An anomalous velocity neutral hydrogen structure near the galactic center

An extensive concentration of neutral hydrogen has been observed in the fourth galactic quadrant, with a mean radial velocity of +44 km s^?1 referred to the local standard of rest. At a distance ofR kpc from the Sun this structure would contain 2.5×10⁴ R ² solar masses of neutral hydrogen. Five possible interpretations of this extensive concentration are considered: (1) part of the shell of a nearby explosive event; (2) a distant spiral arm of the Galaxy; (3) an extragalactic object; (4) material falling into our Galaxy; (5) gas expelled from the galactic center. Arguments are offered against the first three possibilities.