It is rare to see a galaxy being born. One that is forming from gas ripped out of another galaxy is in our back yard. This baby galaxy was hiding in plain sight because it was so well studied.
The discovery was made by a telescope that is not really a telescope in the way you think of them.
A collection of 48 Canon telephoto lens with very sensitive detectors cover a whopping six square degrees of the sky, thirty times the size of the full Moon, and are part of the Dragonfly Array. The advantage of this is that it sees a very large area of sky but can still take really deep images; that is, see very faint detail, and do so at decent resolution. It is less expensive than creating a telescope with a single lens.
The recent discovery of two small galaxies that appear to lack dark matter was made using Dragonfly, and I have written about it a couple of times. The cameras allowed them to see light from across the spectrum, from blue to red.
A new idea is being tested by the team behind Dragonfly. They have created a version of the array using just three lens, but they have a special filter that only allows a very narrow range of colors to be seen. Hydrogen gas in space can be excited by several processes, including hot stars, slamming into other clouds and so on. This gas emits light at a wavelength called H-alpha which is in the red part of the spectrum.
A filter designed to look at just that wavelength weeds out a lot of extraneous light and sees hydrogen gas, which is much fainter since most of their light is being rejected by the filters). A wide-field extremely sensitive telescope that sees hydrogen light can be used to look for star formation and other interesting things in nearby galaxies, but can make sure it sees what is happening all over the place.
This gets fun at that point. The M 81 group is one of the closest in the sky to the Earth, just 12 million light years away. M83 is a weird spiral. You would never guess that it is a spiral galaxy, even though it is seen nearly edge-on. It looks like it is exploding. It is, in fact, a starburst galaxy, one undergoing a very intense episode of star birth. The stellar winds are blowing gas out of the galaxy as many massive stars are being born in the center.
A few hundred million years ago, a close pass with nearby M 81 drew a lot of gas out from M82, the gravity of the bigger galaxy created long streamers from M82 called tidal tails.
The region of the sky is a perfect target for the test version of the Dragonfly array. In the spring of 2020 they made hundreds of observations and created an H-alpha image with a total exposure of 95 hours.
The first emission of H-alpha discovered by the small array was at the edge of the disk. It is very likely to be gas condensing after being torn away because it overlaps one of the tidal tails. The light of 25,000 Suns shines in the narrow color of H-alpha, which is 1000 light years across, and very bright.
They looked at Hubble and GALEX to see what the sky looked like after a hot star. They found small knots in the gas that they think are young star clusters. Radio telescopes show a lot of cold atomic and hydrogen gas there, which indicates that the gas is cooling and forming stars.
The velocities of the two objects were determined by using the large telescope in Hawaii, and they found that the one with the faster speed is very likely not bound to the other. We call it kinematically decoupled if it is independent of M82.
The mass in gas is 50 million times greater than the Suns mass. It's likely that DF-E1 is a galaxy of its own, if a small one.
What will happen to this shiny new galaxy? It may fall back onto M82 since it is not perfectly determined. It will most likely join the other small galaxies in the M 81 Group if it doesn't.
The beauty here is that it will be easy to observe and examine, because it is so close to us. It is a good bet to assume that many more telescopes will be used to look at it and see what we can learn from it. We can watch it as it happens, because it is a brand new baby galaxy.
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