Supermassive black hole wake or bulgeless edge-on galaxy?

Abstract

A recent paper reported the serendipitous discovery of a thin linear object interpreted as the trail of star-forming regions left behind by a runaway supermassive black hole (SMBH) kicked out from the center of a galaxy. Despite the undeniable interest in the idea, the actual physical interpretation is not devoid of difficulty. The wake of a SMBH produces only small perturbations in the external medium, which has to be in exceptional physical conditions to collapse gravitationally and form a long (40 kpc) massive (3 × 109 M⊙) stellar trace in only 39 Myr. Here we offer a more conventional explanation: the stellar trail is a bulgeless galaxy viewed edge-on. This interpretation is supported by the fact that its position–velocity curve resembles a rotation curve, which, together with its stellar mass, puts the object exactly on the Tully–Fisher relation characteristic of disk galaxies. Moreover, the rotation curve (Vmax ∼ 110 km s−1), stellar mass, extension, width (z0 ∼ 1.2 kpc), and surface brightness profile of the object are very much like those of IC 5249, a well-known local bulgeless edge-on galaxy. These observational facts are difficult to interpret within the SMBH wake scenario. We discuss in detail the pros and cons of the two options.