An atomic hydrogen bridge fueling NGC 4418 with gas from VV 655
  The galaxy NGC 4418 harbours a compact ($<20$ pc) core with a very high
bolometric luminosity ($\sim10^{11}$L$_\odot$). As most of the galaxy's energy
output comes from this small region, it is of interest to determine what fuels
this intense activity. An interaction with VV 655 has been proposed, where gas
aquired by NGC 4418 could trigger intense star formation and/or black hole
accretion in the centre. We aim to constrain the interaction hypothesis by
studying neutral hydrogen structures around the two galaxies. We present
observations at 1.4 GHz with the Very Large Array of radio continuum as well as
emission and absorption from atomic hydrogen. Gaussian distributions are fitted
to observed HI emission and absorption spectra. An atomic HI bridge is seen in
emission, connecting NGC 4418 to VV 655. While NGC 4418 is bright in continuum
emission and seen in HI absorption, VV 655 is barely detected in the continuum
but show bright HI emission (M$_\mathrm{HI}\sim10^9$ M$_\odot$). We estimate
SFRs from 1.4 GHz of 3.2 M$_\odot$ yr$^{-1}$ and 0.13 M$_\odot$ yr$^{-1}$ for
NGC 4418 and VV 655 respectively. Systemic HI velocities of 2202$\pm$20 km
s$^{-1}$ (emission) and 2105.4$\pm$10 km s$^{-1}$ (absorption) are measured for
VV 655 and NGC 4418 respectively. Redshifted HI absorption is seen towards NGC
4418, suggesting gas infall. Blueshifted HI-emission is seen north-west of NGC
4418, which we interpret as a continuation of the outflow previously discussed
by Sakamoto et al. (2013). The morphology and velocity structure seen in HI is
consistent with an interaction scenario, where gas was transferred from VV 655
to NGC 4418, and may fuel the activity in the centre.
