An atomic hydrogen bridge fueling NGC 4418 with gas from VV 655

Abstract

The galaxy NGC 4418 harbours a compact ($<20$ pc) core with a very high bolometric luminosity ($\sim 10^{11}$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${}_{\mathrm{HI}}\sim 10^9$ 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.

arXiv comment7 pages, 6 figures

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