A Candidate Runaway Supermassive Black Hole Identified by Shocks and Star Formation in its Wake

The interaction of a runaway supermassive black hole (SMBH) with the circumgalactic medium (CGM) can lead to the formation of a wake of shocked gas and young stars behind it. Here we report the serendipitous discovery of an extremely narrow linear feature in Hubble Space Telescope (HST) Advanced Camera for Surveys images that may be an example of such a wake. The feature extends 62 kpc from the nucleus of a compact star-forming galaxy at z = 0.964. Keck Low-resolution Imaging Spectrometer spectra show that the [O iii]/Hβ ratio varies from ∼1 to ∼10 along the feature, indicating a mixture of star formation and fast shocks. The feature terminates in a bright [O iii] knot with a luminosity of ≈1.9 × 1041 erg s−1. The stellar continuum colors vary along the feature and are well fit by a simple model that has a monotonically increasing age with the distance from the tip. The line ratios, colors, and overall morphology are consistent with an ejected SMBH moving through the CGM at a high speed while triggering star formation. The best-fit time since ejection is ∼39 Myr, and the implied velocity is vBH ∼ 1600 km s−1. The feature is not perfectly straight in the HST images, and we show that the amplitude of the observed spatial variations is consistent with the runaway SMBH interpretation. Opposite the primary wake is a fainter and shorter feature, marginally detected only in [O iii] and the rest-frame far-ultraviolet. This feature may be shocked gas behind a binary SMBH that was ejected at the same time as the SMBH that produced the primary wake.