Analysis shows that these small galaxies are indeed themselves frequently merging to form large galaxies.
An image of this field with the Spitzer Space Telescope shows a lack of infrared emission that would be expected from old stars, indicating that these small galaxies are very young, possibly only a few million years old.
Part of the HUDF is shown above, while one blue building-block galaxy, highly redshifted by the universe so as to appear more yellow, is shown in the upper left inset.
