  Gaia Data Release 2 revealed a population of ultramassive white dwarfs on the
Q branch that are moving anomalously fast for a local disc population with
their young photometric ages. As the velocity dispersion of stars in the local
disc increases with age, a proposed explanation of these white dwarfs is that
they experience a cooling delay that causes current cooling models to infer
photometric ages much younger than their true ages. To explore this
explanation, we investigate the kinematics of ultramassive white dwarfs within
200 pc of the Sun using the improved Gaia Early Data Release 3 observations. We
analyse the transverse motions of 0.95 - 1.25 $M_\odot$ white dwarfs,
subdivided by mass and age, and determine the distributions of the
three-dimensional components of the transverse velocities. The results are
compared to expectations based on observed kinematics of local main-sequence
stars. We find a population of photometrically young ($\sim$ 0.5 - 1.5 Gyr)
ultramassive ($\sim$ 1.15 - 1.25 $M_\odot$) white dwarfs for which the
transverse velocity component in the direction of Galactic rotation is more
dispersed than for local disc stars of any age; thus, it is too dispersed to be
explained by any cooling delay in white dwarfs originating from the local disc.
Furthermore, the dispersion ratio of the velocity components in the Galactic
plane for this population is also inconsistent with a local disc origin. We
discuss some possible explanations of this kinematically anomalous population,
such as a halo origin or production through dynamical effects of stellar triple
systems.
