A candidate runaway supermassive black hole identified by shocksand star formation in its wake

The interaction of a runaway supermassive black hole (SMBH) with the circumgalactic medium (CGM) can lead to theformation of a wake of shocked gas and young stars behind it. Here we report the serendipitous discovery of anextremely narrow linear feature in HST/ACS images that may be an example of such a wake. The feature extends 62kpc from the nucleus of a compact star-forming galaxy at 2=0.964. Keck LRIS spectra show that the (OllVHS'betasratio varies from -1 to -10 along the feature, indicating a mixture of star formation and fast shocks. The featureterminates in a bright (Olll knot with a luminosity of 1.9x10$4115 ergs/s. The stellar continuum colors vary along thefeature, and are wel-fit by a simple model that has a monotonically increasing age with distance from the tip. The lineratios, colors, and the overall morphology are consistent with an eiected SIMBH moving through the CGM at hiah speecwhile triggering star formation. The best-fit time since ejection is -39 Myr and the implied velocity is V-1600 km/s. Theeature is not perfectly straight in the HST images, and we show that the amplitude of the observed spatial variations isconsistent with the runaway SMBH interpretation. Opposite the primary wake is a fainter and shorter feature.marginally detected in (Oll and the rest-frame far-ultraviolet. This feature may be shocked gas behind a binary SMBHthat was eiected at the same time as the SMBH that produced the primary wake.