2023A&A...678A.118S

Context. A recently discovered thin long object aligned with a nearby galaxy could be the stellar wake induced by the passage of a supermassive black hole (SMBH) kicked out from the nearby galaxy by the slingshot effect of a three-body encounter of SMBHs. Alternatively, the object could be a bulgeless edge-on galaxy coincidentally aligned with a second nearby companion. In contrast with the latter, the SMBH interpretation requires a number of unlikely events to happen simultaneously.
Aims. We aim to assign a probability of occurrence to the two competing scenarios.
Methods. The probability that the SMBH passage leaves a trace of stars is factorized as the product of the probabilities of all the independent events required for this to happen (P_SMBH). Then, each factor is estimated individually. The same exercise is repeated with the edge-on galaxy interpretation (P_galax).
Results. Our estimate yields log(P_galax/P_SMBH)≃11.4 ± 1.6, where the error is evaluated considering that both P_galax and P_SMBH are products of a large number of random independent variables. Based on the estimated probabilities, P_SMBH < 6 × 10^–17 and P_galax > 1.4 × 10^–5, we determined the number of objects to be expected in various existing, ongoing, and forthcoming surveys, as well as among all observable galaxies (i.e., when observing between 10⁶ and 2 × 10¹² galaxies). In the edge-on galaxy scenario, there are always objects to be detected, whereas in the SMBH scenario, the expectation is always compatible with zero.
Conclusions. Despite the appeal of the runaway SMBH explanation, arguments based on the Occam's razor clearly favor the bulgeless edge-on galaxy interpretation. Our work does not rule out the existence of runaway SMBHs leaving stellar trails. It tells that the vD23 object is more likely to be a bulgeless edge-on galaxy.