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Where's all the old hydrogen?

by Sarah Scoles


No telescope will ever be able to detect cold hydrogen gas in distant radio galaxies and quasars. Even if venture capitalists funded a telescope with a 500,000,000-foot dish. Which is so big that even Australia and South Africa, put together, couldn't build it.

Why is the simplest element so undetectable in far-away, active galaxies?

Well, because it's just not there. Which is weird. Because neutral hydrogen is, like, everywhere. In all galaxies. All the time. Surrounding them and, in many cases, dwarfing the parts of a galaxy that we normally think of as "the whole galaxy." 

Recent results show that the hydrogen surrounding our galaxy is this big, while the pretty spirals we're used to think of as "our whole selves" is that tiny stuff in the middle. CREDIT: NASA/CXC/M.Weiss; NASA/CXC/Ohio State/A. Gupta,et al .It's also what begets stars. Imagine this: You are a cloud of cold hydrogen gas sitting in space. The gravitational force attracting you toward your own center is perfectly balanced with the pressure from your molecules collisions. So zen, right?



But what if that stopped being true? 

What if you became more massive, making the gravitational force stronger? (It is, after all, the holidays.) Then the gravity would not equal the pressure--gravity would win. You would collapse in on yourself. 

Way to go.

But, no, seriously--way to go. If you did it right, you'll become a star. You'll get contract and collapse and become denser and denser and hotter and hotter and eventually start to fuse your hydrogens together. You're a star!

After you form a star, you fuse heavier and heavier elements, and then, if you are massive enough (and, let's be honest, you are), you explode in a supernova and make even heavier elements. 

And more stars form from the gas cloud you made. And those stars have heavier elements, as do their planets. That's how we got all this gold bling on Earth. This picture was called "BlingBlingNeon.jpg." Credit: PSP.

So if you rewind the clock back to the beginning of the universe, it makes sense that there would be more cold hydrogen gas than there is now, since so much of it has turned into stars and into planets and into, well, the universe as we know it.

So, when, in 2008, Drs. Stephen Curran, Matthew Whiting, and J.K. Webb looked for hydrogen in these distant, energetic galaxies from the beginning of the universe, they were surprised to see...nothing.

How do you "see" hydrogen?

A neutral hydrogen atom emits a radio wave of frequencies 1420.41 MHz when its proton and its electron switch from spinning ("spinning") in the same direction to spinning in opposite directions; this is a transition from a higher energy state to a lower energy state, and the difference in energy must go somewhere (gor a more detailed answer, see this post).

Curran and associates used the Giant Metrewave Telescope in India to look for the hydrogen that they assumed would be more abundant because it hadn't yet collapsed into stars and been consumed.

But since they didn't see any, they had to form an idea as to why.

In a press release from the first paper's debut, Curran suggested that the element's absence was due to the galaxies' supermassive black holes (supermassive black holes: always causing problems): "The intense radiation from the matter accreting into the black hole in these quasars [distant galaxies with active nuclei] is extreme and we believe that this radiation is ripping the electrons from the atoms, destroying the hydrogen gas."

When hydrogen loses its electron, it is ionized and can no longer be seen by radio telescopes, because an electron that's not there can't flip its spin. The is also too hot and energetic to form stars--gravity can't win against the higher pressure and higher temperatures caused by the higher kinetic energy.

But not next time. Credit: myopera.com.

Now, four years later, Curran, Whiting, and a few others have published a model that proves, basically,

"Yep, that disruptive black hole thing was right."

These supermassive black holes are feeding on the material around them, and as the material falls in, it heats up. This hot material emits energetic radiation--specifically, ultraviolet radiation. Because these galaxies are flying away from us, carried by the expansion of the universe, the UV rays are stretched out (redshifted) down to optical wavelengths, so astronomers can literally see the light from stuff swirling around some of the first-ever huge black holes.

While this accretion process is cool, it is too hot to allow the hydrogen gas to remain neutral and form stars. And the heat doesn't just affect some of the atoms--all of them, in these distant galaxies, are ionized.

Why is this interesting?

Well, it means that a long time ago, in galaxies that are now far, far away but that tell us a lot about how galaxies in general may have been a long time ago, stars were not forming because they could not form. But they must have formed sometime, because they are here now. And there is hope for detecting star-forming hydrogen in other, less distant, less active galaxies. As Curran said, “The Square Kilometre Array will excel ... in detecting very cold gas that is too faint to be detected by optical telescopes, which must have existed to give us the stars and galaxies we see today.”

Mysteries! Of the universe. 

Curran and colleagues' conclusions have consistently created a new question for them to answer, which is exactly how science should work.


Curran, S., & Whiting, M. (2012). COMPLETE IONIZATION OF THE NEUTRAL GAS: WHY THERE ARE SO FEW DETECTIONS OF 21 cm HYDROGEN IN HIGH-REDSHIFT RADIO GALAXIES AND QUASARS The Astrophysical Journal, 759 (2) DOI: 10.1088/0004-637X/759/2/117

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Reader Comments (1)

Good one and thanks a lot for sharing!

June 18, 2015 | Unregistered Commenterstanley

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