  We look for simulated star-forming linear wakes such as the one recently
discovered by van Dokkum et al. (2023) in the cosmological hydrodynamical
simulation ASTRID. Amongst the runaway black holes in ASTRID, none are able to
produce clear star-forming wakes. Meanwhile, fly-by encounters, typically
involving a compact galaxy (with a central black hole) and a star-forming
galaxy (with a duo of black holes) reproduce remarkably well many of the key
properties (its length and linearity; recent star formation, etc.) of the
observed star-forming linear feature. We predict the feature to persist for
approximately 100 Myr in such a system and hence constitute a rare event. The
feature contains a partly stripped galaxy (with $M_{\rm gal}=10^9 \sim
10^{10}M_\odot$) and a dual BH system ($M_{\rm BH}=10^5 \sim 10^7\,M_\odot$) in
its brightest knot. X-ray emission from AGN in the knot should be detectable in
such systems. After $100\sim 200\,{\rm Myrs}$ from the first fly-by, the
galaxies merge leaving behind a triple black hole system in a (still) actively
star-forming early-type remnant of mass $\sim 5\times 10^{10}\,M_\odot$.
Follow-up JWST observations may be key for revealing the nature of these linear
features by potentially detecting the older stellar populations constituting
the bright knot. Confirmation of such detections may therefore help
discriminate a fly-by encounter from a massive BH wake to reveal the origin of
such features.
