The Hubble Deep Field is an emblematic image taken by the Hubble Space Telescope in visible light, which shows that in a rather 'dark' region of the sky, you can observe with the telescope a large number of galaxies whose existence was previously unknown. Most of the galaxies detected with Hubble in this image have a redshift of less than 1.5, which indicates that they are rather 'close'.
Most celestial bodies radiate energy at various wavelengths, including those in the millimeter and submillimeter wavelength, which ALMA can capture. On the other hand, due to the expansion of the Universe, emissions from more distant objects reach us on wavelengths changed towards the red. This phenomenon is known as redshift. In order to measure it, astronomers use an index (z).
The lower line shows that with an optical image, such as the Hubble telescope’s deep field, most detections are of galaxies within z<1,5. Unlike optical images, around 80% of galaxies detected by ALMA are situated at high redshifts.