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-- Context. A recently discovered thin long object aligned with a nearbygalaxy could be the stellar wake induced by the passage of a supermassive blackhole (SMBH) kicked out from the nearby galaxy by the slingshot effect of athree-body encounter of SMBHs. Alternatively, the object could be a bulgelessedge-on galaxy coincidentally aligned with a second nearby companion. Incontrast with the latter, the SMBH interpretation requires a number of unlikelyevents to happen simultaneously. -- Aims. We aim to assign a probability ofoccurrence to the two competing scenarios. -- Methods. The probability that theSMBH passage leaves a trace of stars is factorized as the product of theprobabilities of all the independent events required for this to happen(PSMBH). Then, each factor is estimated individually. The same exercise isrepeated with the edge-on galaxy interpretation (Pgalax). -- Results. Ourestimate yields log(Pgalax/PSMBH) simeq 11.4 pm 1.6, where the error isevaluated considering that both Pgalax and PSMBH are products of a large numberof random independent variables. Based on the estimated probabilities, PSMBH <6 x 10**-17 and Pgalax > 1.4 x 10**-5, we determined the number of objects tobe expected in various existing, ongoing, and forthcoming surveys, as well asamong all observable galaxies (i.e., when observing between 10**6 and 2 x10**12 galaxies). In the edge-on galaxy scenario, there are always objects tobe detected, whereas in the SMBH scenario, the expectation is always compatiblewith zero. -- Conclusions. Despite the appeal of the runaway SMBH explanation,arguments based on the Occam's razor clearly favor the bulgeless edge-on galaxyinterpretation. Our work does not rule out the existence of runaway SMBHsleaving stellar trails. It tells that the vD23 object is more likely to be abulgeless edge-on galaxy.

Supermassive black hole wake or bulgeless edge-on galaxy? II: Order-of-magnitude analysis of the two physical scenarios