only two galaxies studied in cross-section both look the same,
that’s pretty strong evidence in favor of it being a typical arrangement.
Which would suggest that maybe the Milky Way didn’t form
from a rare early collision, but rather followed some path not yet understood.
Now speaking of spiral galaxies, another research team reported last week
that they had discovered one that is older than any previous known example.
In a paper published in the journal Science, the authors described BRI 1335-0417,
a spiral galaxy that seems to have formed
less than 1.4 billion years after the Big Bang.
That’s a billion years younger than the oldest known galaxy of any kind,
but more than a billion years older than the previous record-holder for
the oldest spiral, one discovered with the ALMA radio telescope in 2019.
This new discovery was also made using ALMA,
but it didn’t quite happen in the traditional way.
The study’s authors didn’t set out to observe 0417; instead, they noticed
an observation was already sitting in the telescope’s public data archive.
So hooray for publicly-available data!
The researchers measured the galaxy’s redshift,
which tells astronomers how much the light that’s reaching us
has been stretched out as the galaxy moved away from us,
sort of how a train or a siren that moves away from you sounds lower and deeper.
Based on that number, the team realized that
the galaxy was extraordinarily far away.
And because light doesn’t travel instantaneously, astronomers see far-off objects
not as they would appear in the present,
but how they looked when those bits of light first left.
While examining the data, the researchers keyed in on
frequencies of light emitted by electrically-charged carbon.
The way those frequencies changed across the galaxy implied
not only a bunch of ionized carbon, but a bunch of carbon in motion,
with a bulge near the center and a pair of spiral arms.
In short, the rough signal of a spiral galaxy.
Which is kind of surprising.
Even though spirals make up most visible galaxies today,
their number drops dramatically as you go back in time.
Astronomers aren’t really sure why there are more spirals now,
and the answer could help explain how they form.
Which, come to think of it, could be a key clue in
answering some other spiral galaxy-related questions raised this week.
So, all in all, this has been a pretty decent week
if you happen to be a galactic astronomer, or just a lover of galaxies.
Or a galaxy that’s just trying to understand itself.
We’ve got a puzzling new mystery to solve, but also an exciting new,
or should I say really, really old, piece of evidence.
But solving the puzzles and mysteries of today
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