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

TL;DR

This study compares two scenarios for a peculiar astronomical object: a stellar wake caused by a supermassive black hole or a bulgeless edge-on galaxy, concluding the galaxy interpretation is far more probable based on probability estimates.

Contribution

The paper provides a quantitative probability analysis favoring the edge-on galaxy scenario over the SMBH wake hypothesis for the observed object.

Findings

Probability of SMBH wake scenario is extremely low (<6 x 10^-17).

Probability of edge-on galaxy scenario is significantly higher (>1.4 x 10^-5).

Observational expectations differ greatly between the two scenarios.

Abstract

-- 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 (PSMBH). Then, each factor is estimated individually. The same exercise is repeated with the edge-on galaxy interpretation (Pgalax). -- Results. Our estimate yields log(Pgalax/PSMBH) simeq 11.4 pm 1.6, where the error is evaluated considering that both Pgalax and PSMBH are products of a large number of 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 to be expected in various existing, ongoing, and forthcoming surveys, as well as among all observable galaxies (i.e., when observing between 10**6 and 2 x 10**12 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.