Green pea galaxies may have been responsible for re-ionizing the Universe
The “green pea” galaxies were one of the first discoveries of the Galaxy Zoo; they were first noticed by several of our early volunteers, and appeared in a paper led by Carie Cardamone in 2009 (with over 100 citations so far!). They’ve been the subject of a great deal of follow-up research since then, much of which we’ve tried to follow on this blog.
A new paper on the Green Peas has just appeared in Nature, one of the most prestigious and widely-read journals in science. A truly international team of researchers (working in Ukraine, Czech Republic, Switzerland, France, Germany, and the United States) made observations of one green pea galaxy, known as J0925+1403, using an ultraviolet spectrograph on the Hubble Space Telescope. They were able to measure emission from what astronomers call “Lyman continuum” photons; this is light produced by massive stars that are solidly in the ultraviolet wavelengths.
The reason this is so important and interesting relates to one of the most fundamental steps in the history of the Universe that astronomers know of. The majority of matter in the Universe is hydrogen (formed shortly after the Big Bang), and much of it exists in diffuse clouds between galaxies, which is called the intergalactic medium. We know from observations that almost all of that hydrogen is currently ionized – that means instead of consisting of a neutral atom with one proton and one electron orbiting it, the average hydrogen atom between galaxies has had its electron stripped away from the proton. This is a big difference because neutral atoms interact with light differently than ionized atoms. If the hydrogen between galaxies were neutral, it would absorb much of the light coming from individual stars and galaxies, making a huge difference in our ability to observe distant objects.
It’s been known for years the Universe is currently ionized; however, about 700 million years after the Big Bang, we know that the Universe used to be neutral. That’s pretty well-established — however, there’s a great deal of debate about what caused the sudden reionization. Something must have produced large numbers of photons that traveled into the intergalactic medium and ionized all of the hydrogen fairly quickly. There have been lots of papers proposing different possible sources for this, including dwarf galaxies, active galactic nuclei, quasars, very early and massive stars, etc.
This new paper proposes that green pea galaxies could be responsible for re-ionizing the early Universe. The measurements from this paper show that at least one green pea galaxy is actively emitting photons with sufficient energy to ionize neutral hydrogen. Lots of galaxies can create such radiation, but one unique aspect of the peas is that the photons are escaping the galaxy where they’re being formed. Usually they’re absorbed by dust or gas clouds within the galaxy before they can affect the rest of the Universe. This is the first time that it’s been demonstrated to occur for a green pea galaxy.
The paper (Izotov et al. 2016) is available online. Nature has also published a nice summary at a slightly less technical level to accompany the article that I’d recommend – you can read that here. Please post if you have any questions or want to discuss more about what this means. We’re extremely excited that your discoveries are still yielding new and interesting science!