H [CSC]i[/CSC] in the Shell Elliptical Galaxy NGC 3656

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Abstract
Very Large Array** neutral hydrogen observations of the shell elliptical galaxy NGC 3656 reveal an edge-on, warped minor-axis gaseous disk (M ~ 2 × 109 M⊙) extending 7 kpc. H I is also found outside the optical image, on two complexes to the northeast and northwest that seem to trace one or two tidal tails, or possibly an outer broken H I disk or ring. These complexes link with the outer edges of the inner disk and appear displaced with respect to the two optical tails in the galaxy. The disk kinematics is strongly lopsided, suggesting recent or ongoing accretion. Integral-field optical fiber spectroscopy at the region of the bright southern shell of NGC 3656 has provided a determination of the stellar velocities of the shell. The shell, at 9 kpc from the center, has traces of H I with velocities bracketing the stellar velocities, providing evidence for a dynamical association of H I and stars at the shell. Within the errors the stars have systemic velocity, suggesting a possible phase-wrapping origin for the shell. We probed a region of 40' × 40' (480 kpc × 480 kpc) × 1160 km s-1 down to an H I mass sensitivity (6 σ) of 3 × 107 M⊙ and detect five dwarf galaxies with H I masses ranging from 2 × 108 to 2 × 109 M⊙, all within 180 kpc of NGC 3656 and all within the velocity range (450 km s-1) of the H I of NGC 3656. The dwarfs had been previously cataloged, but none had a known redshift. For the NGC 3656 group to be bound requires a total mass of (3–7.4) × 1012 M⊙, yielding a mass-to-light ratio from 125 to 300. The overall H I picture presented by NGC 3656 supports the hypothesis of a disk-disk merger origin or possibly an ongoing process of multiple mergers with nearby dwarfs.
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