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.

Images of Green Peas

Examples of the green pea galaxies originally discovered in Galaxy Zoo. Top Row: Sloan Digital Sky Survey images. Bottom Row: Hubble Space Telescope images.

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.


Diagram illustrating how individual galaxies may have re-ionized the neutral hydrogen in the Universe shortly after the Big Bang. From Erb (2016).

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!

About Kyle Willett

Kyle Willett is a postdoc and astronomer at the University of Minnesota. He works as a member of the Galaxy Zoo team, and gets to study galaxy morphology and evolution, AGN, blazars, megamasers, citizen science engagement, and many other cool things.

6 responses to “Green pea galaxies may have been responsible for re-ionizing the Universe”

  1. sisifolibre says :

    great! very exciting! have read somewere that green peas are Wolf Rayet galaxys, I’m right? can help in this way the study of more near WR stars and galaxies. Will be needed a search of Green Peas in z=6?

    • Kyle Willett says :

      Excellent memory. There have been 1 or 2 papers studying this; I would characterize the conclusions as “tentative” so far. Features in the spectra of green peas that would be consistent with Wolf-Rayet stars have been found, but at relatively low significance (only in the stacked spectra). I don’t think we know yet for sure, although Wolf-Rayet stars are a possibility. See if you want to look at the original paper.

      z=6 galaxies of any kind are quite challenging, although we’re getting better with telescopes like Hubble. Galaxy Zoo hasn’t had any images yet going out to that high of a redshift, largely because the numbers of galaxies are so small that they can still be tackled by individual science teams.

      • Peter Dzwig says :

        Is there any way that we can use the numbers available out to say z=4?

        If I recall correctly we had z~3 in GZH, and it’d be interesting to see what changes happen in morphology in that range and how consistent that is with what we have seen to date.

  2. Kyle Willett says :

    Hi Peter,

    Thanks for your interest. I’m not sure I quite understand the question – could you say more about what you mean by “use the numbers available”?

    • Peter Dzwig says :

      Sorry very poorly expressed!

      What I meant was: can we use other sources of data and/or data that we have elsewhere in the surveys to take the search out perhaps as far as z = 4 (say?).


      • Kyle Willett says :

        Yes, definitely. Going out to redshifts of z=4 basically means using Hubble. There are two basic ways of finding the powerful oxygen 5007 A lines that made them appear green in the SDSS bands. The first way would be to use narrow photometric bands, just like we did for SDSS – at high redshift, though, the 5007 A line will be Doppler shifted into longer wavelength bands as viewed on Earth. So you’d need to a) know the redshift of the galaxy, and then b) search for galaxies with multiple narrow-band photometric measurements and find candidates that have an excess of flux at rest-frame 5007.

        The second way would simply be to search galaxies that have been observed spectroscopically with Hubble (significantly fewer targets, since that’s harder) and make some sort of cut on the OIII/continuum flux.

        Surveys such as CANDELS and COSMOS are good candidates for exploring this further.

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