We’ve Mapped Hydrogen in the Milky Way. Could Helium be Next?

Here’s how we can go beyond the 21 cm line in radio astronomy.

Hydrogen regions of the electromagnetic spectrum may not seem like the most exciting places in the universe. Unlike their more dramatic ionized cousins, these H I regions are composed almost entirely of neutral hydrogen. Far from any star, they emit virtually no visible light, with the exception of spectral lines from trace amounts of heavier elements. They simply lurk in interstellar space, hidden from the human eye.

A colorized version of Figure 1, Yun et al. 1994. On the left is an optical image of the M81 galaxy group, taken by the Palomar Sky Survey. On the right is a map of 21 cm emission from the galaxies, revealing tidal tails of gas that provide evidence for intense past and ongoing intergalactic interactions. Image credit: NRAO/AUI/NSF.

To a radio astronomer, however, H I regions are bright! They emit photons with a wavelength of 21 centimeters, producing a useful spectral feature sensibly known as the 21 centimeter line. This 21-cm radiation is important because it allows astronomers to trace the location of the interstellar gas clouds that make up a large fraction of most galaxies.

In fact, we can thank H I regions and the 21-cm line for part of our early understanding of the structure of our galaxy. More than six decades ago, Oort, Kerr and Westerhout searched for 21-cm emission and showed that the Milky Way indeed has a number of spiral arms, which are traced by neutral hydrogen. Other astronomers had recently shown that a number of bright, massive stars were arranged in a spiral structure, but the neutral hydrogen observations lent even more credence to the idea that the Milky Way was a spiral galaxy.

This early map shows the number density distribution of neutral hydrogen throughout much of the Milky Way. The empty wedges are regions where the differential rotation of the gas is too small to allow accurate determinations of its motion — and, via the Doppler effect, its location in the Milky Way. The Solar System lies approximately midway up the image, 8 kiloparsecs from the center of the galaxy. Image credit: Figure 4, Oort et al. 1958.

Most spectral lines arise from either electrons jumping from one energy state to another or molecules changing vibrational or rotational modes. The 21-cm line, however, is completely different. We see 21-cm emission when a hydrogen atom undergoes a spin-flip transition.

All particles have intrinsic angular momentum vectors called spin, and in a hydrogen atom, the proton and electron can have their spin vectors aligned (a higher-energy state) or anti-aligned (a lower-energy state). Once in a long while, an electron in the higher-energy state will flip its spin from aligned to anti-aligned, emitting a photon in the process. If you do the calculations out, you can show that this photon will have a wavelength of 21 cm.

You might be wondering whether the same sort of “hyperfine” transition can occur in other atoms — perhaps in helium, the second most abundant element in the universe— and you’d be right! Singled-ionized helium-3 is known to undergo a spin-flip transition at 3.46 cm, or 8666 MHz. Although it’s higher-energy than the 21-cm hydrogen line, it’s also much less likely to occur in a given atom. Combine that with the fact that helium is still significantly less common than hydrogen, and suddenly the 3.46 cm line seems like a much less helpful observational tool.

The H II region W51 (data from Koo & Moon 1997). Predmore et al. only put upper limits on the 3.46 cm line, but Rood et al. were able to definitively detect helium in G 49.5–0.4, the dense subregion seen at the lower left. Image credit: Carpenter & Sanders 1998

That doesn’t mean that it’s impossible to use the helium-3 spin-flip line to make some interesting discoveries, though. In the 1960s and 1970s, as the 21-cm line was coming into wider use, astronomers suggested looking for the helium-3 line in H II regions, where hot stars ionize hydrogen — and helium.

In 1971, Predmore, Goldwire and Walters used the 140 foot radio telescope at Green Bank, West Virginia, to probe the H II regions known as M17, W3, W51 and the Orion Nebula. They weren’t able to detect helium-3, but their non-detection still put an upper limit on the ratio of helium to hydrogen in those clouds: approximately 50 parts per million.

Eight years later, another team hit the jackpot. Robert Rood, T.L. Wilson and Gary Steigman used the enormous Effelsberg 100-m Radio Telescope in western Germany, then the largest single-dish fully-steerable telescope in the world (and, as of this writing, still the second-largest). They observed seven sources, overlapping with the Green Bank group, but this time one of their observations was successful. G49.5–0.4, an H II region lying in W51, seemed to produce what the astronomers called a “tentative” 3.46 cm signature. Although it lies 7.5 kpc away from Earth — almost as far as the galactic center — it was bright enough for a detection.

The 100-meter diameter radio telescope in Effelsberg, Germany, remains one of the largest telescopes of any kind in the world. With a collecting area of over 84,000 square feet, you could fit almost one and half football fields inside its dish. Its size almost provides excellent angular resolution — perfect for observing and localizing distant sources. Image credit: Xorx (cc).

We won’t be mapping the Milky Way with hyperfine helium emission anytime soon — there simply isn’t enough helium for it to be feasible, and the line itself is fairly weak. However, it might be a useful tool for peering into the early universe.

In 2009., J.S. Bagla and Abraham Loeb identified several high-redshift sources of 3.46 cm radiation. Hot Lyman-alpha blobs, enormous overdensities of gas several times the size of the Milky Way, are potential sources of hyperfine emission by helium.

Another possibility is H II regions ionized by massive stars in the earliest galaxies. Bagla and Loeb also suggested that quasars could have created regions of double-ionized helium, which would then recombine to form more stable clouds rich in the singly-ionized helium that forms the 3.46 cm line. All of these are promising targets for the next generation of radio telescopes and surveys.

Graham Doskoch is an astrophysics undergrad, currently looking at the skies through gamma-rays and x-rays. He writes about deep sky objects and the future of observational astronomy.

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