Method used to compute the formation redshift. The observed redshift is z_obs and the age of the Universe at this redshift is given by A_U(z_obs). The age of the galaxy measured by GOSSIP+ is Age(GOSSIP+). A_U(z_f) is the age of the Universe at z_f. The black curve represents the age of the Universe as a function of redshift in the ΛCDM model. From the measurement of z_obs and the age given by GOSSIP+, we can infer the formation redshift by successive iterations (see text).