Class numbers of cyclotomic function fields

Let $q$ be a prime power and let ${\mathbb F}_q$ be the finite field with $q$ elements. For each polynomial $Q(T)$ in ${\mathbb F}_q [T]$, one could use the Carlitz module to construct an abelian extension of ${\mathbb F}_q (T)$, called a Carlitz cyclotomic extension. Carlitz cyclotomic extensions play a fundamental role in the study of abelian extensions of ${\mathbb F}_q(T)$, similar to the role played by cyclotomic number fields for abelian extensions of ${\mathbb Q}$. We are interested in the tower of Carlitz cyclotomic extensions corresponding to the powers of a fixed irreducible polynomial in ${\mathbb F}_q [T]$. Two types of properties are obtained for the $l$-parts of the class numbers of the fields in this tower, for a fixed prime number $l$. One gives congruence relations between the $l$-parts of these class numbers. The other gives lower bound for the $l$-parts of these class numbers.