  Pair instability supernovae are thought to restrict the formation of black
holes in the mass range ~50 - 135 solar masses. However, black holes with
masses within this "high mass gap" are expected to form as the remnants of
binary black hole mergers. These remnants can merge again dynamically in
densely populated environments such as globular clusters. The hypothesis that
the binary black hole merger GW190521 formed dynamically is supported by its
high mass. Orbital eccentricity can also be a signature of dynamical formation,
since a binary that merges quickly after becoming bound may not circularize
before merger. In this work, we measure the orbital eccentricity of GW190521.
We find that the data prefer a signal with eccentricity $e \geq 0.1$ at 10 Hz
to a non-precessing, quasi-circular signal, with a log Bayes factor $\ln{\cal
B}=5.0$. When compared to precessing, quasi-circular analyses, the data prefer
a non-precessing, $e \geq 0.1$ signal, with log Bayes factors $\ln{\cal
B}\approx2$. Using injection studies, we find that a non-spinning, moderately
eccentric ($e = 0.13$) GW190521-like binary can be mistaken for a
quasi-circular, precessing binary. Conversely, a quasi-circular binary with
spin-induced precession may be mistaken for an eccentric binary. We therefore
cannot confidently determine whether GW190521 was precessing or eccentric.
Nevertheless, since both of these properties support the dynamical formation
hypothesis, our findings support the hypothesis that GW190521 formed
dynamically.
