Do big star-rich galaxies form slowly over long periods of time, with slow absorption of smaller galaxies and stately formation of stars? Or do they form swiftly when smaller galaxies crash together in a furious burst of star formation? Astrophysicists from the United Kingdom, America, France and South Africa – including the University of the Western Cape’s (UWC) Astrophysics Group – may have the answer in a recent paper published in the 22 May online issue of Nature.
When the source HXMM01 was first identified by the Herschel Space Observatory it seemed to be just one massive, sub-millimetre-bright galaxy. Such a strong sub-millimetre emission (that is, with wavelengths of under a millimetre but larger than visible light) is often due to a single distant galaxy forming stars at a high rate, and possibly to the gravitational lensing of a background galaxy by intervening (foreground) galaxies. However, further observations by several telescopes on the ground and in space as part of a co-ordinated multi-wavelength campaign, showed that it was actually composed of two smaller (though still massive) colliding spiral galaxies, connected by a bridge of material. A careful analysis of the various images showed that the gas from the two galaxies is rapidly coalescing into stars, at a rate of about 2 000 Sun-sized stars formed per year – more than a thousand times the rate of star formation in our own Milky Way galaxy.
By measuring the rate of star formation in the two merging galaxies, as well as the amount of gas that still remained in them (about 200 billion solar masses each), the researchers determined how long it would take before the gas resources were exhausted and the galaxies settled down into one big (super-giant, in fact) elliptical galaxy: around 200 million years.
And while that may sound like a very long time, it’s really very short when compared to the age of the Universe as a whole (13,8 billion years) or even of our own Sun (around 5 billion years). And since HXMM01 is 11 billion light years away, this merger happened during a time when our galaxy was only about 3 billion years old.
“Largely thanks to the launch of two long-wavelength satellites, Nasa’s Spitzer in 2003 and ESA’s Herschel in 2009, one of the most important astrophysical quests over the last ten years has been to try and map these processes as we look back in time,” said Dr Mattia Vaccari, study co-author and SKA Postdoctoral Fellow at UWC’s Astrophysics Group. “Because light takes time to reach us, observing galaxies far away in space allows us to look back into the past phases of galaxies’ lives. We can use the sensitive instruments at our disposal to probe faint distant galaxies and thus see the various stages of their formation.”
These findings may help explain a persistent astronomical mystery – why are there so many large and reddish ellliptical galaxies in the young Universe? For years, scientists have debated how these elliptical galaxies formed from smaller spiral galaxies (such as the Milky Way our solar system inhabits). One hypothesis has it that spiral galaxies slowly grew into ellipticals by slowly absorbing many other galaxies with low star formation rates. Another holds that powerful collisions between galaxies led to increased star formation rates.
This work provides powerful support for the merger/collision idea: it seems that when it comes to star formation, the big galaxies of the early days of the Universe grew up very quickly – and the biggest galaxies may be the result of smash-ups rather than slow accumulation.