Maximal ideal space of a quotient or Banach subalgebra Let $\mathcal{A}$ be a commutative unital Banach algebra, $\mathcal{B} \subset \mathcal{A}$ a closed unital subalgebra, $\mathcal{I} \subset \mathcal{B}$ a closed ideal.
Is there in general a way to "identify" (soft question, perhaps) the maximal ideal spaces  $\Sigma(\mathcal{B})$ and $\Sigma(\mathcal{A}/\mathcal{I})$, in terms of $\Sigma(\mathcal{A})$ and possibly some sort of other data?  A couple of examples of the sort of thing I have in mind:


*

*If $\mathfrak{X}$ is a Banach space and $M \subset \mathfrak{X}$ a closed subspace, then $M^* \simeq \mathfrak{X}^*/M^\perp$ and $(\mathfrak{X}/M)^* \simeq M^\perp$, where 
$$
M^\perp = \{f \in \mathfrak{X}^* \mid \forall m \in M: \, f(m) = 0\}.
$$

*In the special case where $\mathcal{A}$ is a $C^*$-algebra, the contravariant equivalence of categories with (compact Hausdorff spaces, continuous maps) implies that $C^*$-subalgebras of $\mathcal{A}$ correspond to quotients of $\Sigma(\mathcal{A})$, and quotients of $\mathcal{A}$ correspond to closed subspaces of $\Sigma(\mathcal{A})$. 


Are there some sort of analogous relationships with commutative Banach algebras?  An example: Viewing the disc algebra $A(\mathbb{D})$ as a closed subalgebra of $C(\mathbb{T})$, the above analogies might lead us to expect that $\Sigma(A(\mathbb{D}))$ is a quotient of $\Sigma(C(\mathbb{T}))$.  But $\Sigma(A(\mathbb{D})) \simeq \overline{\mathbb{D}}$ while $\Sigma(C(\mathbb{T})) \simeq \mathbb{T}$, so it looks like in this case the relationship is a subspace rather than a quotient (and going the other direction).
 A: Placeholder until I can come back with a more thought-out answer.
Take B to be the Jacobson radical of A to see that the natural map from max ideal space of A to that of B need not be injective.
Take B to be the disc algebra and A to be C(T), as you did, to see that said map need not be surjective.
As Jonas has mentioned in his comment, the natural map from max ideal space of A/I to that of A will be injective with closed range.
In the non-unital setting, note that one can have commutative Banach algebras with trivial Jacobson radical which quotient onto radical Banach algebras. The standard example is the Volterra algebra arising as a quotient of the convolution algebra L^1(R_+).
A: I'm not really sure what you're asking. $\Sigma$ is still a contravariant functor from commutative Banach algebras to compact Hausdorff spaces (it just isn't an equivalence), so from the sequence of morphisms
$$B \to A \to A/I$$
you get a sequence of morphisms in the other direction
$$\Sigma(A/I) \to \Sigma(A) \to \Sigma(B)$$
but I don't think there's much you can say anything in general about the corresponding morphism $\Sigma(A/I) \to \Sigma(B)$ without more information. 
A: Thanks everyone.  Here's what I understand so far:


*

*Regarding the maximal ideal space of a quotient, one has (in the unital case) the identification
$$
\Sigma(\mathcal{A}/I) \simeq \text{hull}(I) = \{\omega \in \Sigma(\mathcal{A}) \mid \omega = 0 \text{ on } I\}.
$$

*Regarding the maximal ideal space of a subalgebra, things aren't as clean.  Denote by $E(\mathcal{B}) \subseteq \Sigma(\mathcal{B})$ the subspace of homomorphisms which are extendible to $\mathcal{A}$, and by $\sim$ the equivalence relation on $\Sigma(\mathcal{A})$ induced by restriction to $\mathcal{B}$.  Then
$$
E(\mathcal{B}) \simeq \Sigma(\mathcal{A})/\sim.
$$
Some examples:


(1) $\mathcal{A} = C(\mathbb{T})$, $\mathcal{B} = A(\mathbb{D})$ shows that $E(\mathcal{B})$ can be a proper subspace of $\Sigma(\mathcal{B})$.  The multiplicative linear functionals on $\mathcal{B}$ correspond to evaluation at points in the closed disc $\overline{\mathbb{D}}$, but only those corresponding to points in $\mathbb{T}$ are extendible to $\mathcal{A}$.
(2) $\mathcal{B}$ could be the Jacobson radical of $\mathcal{A}$, showing that $\sim$ can be nontrivial.  
(3) Another (unital) example where $\sim$ is nontrivial is $\mathcal{A} = C(K)$ and $\mathcal{B} = C(K/\approx)$ where $\approx$ is a (nontrivial) equivalence relation on the compact Hausdorff space $K$.  Then $\sim$ is the same as $\approx$, modulo the identification of $K$ with $\Sigma(C(K))$ and $K/\approx$ with $\Sigma(C(K/\approx))$.  Forgive my sense of humor, but I can't pass up the opportunity to write $\sim \simeq \approx$ and have it almost mean something.
(4) Let $\mathcal{B} \subseteq A(\mathbb{D})$ be the subalgebra generated by $z^2$, i.e. the functions whose odd Taylor coefficients are all zero.  Then $\sim$ is the antipodal equivalence on $\Sigma(\mathcal{A}) \simeq \mathbb{T}$, and $\Sigma(\mathcal{B})$ is the quotient of the disc under the antipodal map.  In this example we have both that $E(\mathcal{B})$ is properly contained in $\Sigma(\mathcal{B})$, and that $\sim$ is nontrivial.
Rather elementary considerations, but I hadn't really thought through them before.  Thanks for your patience.
