![]() The galaxies are linked by bridge of gas, indicating that they are merging. Follow-up observations showed that it is in fact two galaxies, each boasting a stellar mass equal to about 100 billion Suns and an equivalent amount of gas. The galaxy pair was initially identified in the Herschel data as a single bright source, named HXMM01. In a new study using Herschel data, astronomers have captured the onset of this process between two massive galaxies, seen when the Universe was just 3 billion years old. One idea is that two spiral galaxies might collide and merge to produce a vast elliptical galaxy, with the collision triggering such a massive burst of star formation that it would rapidly deplete the gas reservoir. Somehow, on short cosmological timescales, these galaxies had rapidly assembled vast quantities of stars and then ‘switched off’. So the discovery in the last decade that very massive elliptical galaxies had managed to form during just the first 3–4 billion years of the Universe’s history posed something of a conundrum. The theory held that the gas in those galaxies would gradually be converted into cool, low-mass stars, so that by today they would have exhausted all of their star-forming material, leaving them ‘red and dead’. It was long assumed that the large elliptical galaxies seen in the Universe today built up gradually over time via the gravitational acquisition of many small dwarf galaxies. Most large galaxies fall into one of two major categories: spirals like our own Milky Way that are full of gas and actively forming stars, or gas-poor ellipticals, populated by old cool red stars and showing few signs of ongoing star formation.
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