An enormous tail of gas 300,000 light-years long has been detected trailing behind a galaxy. The hydrogen tail is three times the length of our own galaxy and five times longer than NGC 4569, the galaxy from which it comes. The find explains a mystery – why is NGC 4569 so depleted in hydrogen gas?
At 55 million light-years away, NGC 4569 (Messier 90) is relatively close by galactic standards, and is part of the famous Virgo cluster of galaxies. It is notable both because it is moving at 1,200 kilometers (745 miles) per second relative to that huge cluster as a whole, and because it is highly depleted in gas.
Astronomers suspected that the two facts were related, with NGC 4569 being stripped of gas in its travels. As a paper in Astronomy and Astrophysics noted, “Galaxies in rich environments are systematically different from those located in the field.” One common difference is reduced concentrations of hydrogen gas, indicating that something in the cluster strips gas from passing galaxies.
Cortese and colleagues think they have changed this. “Now, with these observations, we’ve seen a huge amount of gas that creates a stream trailing behind the galaxy for the first time,” he said. “What’s very nice is that if you measure the mass of the stream, it’s the same amount of gas that is missing from the galaxy’s disc.”
Detecting the missing hydrogen gas was a challenge because, left behind from its galaxy, there is nothing to light it up. Nevertheless, Cortese and his colleagues were able to observe it using the Canada France Hawaii Telescope, equipped with a filter to detect H-alpha radiation from ionized hydrogen.
The ionized gas is estimated to have a mass equal to 3.2 billion times that of the Sun, but no star-forming regions have been detected. This eliminates high-energy radiation from stars as a mechanism for the gas' ionization. The authors suggest several other possible ionization processes, with some evidence in favor of powerful magnetic fields.
One popular theory is that galaxies in clusters lose gas because the gravitational pull of large galaxies steals material from those that get too close. In this case, however, the authors favor the alternative explanation, blaming gas concentrations between galaxies within the cluster.
“We know that big clusters of galaxies trap a lot of hot gas,” said Cortese. “So when a galaxy enters the cluster it feels the pressure of all the gas, like when you feel the wind on your face, and that pressure is able to strip matter away from the galaxy.” The faster a galaxy like NGC 4569 is traveling compared to the gas within the cluster, the more pressure it will experience, forcing unbound hydrogen to trail behind it.