Convergence of a radial singular integral

Let $D = \{(x, y) \in \mathbb{R}^2 : 0 < x^2 + y^2 \le 1\}$. For $\alpha \ge 0$, consider the integral $I_\alpha = \iint_D \frac{1}{(x^2 + y^2)^{\alpha}} \, dx \, dy$. Let $N_0 = \sup\{\alpha \ge 0 : I_\alpha < \infty\}$. Then the value of $N_0$ equals ________ (answer in integer).