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

#### 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?

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#### 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.]

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.

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 [Wikipedia]. 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!

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