Some quasars show several sets of absorption lines whose redshifts are less than the redshift of the quasars’ emission lines. For example, the quasar PKS XXXXXXXXXXhas five sets of absorption lines,...

Some quasars show several sets of absorption lines whose redshifts are less than the redshift of the quasars’ emission lines. For example, the quasar PKS 0237-23 has five sets of absorption lines, all with redshifts somewhat less than the redshift of the quasar’s emission lines. Propose an explanation for these sets of absorption lines.