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(Intro)

When you think of the universe, you probably think of all the bright stuff, stars and galaxies that light up the night sky, and that's understandable.  I mean, in between those things, it looks like there's a lot of emptiness, but more than half of all the matter in the universe is out there in the dark, filling that empty space, and I'm not talking about dark matter or anything like that.  I'm talking about the intergalactic medium.  It's the thinly spread gas in the space between galaxies and even though it's basically invisible, it has a lot to tell us about the stuff we can see.

The intergalactic medium or IGM glows extremely glows extremely faintly, so it's invisible to most telescopes.  In fact, it wasn't even discovered until the 1960s.  That's when scientists first discovered quasars, incredibly bright objects in the old and distant universe.  Oddly, when they looked at these quasars, they found lots of wavelengths missing from their light.

Normally, light from stars is a continuum of all wavelengths and by splitting that light apart, kind of like you do with a prism, you can see a continuous rainbow or a spectrum, but there are often gaps in that spectrum.  You get gaps when there's something sitting between the light source and your telescope, absorbing specific wavelengths of light before they reach you.  That's what astronomers were seeing in the spectra from these quasars.  So they knew there had to be invisible matter sitting between us and these distant objects.  They could even tell what it was made of.

The patterns of absorbed lines told astronomers that this gas was mostly hydrogen, with some heavier elements thrown in like carbon, silicon, and oxygen.  In general, the IGM is extremely thin.  On average, it has less than one atom per cubic meter.  But around galaxies, gravity is able to hang on to a somewhat denser halo of gas, which scientists call the circumgalactic medium, and even though the gas in both of these regions is thin and invisible, it's closely tied to the life cycle of galaxies and has a lot to tell us about how they form and evolve.  

These days, astronomers have some new tools for studying the IGM, but a lot of the time, they still use the same old technique.  After all, studying the gaps in spectra can tell us about the temperature, distance, age, chemistry, and motion of gas in front of the light source, which is a lot to know about an invisible gas billions of light-years away.  The really cool thing is, since quasars are so old, by exploring their spectra, we can actually explore gases from the early universe.  Their light is carrying information that's billions of years old.

Astronomers are even able to look back to when the early universe gave birth to the first stars and galaxies, just hundreds of millions of years after the Big Bang.  Back then, clouds of gas were made of pure hydrogen and in them, astronomers can see clumps, some slightly denser and hotter spots, separated by thinner gas in between, and those hot clumps of hydrogen?  Those are the beginnings of the universe's first galaxies.

By using those clumpy spots as a starting point, scientists have been able to simulate the evolution of the universe and better understand how we got the universe we live in today.  These days, the IGM still makes up 60% of matter in the universe, but it's not the same old gas that was there all along.  Now it has a lot more heavy elements that were forged in stars because galaxies are constantly trading material with the IGM.

Gas likely gets blown out of active galaxies by things like violent supernovas, black hole jets, and solar winds.  Violent events like galactic mergers can also toss a huge amount of dust out of a galaxy.  This seems to explain how the intergalactic medium got its sprinkle of heavy elements, but galaxies aren't just blowing out gas.  They're also pulling it in.

Once the circumgalactic gas moves away from the hot, turbulent environment of the galaxy, it cools down.  As it slows, it becomes more susceptible to the galaxy's immense gravitational pull and some of it falls back into the galaxy.  This cycle of inflowing and outflowing gas is known as galactic recycling.  Models suggest that galaxies typically pull in about one solar mass of gas a year, and this year inflowing gas is what keeps a galaxy alive.  That's how a galaxy can keep making new stars.  

Recycling doesn't seem to be a perfect loop, though.  A 2019 study of our own galaxy revealed that more gas was flowing in than out, so the Milky Way must be getting a boost of gas from somewhere.  Since our galaxy is one of the biggest in the neighborhood, it may be leeching circumgalactic gas from smaller neighboring galaxies or it's possible that this gas is blowing in from deeper in the intergalactic medium.  

Either way, we don't have all the answers yet, but scientists hope that similar measurements from other galaxies could tell us more about how galaxies interact with intergalactic gas.  Measurements like this could also help answer the question of why galaxies die, because it's not exactly obvious.  As long as galaxies are recycling gas, it seems like they could keep making stars forever, but they don't.

We see lots of galaxies whose stars are all red and old with no youngsters in the mix, but it's not clear why  Observations of the area around dead galaxies show that there is still gas that's cool enough to fall in, but for some reason, it doesn't.  Scientists don't know what's stopping it, but whatever the answer is, it's probably in the gas between galaxies.  Researchers hope that future surveys of dead galaxies and their circumgalactic halos will offer more clues. 

The space between the stars is still pretty murky territory, but as we get better at exploring it, the intergalactic medium will have a lot to tell us about how galaxies live and die. 

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