James Webb Space Telescope finds tiny early galaxy packing big star-forming punch

Astronomers used the James Webb Space Telescope to look more than 13 billion years into the past to discover a unique, minuscule galaxy that could help astronomers learn more about galaxies that were present shortly after the Big Bang. (Image credit: ESA/Webb, NASA & CSA, P. Kelly)

The James Webb Space Telescope (JWST) has discovered a tiny galaxy in the early universe that is growing rapidly as it forms stars at a tremendous rate, revealing more about the progenitors of galaxies such as our own.

The galaxy, referred to as RX J2129-z95, is seen at a redshift of 9.51. That number, which refers to the extent to which the galaxy's light has been stretched by the expansion of the universe, means that we are seeing it as it existed just 510 million years after the Big Bang.

Because it is so distant, RX J2129-z95 is exceptionally faint. However, its light received a boost from the gravitational-lensing effect of a massive foreground galaxy cluster called RX J2129.6+0005, which is located about 2.5 billion light-years from Earth along the same line of sight. The gravity of the 150-trillion-solar-mass cluster amplified the light of RX J2129-z95, as well as splitting it into three images.

Related: 12 amazing James Webb Space Telescope discoveries

The galaxy's redshift was confirmed by JWST's NIRSpec (Near-Infrared Spectrometer) instrument, which also detected strong emission from hydrogen and oxygen gas clouds within RX J2129-z95. These emission lines in the galaxy's spectrum have revealed some of RX J2129-z95's extraordinary properties.

For example, RX J2129-z95 is just 105.6 light-years across, which is tiny compared to the 100,000 light-year-diameter of our Milky Way galaxy or even modern dwarf galaxies that span several thousand light years. Yet despite having a volume a thousand times less than the Milky Way, RX J2129-z95's rate of star formation is the same as our galaxy's, meaning it is much more intense.

JWST is finding that such high star-formation rates are a fairly typical trait of galaxies in the early universe, but RX J2129-z95 is extreme even as far as high-redshift galaxies go. 

"The star-formation rate is similar to other high-redshift galaxies confirmed with NIRSPec, but the radius of the galaxy is at least three times smaller than those other galaxies," said Hayley Williams, a PhD student at the University of Minnesota who led the research, in an interview with Space.com. "This means that a ton of star formation is packed into a very tiny volume."

The fact that all these early galaxies are small, despite their high luminosity, favors the popular model of hierarchical galaxy formation, which predicts that small galaxies formed first, before merging and growing into larger galaxies such as the Milky Way.

Williams' team also found no evidence for an active supermassive black hole at the center of RX J2129-z95. This could be important, since RX J2129-z95 exists near the end of the "cosmic dark ages," when radiation ionized most of the vast ocean of neutral hydrogen gas that filled the universe. One of cosmology's biggest questions has been what led to the end of the dark ages — was it radiation from hungrily growing black holes, or from massive bouts of star formation? If RX J2129-z95 is typical, then the lack of an obvious black hole could be pointing toward radiation from plentiful hot, young stars as being the prime mover in ending the dark ages.

So far, the amount of ionizing radiation being produced by RX J2129-z95 has not been quantified. However, "if there is a substantial population of similar galaxies, then perhaps they play a key role in the ionizing photon budget," said Williams. 

More observations of galaxies at similar redshifts are required, but with a handful of very high-redshift luminous galaxies already confirmed by JWST and many more such finds likely to follow, astronomers will soon have a large sample to draw from.

Until then, "it is difficult to make conclusions about these things with such a small sample size," said Williams.

The findings were published on April 13 in the journal Science.

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Keith Cooper
Contributing writer

Keith Cooper is a freelance science journalist and editor in the United Kingdom, and has a degree in physics and astrophysics from the University of Manchester. He's the author of "The Contact Paradox: Challenging Our Assumptions in the Search for Extraterrestrial Intelligence" (Bloomsbury Sigma, 2020) and has written articles on astronomy, space, physics and astrobiology for a multitude of magazines and websites.

  • rod
    This is a very tiny angular size galaxy reported :) My calculations indicate perhaps 4.5 mas angular size. Consider the article says it is about 2.5 billion light years distance (about 767 Mpc).

    Ref cited - https://www.science.org/doi/10.1126/science.adf5307, "...The measured redshift is z = 9.51 ± 0.01, corresponding to 510 million years after the Big Bang. The galaxy has a radius of 16.2+4.6−7.2 parsecs, substantially more compact than galaxies with equivalent luminosity at z ~ 6 to 8, leading to a high star formation rate surface density."

    There is the comoving radial distance for this object today where space expands much faster than c velocity and we do not know what the object really looks like at those distances. However, what galaxy gets the smallest angular size seen in the sky and smallest diameter based upon estimated distance? Perhaps this galaxy :)

    Correction. The radius is about 16.2 pc so diameter near 32-33 pc. Angular size about 9 mas at 767 Mpc. With a diameter of about 32-33 pc for this galaxy, what is the average size of open star clusters in the MW and globular clusters? :)
  • rod
    Another correction here after more review :) The lensing object is about 2.5 Gly distance so the galaxy angular size is smaller than 9 mas. Cosmology calculators indicate look back distance is 13.21 Gly for z=9.51, so a galaxy about 32-33 pc diameter is very tiny angular size. The look back distance is about 4.05 x 10^9 pc and the galaxy angular size about 1.65 mas. Google says the average size of globular clusters in the MW is about 20-22 pc diameter. This tiny galaxy is only a bit larger than many GCs seen in the MW today.]
  • Torbjorn Larsson
    Good to see that gravitational lensing helps so soon! The earliest galaxies seems to be earlier than expected and even Webb is a bit stretched to cope with this important cosmological era.
  • Torbjorn Larsson
    rod said:
    The look back distance is about 4.05 x 10^9 pc and the galaxy angular size about 1.65 mas. Google says the average size of globular clusters in the MW is about 20-22 pc diameter. This tiny galaxy is only a bit larger than many GCs seen in the MW today.]

    The gravitational lensing is nonlinear and the paper report their estimate from two different methods. G2 is one of the three lensed images, and the supplement tells us they estimate a magnification of 20.2 ± 3.8. A linear estimate is then that the angular size is ~ 0.04/20 or 2 mas.

    We use the Lenstruction software (24, 25) to reconstruct the F150W image of G2, correcting for the effects of gravitational lensing and the NIRCam point spread function (PSF). We fitted the reconstructed image with a surface brightness model, consisting of an elliptical Sèrsic profile with index n fixed to 0.5 (n determines the degree of curvature of the profile, with n = 0.5 being a Gaussian profile). This indicates the intrinsic half-light radius of the reconstructed source is Re,intrinsic=16.2+4.6−7.2�e,intrinsic=16.2−7.2+4.6 parsecs (pc) (Figure S7). We also fitted the observed F150W image directly, using the Galight software (26), which indicates an observed angular size of θe,observed = 0.04 ± 0.01 arcseconds (21).

    Comparing roughly the same measures, the half-light radius of galaxy clusters distributes closer to less than 10 pc in consistency with what you claim but tails off at 25 pc . An old hypothesis is that GCs are remnants from early galaxy formation, and it is a likely prior that this first find is biased large despite the random lensing factor but is still a feasible candidate compared to a 50 pc diameter tail end. (And then we have the errors and differences between their estimate and half-radius estimate methods et cetera.) A nice observation!