A low H I column density filament in NGC 2403: signature of interaction or accretion^⋆

¹ Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, The Netherlands,
e-mail: blok@astron.nl
² Astrophysics, Cosmology and Gravity Centre, Department of Astronomy, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ Rincon Research Corporation, Tucson, AZ 85711, USA
⁵ WVU Dept. of Physics & Astronomy, PO Box 6315, Morgantown, WV 26506, USA
⁶ Department of Physics and Astronomy, University of Bologna, via Berti Pichat 6/2, 40127 Bologna, Italy
⁷ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁸ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁹ Institut für theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert-Ueberle Str. 2, 69120 Heidelberg, Germany
¹⁰ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA

Received: 25 March 2014
Accepted: 12 July 2014

Abstract

Observed H i accretion around nearby galaxies can only account for a fraction of the gas supply needed to sustain the currently observed star formation rates. It is possible that additional accretion occurs in the form of low column density cold flows, as predicted by numerical simulations of galaxy formation. To constrain the presence and properties of such flows, we present deep H i observations obtained with the NRAO Green Bank Telescope of an area measuring 4° × 4° around NGC 2403. These observations, with a 5σ detection limit of 2.4 × 10¹⁸ cm^-2 over a 20 km s^-1 linewidth, reveal a low column density, extended cloud outside the main H i disk, about 17′ (~ 16 kpc or ~ 2 R₂₅) to the NW of the center of the galaxy. The total H i mass of the cloud is 6.3 × 10⁶ M_⊙, or 0.15 percent of the total H i mass of NGC 2403. The cloud is associated with an 8 kpc anomalous-velocity H i filament in the inner disk, that was previously observed in deep VLA observations. We discuss several scenarios for the origin of the cloud, and conclude that it is either accreting from the intergalactic medium, or is the result of a minor interaction with a neigboring dwarf galaxy.