Canonical metrics and ambiK\"ahler structures on 4-manifolds with $U(2)$ symmetry

Abstract

For $U(2)$-invariant 4-metrics, we show that the $B^t$-flat metrics are very different from the other canonical metrics (Bach-flat, Einstein, extremal K\"ahler, etc). We show every $U(2)$-invariant metric is conformal to two separate K\"ahler metrics, leading to ambiK\"ahler structures. Using this observation we find new complete extremal K\"ahler metrics on the total spaces of $\mathcal{O}(-1)$ and $\mathcal{O}(+1)$ that are conformal to the Taub-bolt metric. In addition to its usual hyperK\"ahler structure, the Taub-NUT's conformal class contains two additional complete K\"ahler metrics that make up an ambi-K\"ahler pair, making five independent compatible complex structures for the Taub-NUT, each of which has a conformally K\"ahler (1,1) form.