A low H I column density filament in NGC 2403: signature of interaction or accretion W. J. G. de Blok, K. M. Keating, D. J. Pisano, F. Fraternali, F. Walter, T. Oosterloo, E. Brinks, F. Bigiel and A. Leroy A&A, 569 (2014) A68 Published online: 25 September 2014 DOI: 10.1051/0004-6361/201423880