Young Galaxies Are Surrounded by Huge ‘Atmospheres’
New discoveries

Young Galaxies Are Surrounded by Huge ‘Atmospheres’

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Planet Earth is surrounded by an atmosphere – a thin layer of gas. Some other planets, and most stars, also have atmospheres. In many cases, they’re much more extended than the atmosphere of Earth. ALMA has now discovered two young galaxies that really take the biscuit: their ‘atmospheres’ extend for tens of thousands of light-years beyond their visible edges. By the way, they don’t consist of oxygen and nitrogen, like the Earth’s atmosphere. Instead, it’s all cold hydrogen gas.

The huge atmospheres of the remote galaxies are known as ‘super-halos’. These huge clouds of cool hydrogen gas are backlit by galaxies that are even further away. The background galaxies have very luminous cores; they are known as quasars. On its way to Earth, the quasar light passes through the gas cloud, just like a distant street light may shine through an intervening layer of fog. That’s how astronomers already knew about the existence of the ‘super-halos’.

However, the galaxies themselves had not yet been seen. That’s because they’re much fainter than the bright quasars, which appear nearby on the sky – it’s like trying to see a candle flame in bright daylight. ALMA has now detected the galaxies for the first time, not in visible light but at millimeter wavelengths. The radiation that ALMA sees is emitted by star-forming regions inside the main disks of the galaxies.

Surprisingly, the galaxies were not seen very close to the background quasars, as astronomers expected. Instead, they appeared quite offset. This can only mean one thing: the surrounding cloud of hydrogen gas – the galaxy’s ‘atmosphere’ must be huge – otherwise it wouldn’t be backlit by the quasar.

The astronomer does not know if the hydrogen gas is in the form of a real halo (surrounding the galaxy on all sides) or in the form of an extended flattened disk. Either way, the amount of hydrogen gas must be huge.

Because the galaxies are at some 12 billion light-years from Earth, they are seen as they were 12 billion years ago, when the Universe was still in its infancy. In the future, the gas from the ‘super-halos’ may fall into the galaxies, causing them to grow.


The galaxies discovered by ALMA are now known by their new catalog numbers: ALMA J081740.86+135138.2 and ALMA J120110.26+211756.2. The first one is in the constellation Ophiuchus, the Serpent Bearer; the second one is in a small constellation known as Coma Berenices (Berenice’s Hair). Both galaxies are located at some 12 billion light-years from Earth. In other words: their light took some 12 billion light-years to reach us, so we see them as they were 12 billion years ago, when the Universe was only 8 percent of its present age. The background quasars are even further away: some 12.5 billion light-years.


The new ALMA observations were carried out by a team of astronomer led by Marcel Neeleman of the University of California at Santa Cruz. Marcel worked together with Xavier Prochaska, also at Santa Cruz, and with Nissim Kanekar (India), Marc Rafelski, Chris Carilli, and Arthur Wolfe (all three from the United States). Unfortunately, Arthur Wolfe died before the team published their results in the weekly magazine Science.