  Blobs, or quasi-spherical emission regions containing relativistic particles
and magnetic fields, are often assumed ad hoc in emission models of
relativistic astrophysical jets, yet their physical origin is still not well
understood. Here, we employ a suite of large-scale two-dimensional
particle-in-cell simulations in electron-positron plasmas to demonstrate that
relativistic magnetic reconnection can naturally account for the formation of
quasi-spherical plasmoids filled with high-energy particles and magnetic
fields. Our simulations extend to unprecedentedly long temporal and spatial
scales, so we can capture the asymptotic physics independently of the initial
setup. We characterize the properties of the plasmoids that are continuously
generated as a self-consistent by-product of the reconnection process: they are
in rough energy equipartition between particles and magnetic fields; the upper
energy cutoff of the plasmoid particle spectrum is proportional to the plasmoid
width w, corresponding to a Larmor radius ~0.2 w; the plasmoids grow in size at
~0.1 of the speed of light, with most of the growth happening while they are
still non-relativistic (first they grow); their growth is suppressed once they
get accelerated to relativistic speeds by the field line tension, up to the
Alfven speed (then they go). The largest plasmoids, whose typical recurrence
interval is ~2.5 L/c, reach a characteristic size w ~ 0.2 L independently of
the system length L, they have nearly isotropic particle distributions and they
contain the highest energy particles, whose Larmor radius is ~0.03 L. The
latter can be regarded as the Hillas criterion for relativistic reconnection.
We briefly discuss the implications of our results for the high-energy emission
from relativistic jets and pulsar winds.
