October 17, 2011 | 1
If you’re reading this at night, look outside. Even in a city you’ll be able to see a few stars, if it’s not too cloudy and your eyes are up to it. If you’re lucky, the view from your window or garden will include a whole host of stars. Either way, you will be looking out on the vast blackness of space and seeing pinpricks of light that have traveled for millions of years before reaching your eyes.
This is only possible because of something that happened in the early universe. Shortly after the Big Bang, the universe quickly filled with a glowing plasma of mainly hydrogen and helium. As it expanded and cooled, atomic nuclei and electrons combined to make a dense fog of neutral gas. At this point, photons — particles of light — were freed and became the Cosmic Microwave Background Radiation that astronomers are studying today. But we’re not interested in that. It’s the opaque fog the photons left behind that would get in the way of seeing stars. Of course, it doesn’t, because it no longer exists — so where did it go?
Reionisation is the name of the process that took the universe out of the cosmological dark ages and into the light. In all, the dark ages lasted about half a billion years. They were ended by the formation of the first galaxies.
Astronomers believe that star-forming galaxies are the most likely suspects in the search for the source of radiation that was needed to clear away the cosmic cobwebs — or, in more technical terms, turn the neutral intergalactic medium, composed mainly of hydrogen, into the charged plasma that it is today. Until now, though, astronomers weren’t sure how the radiation could have got out of the star-formaing galaxies in which it was created.
The galaxies that astronomers think are the source of the radiation needed to reionise the universe are known as starburst galaxies. Unsurprisingly, this is because they go through intense bursts of star formation. Starburst galaxies are rare today, but astronomers think they were much more common in the early universe. One starburst galaxy in particular, NGC 5253 — a dwarf starburst not too far away, cosmologically speaking — has been helping astronomers with their inquiries into exactly how the radiation required to clear the early universe made its escape.
NGC 5253 has just undergone an episode of star formation that resulted in huge clusters of massive, young stars. Massive stars can create a wind that expands out through the galaxy. As it goes, the wind removes any obstacles in the way of radiation created by the stars, allowing it to pour out of the galaxy along the same path.
It’s ultraviolet light which is the radiation under scrutiny, so Jordan Zastrow, a graduate student at the University of Michigan, and her colleagues looked for signs of ultraviolet light coming from NGC 5253. They didn’t look for the light itself, but its interaction with other gas close to the galaxy.
In a paper published in Astrophysical Journal Letters last week Zastrow and her colleagues describe results that show ultraviolet light is emanating from the galaxy in a narrow cone, and making gas in the interstellar medium evaporate as it goes. Outpourings like this one could be what started reionisation.
Zastrow and her colleagues cannot completely rule out an active galactic nucleus as the source of the radiation. Astronomers think active galactic nuclei are caused by the accretion of mass on to supermassive black holes at the centre of a galaxies. AGN sometimes eject radiation, which could look a lot like what Zastrow and her colleagues see around NGC 5253. But Zastrow thinks massive stars are much more likely to be causing the outpouring in this case.
That the radiation escapes the starburst galaxy in a narrow cone is important, because it means that, unless it is oriented in the right way, we won’t be able to tell whether or not a starburst galaxy is emitting ultraviolet light. As well as explaining why we have not seen this phenomenon before, this could mean that NGC 5253 may be the only nearby starburst galaxy that can teach us something about how reionisation began.
Jordan Zastrow, M. S. Oey, Sylvain Veilleux, Michael McDonald, & Crystal L. Martin (2011). An Ionization Cone in the Dwarf Starburst Galaxy NGC 5253 Asrophysical Journal Letters arXiv: 1109.6360v1