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So Hyperfine

Hydrogen is the most abudant element in the universe because it is the simplest element in the universe. One proton, one electron. Balance on the smallest possible scale.

Neutral hydrogen
a) is cold
b) hangs out in the space between stars
c) is part of the structure of many galaxies
d) makes a good cake frosting
e) a, b, and c but not d
f) d and e
g) wait, what?

Anyway, the answer is e. Now you know a lot about neutral hydrogen (or HI, pronounced not "hi" but "aitch one"). The other thing you need to know is that HI emits a radio wave, which means that those of us with radio telescopes can study it.

The proton and electron in an HI atom can be spinning (and here, because we're talking about quantum mechanics, "spinning" is really just a metaphor) in the same direction or in opposite directions. The same-direction spin is a higher energy state than the opposite-direction spin.

Sometimes, an HI atom will switch from the high-energy state to the low-energy state. But we all know, because of car accidents and atomic bombs, that energy is conserved. So the difference between the high-energy state and the low-energy must go somewhere. It can't just disappear.
The HI atom, it turns out, emits a light wave that has the same amount of energy as the difference between the two states. You can see all of this illustrated in the convenient picture to your right.

The transition is called "spin-flip" or "hyperfine" or "so fly" (I'm the only one who calls it that last one). The radio wave has a wavelength of about 21 cm, which corresponds to a frequency of 1420.41 MHz.

This HI transition is "forbidden," which, like many forbidden things, such as murder and stealing cookies, doesn't mean it never happens. When you look at an individual HI atom, it isn't very likely to undergo this transition; in fact, it only happens to Harry the Hydrogen once every 10 million years. But when you get Harry together with all his friends, some of his friends will be spin-flipping, just because Harry has so many friends and statistics works the way statistics does.

We can see hydrogen in our own galaxy, and it can tell us which way and how fast Milky Way's arms are flailing around. But our galaxy isn't special. It isn't the only one whose HI can help astronomers look smart.

Using HI, astronomers can learn about the rotation rates of other galaxies, as well as their recessional velocities, as well as how their luminosity (optical light) compares to the radio brightness of their hydrogen, which can reveal how much cold gas (HI) there is relative to hot material (like stars), and where the two are relative to each other.

With that in mind, members of the HI community got together, high-fived each other, and came up with the idea for the ALFALFA Survey. This survey uses a seven-pixel receiver (basically a radio CCD [and seven pixels is about six more than most radio receivers]) mounted on the Arecibo Telescope. The receiver is 'tuned' to 1420.41 MHz, since it is looking for hydrogen. It takes data on seven adjacent points in the sky and then moves on to seven more points. Astronomers then search through the data to see if they can identify galaxies.

Thus far, they've gone through 40% of the total data and have found 12,000 galaxies.

I repeat: they have found 12,000 galaxies.

These galaxies, however, are not evenly distributed in space, but are clumpy, like the condensed soup that I somehow failed to make properly last night. 
Credit: DOCTOR Ann Martin.
In this conical diagram, each line going radially outward represents one line-of-sight through the sky, and the farther a dot is from the center of the cone, the farther it is from us.
You can see how these galaxies cluster around each other and leave large empty spaces. Why? Why this particular arrangement? How are galaxies that are very close together different from galaxies that are not in dense regions? Did they start out different (and/or differently), or were they made different (and/or differently) by their environments? Why are there voids? How does the HI mass of a galaxy relate to its position in either a dense or a not-dense region?

And how, exactly, do you define "dense" and "not dense"? It's not like the universe is neatly and discretely arranged. There has to be some cutoff, but will the cutoff be phenomenological (meaning that the two kinds of regions are split this way because they house fundamentally different processes), or will the cutoff be human-imposed and arbitrary?

These questions, and more, are being investigated by the ALFALFA Survey Team, which includes my friend Dr. Ann Martin, who got her PhD last week by talking about what she's discovered about the nature and nurture of HI-selected galaxies. 

The galaxies that contain the most HI tend not to be found in clusters, but hanging out relatively alone. The clustered galaxies tend to contain older, redder stars than the lonely galaxies, and also tend to be optically brighter.

I would say more about her results, but she's going to do a guest post soon, so I wouldn't want to spoil all the fun. This is just an intro to what HI is and what kinds of questions HI astronomers are asking. If you want to read up and get ahead and sit in the front row and get made fun of by all the other kids, check out this paper:

Martin, A.; Papastergis, E; Giovanelli, R.; Haynes, M.; Springob, C.; Stierwalt, S. The Arecibo Legacy Fast ALFA Survey. X. The H I Mass Function and Ω_H I from the 40% ALFALFA Survey. 2010ApJ, 723(1359). 

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    Response: why not look here
    Summary and analysis of new scientific discoveries in biology and astronomy. by Sarah Scoles and Brooke Napier.
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    Many people are not aware of this information hydrogen is the most abundant element in the entire universe because it is simplest element. Thanks for sharing such scientific information. It’s really very impressing knowledge of all the time.
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