Atomic hydrogen bridge fueling NGC 4418 with gas from VV 655

Artikel i vetenskaplig tidskrift, 2017

Context. The galaxy NGC4418 harbours a compact (< 20 pc) core with a very high bolometric luminosity (similar to 10(11) L-circle dot). As most of the galaxy energy output comes from this small region, it is of interest to determine what fuels this intense activity. An interaction with the nearby blue irregular galaxy VV655 has been proposed, where gas acquired by NGC4418 could trigger intense star formation and / or black hole accretion in the centre. Aims. We aim to constrain the interaction hypothesis by studying neutral hydrogen structures that could reveal tails and debris connecting NGC4418 to the nearby galaxy VV655. Methods. We present observations at 1.4GHz with the Very Large Array (VLA) of the radio continuum as well as emission and absorption from atomic hydrogen. Gaussian distributions are fitted to observed HI emission and absorption spectra. We estimate the star formation rates (SFRs) of NGC4418 and VV655 from the 1.4 GHz radio emission and compare them with estimates from archival 70 mu m Herschel observations. Results. An atomic HI bridge is seen in emission, connecting NGC4418 to the nearby galaxy VV655. An HI tail is also seen extending south-west from VV655. While NGC4418 is bright in continuum emission and seen in HI absorption, VV655 is barely detected in the continuum, but shows bright HI emission (M-HI similar to 10(9) M-circle dot). We estimate SFRs from the 1.4 GHz continuum of 3 : 2 M circle dot yr(-1) and 0 : 13 M(circle dot)yr(-1) for NGC4418 and VV655, respectively. Systemic HI velocities of 2202 +/- 20 km s(-1) (emission) and 2105 : 4 +/- 10 km s(-1) (absorption) are measured for VV655 and NGC4418, respectively. Redshifted HI absorption is seen (v(c) = 2194 : 0 +/- 4 : 4 km s(-1)) towards NGC4418, suggesting gas infall. North-west of NGC4418, we detect HI in emission, blueshifted (v(c) = 2061 : 9 +/- 5 : 1 km s(-1)) with respect to NGC4418, consistent with an outflow perpendicular to the galaxy disk. We derive a deprojected outflow speed of 178 km S-1, which, assuming a simple cylindrical model, gives an order-of-magnitude estimate of the HI mass outflow rate of 2 : 5 M(circle dot)y(r-)1. Conclusions. The morphology and velocity structure seen in HI is consistent with an interaction scenario where gas was transferred from VV655 to NGC4418. We argue that the galaxies have passed each other once, about 190 M-circle dot ago, and that this interaction has caused the tidal HI bridge and HI tail seen today. Some gas is falling towards NGC4418, and may fuel the activity in the centre. We interpret blueshifted HI-emission north-west of NGC4418 as a continuation of the outflow previously reported on smaller scales, powered by star formation an /or black hole accretion in the centre.