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We present late-time radio/millimeter (as well as optical/UV and X-ray)detections of the tidal disruption event (TDE) AT2018hyz, spanning $970 - 1300$d after optical discovery. In conjunction with earlier deeper limits, includingat $\approx 700$ d, our observations reveal rapidly rising emission at$0.8-240$ GHz, steeper than $F_\nu\propto t^5$ relative to the time of opticaldiscovery. Such a steep rise cannot be explained in any reasonable scenario ofan outflow launched at the time of disruption (e.g., off-axis jet, suddenincrease in the ambient density), and instead points to a delayed launch. Ourmulti-frequency data allow us to directly determine the radius and energy ofthe radio-emitting outflow, showing that it was launched $\approx 750$ d afteroptical discovery. The outflow velocity is mildly relativistic, with$\beta\approx 0.25$ and $\approx 0.6$ for a spherical and a $10^\circ$ jetgeometry, respectively, and the minimum kinetic energy is $E_K\approx 5.8\times10^{49}$ and $\approx 6.3\times 10^{49}$ erg, respectively. This is the firstdefinitive evidence for the production of a delayed mildly-relativistic outflowin a TDE; a comparison to the recently-published radio light curve ofASASSN-15oi suggests that the final re-brightening observed in that event (at asingle frequency and time) may be due to a similar outflow with a comparablevelocity and energy. Finally, we note that the energy and velocity of thedelayed outflow in AT2018hyz are intermediate between those of pastnon-relativistic TDEs (e.g., ASASSN-14li, AT2019dsg) and the relativistic TDESw\,J1644+57. We suggest that such delayed outflows may be common in TDEs.

A Mildly Relativistic Outflow Launched Two Years after Disruption in the Tidal Disruption Event AT2018hyz