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The wake of a supermassive black hole?

A serendipitous streak discovered in Hubble Space Telescope images might come from a runaway black hole hurtling through the atmosphere of a distant galaxy.

Published onFeb 27, 2023
The wake of a supermassive black hole?

An image (top) contains a galaxy seen by HST and a long, streak-like feature pointing toward it. The streak has a brighter knot at its tip, which is facing away from the galaxy. Spectroscopy along the bottom show that along the slit, there is a gradient in velocity and strong variations in the emission line strengths of OIII and H-beta.

Hubble imaging (top) and Keck/LRIS spectroscopy (bottom) of a candidate runaway supermassive black hole. The streak seen in the Hubble imaging was confirmed via spectroscopy to have shocks and star formation, traced by stellar continuum and emission-line ratios. The gas seen in emission is in the circumgalactic medium of the z~1 host galaxy, and we interpret the streak as a perturbation resulting from the black hole’s passage.

The images above show a strange streak in our Hubble Space Telescope (HST) observations that we suspect might be a runaway supermassive black hole shooting through a galactic atmosphere. Our paper on it [1] describes how what initially appeared to be a cosmic ray resolved, on closer inspection in multiple HST bands, into a distinct object with a measurable width, surface brightness variations, and an unusual knot of emission at one end.

The streak is aligned with a small galaxy that appears morphologically disturbed. We thought the streak might be an optical jet of some kind, but one would expect such a jet to become wider at greater distances from the galaxy, whereas this feature seems to become narrower.

We obtained Keck/LRIS spectroscopy along the streak and were surprised to discover emission lines (Hβ\beta and [OIII]). The figure’s lower panels show those long-slit spectroscopic images. Our spectroscopy confirmed that the streak and the galaxy are related, both with a redshift of z=0.964z=0.964.

The streak itself has a velocity gradient, with a roughly 300 km/s difference in line-of-sight velocity between its base (near the systemic velocity of the galaxy) and its tip (which is blueshifted with respect to the galaxy). Moreover, its [OIII]/ Hβ\beta ratio varies strongly with position, going from ~1 near the base to ~10 near the tip.

Considering both the spectroscopy and the imaging, we concluded that the data best fit a scenario in which a supermassive black hole ejected from its host galaxy (because of a merger) subsequently “flew through” the circumgalactic medium (CGM) of the host galaxy. A black hole’s passage would trigger a trailing wake in the galaxy’s atmosphere, leading to shocks and star formation that are consistent with the emission line ratios and young stellar populations observed in the streak.

So-called “runaway” (ejected) supermassive black holes have long been predicted (e.g., [2] [3] [4]) but have not previously been detected. Other possible explanations include an optical jet or a jet-CGM interaction inducing star formation. However, the host galaxy shows no current signs of AGN activity, and an optical jet should have a power-law spectrum. Modeling of the the gas velocities and stellar populations forming along the streak therefore seems most consistent with the runaway black hole scenario.

The story grew more complicated when we noted a knot of [OIII] and optical emission in the opposite direction from the streak. Followup u-band imaging with the Canada-France-Hawaii Telescope (CFHT) detected hints of an extended feature running from the galaxy to the knot. However, there is no gap in the primary streak; u-band emission is seen from the galaxy to the tip.

It is possible that the second knot indicates a merger event that ejected a lone black hole in one direction and a bound pair of black holes in the other direction. Further followup spectroscopy and imaging will help determine whether this “counter-wake” is real and the dual-ejection scenario is likely. Additionally, the “smoking gun” for the entire model would be detection of the black holes themselves — which can be achieved via X-rays with Chandra.

Thus far, these results have been interpreted with a focus on assessing the runaway black hole scenario. But regardless of what created the features, it is almost certain that the emission streak represents a ~62 kpc probe of the atmospheric gas around this z1z\sim1 galaxy, a unique opportunity for further studying its circumgalactic medium.

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