Bifurcation measures and quadratic rational maps

We study critical orbits and bifurcations within the moduli space of quadratic rational maps on $\mathbb{P}^1$. We focus on the family of curves, $Per_1(\lambda)$ for $\lambda$ in $\mathbb{C}$, defined by the condition that each $f\in Per_1(\lambda)$ has a fixed point of multiplier $\lambda$. We prove that the curve $Per_1(\lambda)$ contains infinitely many postcritically-finite maps if and only if $\lambda = 0$; addressing a special case of [BD2, Conjecture 1.4]. We also show that the two critical points of a map $f$ define distinct bifurcation measures along $Per_1(\lambda)$.