Astronomers have mapped atomic neutral hydrogen across the entire sky, creating an unprecedented portrait of our galaxy and some of its nearest neighbors.

HI4PI maps Milky Way's neutral hydrogen
This map depicts the radiation from neutral atomic hydrogen (HI) across the entire sky, as seen by the Parkes and Effelsberg radio telescopes. Our galaxy, the Milky Way, appears as a luminous band across the sky with the Galactic Center in the middle. The Large and Small Magellanic Clouds are prominently visible in orange below the Galactic plane. They are surrounded by huge clouds of gas, forcefully disrupted from their hosts by gravitational interaction with the Milky Way. The HI emissions of the Andromeda galaxy (M31) and its neighbor, Triangulum (M33), are also easy to spot as bright purple ellipses in the lower left. The gas motion is color-coded to represent the gas's motion, and the visual brightness in the image relates to how much neutral hydrogen is present. Click for bigger image.
HI4PI Collaboration

Hydrogen is the single most abundant element in the universe. The simple pairing of a proton and electron is so reactive that atomic hydrogen doesn’t occur naturally on Earth — it reacts with itself or other elements to form molecules instead. But in the large, mostly empty space between stars floats a copious amount of neutral atomic hydrogen.

We only know it’s there when the atom’s lone electron very occasionally flips from an “up” state to a “down” state, releasing a single 21-centimeter radio wave. Modern radio telescopes can easily pick up the faint signal, which multiplies thanks to the wealth of hydrogen atoms. What’s hard is to map that signal across the entire sky — and that’s exactly what astronomers have done in unprecedented detail.

Using two of the world’s largest fully steerable radio dishes, the 100-meter Effelsberg dish near Bonn, Germany, and the 64-meter Parkes dish west of Sydney, Australia, astronomers have generated a survey they’ve dubbed HI4PI. Pronounced “hi four pie,” the survey refers to the abbreviation for neutral hydrogen (HI) and the geometrical reference to the whole sky (4PI, or 4π).

Over thousands of hours on the sky, the dishes took more than a million individual observations. Thousands more hours went into processing dozens of terabytes of data, removing radio interference from broadcast stations, military radar, and other Earthbound sources, then stitching everything together into a seamless map of the hydrogen sky.

The resulting image is incredible: the ultrafine detail reveals fine threads and diffuse clouds in the interstellar medium never seen before. And for astronomers studying faraway X-ray or gamma-ray sources, the map will prove essential in cleaning the window to the distant universe.

The map shows not only the amount of hydrogen at each celestial location, but also the motion of hydrogen across the sky: the 21-centimeter radio signal shifts to slightly lower or higher energies as the hydrogen gas moves toward or away from us, respectively. Watch the video below to take a stepwise look through the data, seeing first the most negative velocities (infalling gas, moving toward Earth) and ending with the most positive radial velocities (receding gas from Earth’s perspective).

HI4PI data is available to scientists through the Strasbourg astronomical Data Center.


Image of Robert-Casey


October 26, 2016 at 3:21 pm

Looks like you can see the Sun's motion around the galaxy along with the hydrogen clouds. Just to the right of center, looks like the clouds are moving towards us, and to left of center receding away. Extreme left, the clouds are moving towards us, and extreme right, receding away. What you'd see in a system where objects closer to the galaxy's center would orbit faster, and objects further away than we are orbit slower. Though the dark matter halo around the galaxy would make this effect lower amplitude. Looks like the Sun and us is heading to a point halfway to the left.

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Image of Anthony Barreiro

Anthony Barreiro

October 26, 2016 at 7:46 pm

In this equal-area map of the sky, the parts of the Milky Way to the left of galactic center are closer to us than the parts to the right of galactic center. (Remember that we would stitch the edges of this projection to one another to recreate the celestial sphere, so the parts of the galactic central plane that are farthest to the left and farthest to the right are actually right next to one another.) It seems to me that the parts of the galaxy that are closer to us don't have much velocity toward or away from us, while those that are farther away are moving fastest away from us in their orbit around the galactic center. At least that's how I'm thinking about this. I'd be interested to hear from somebody who actually understands this stuff.

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Image of Greg-Trigg


October 29, 2016 at 2:26 pm

I think Venus is a good example. When it passed between us and the sun, it wasn't moving toward or away from us. It's greatest velocity towards us was at its lowest point on the horizon before that and the greatest velocity away would be at the lowest point on the horizon afterwards. Of course, that's an example of a body that is closer to the center of rotation not one such as Mars.

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Image of Bob


October 31, 2016 at 12:34 am

Thousands of hours, million observation, terabytes of data. Quite impressive.

Thanks for the article.

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