Atomic hydrogen bridge fueling NGC 4418 with gas from VV 655^⋆,⋆⋆

¹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: varenius@chalmers.se
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Cornell Center for Astrophysics and Planetary Science, Space Sciences Building, Ithaca, NY 14853, USA
⁴ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
⁵ Departamento de Astronomía, Universidad de Concepción, Concepción, Chile

Received: 30 September 2016
Accepted: 11 September 2017

Abstract

Context. The galaxy NGC 4418 harbours a compact (<20 pc) core with a very high bolometric luminosity (~ 10¹¹L_⊙). 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 VV 655 has been proposed, where gas acquired by NGC 4418 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 NGC 4418 to the nearby galaxy VV 655.

Methods. We present observations at 1.4 GHz 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 NGC 4418 and VV 655 from the 1.4 GHz radio emission and compare them with estimates from archival 70 μm Herschel observations.

Results. An atomic HI bridge is seen in emission, connecting NGC 4418 to the nearby galaxy VV 655. An HI tail is also seen extending south-west from VV 655. While NGC 4418 is bright in continuum emission and seen in HI absorption, VV 655 is barely detected in the continuum, but shows bright HI emission (M_HI ~ 10⁹ M_⊙). We estimate SFRs from the 1.4 GHz continuum of 3.2 M_⊙ yr^-1 and 0.13 M_⊙ yr^-1 for NGC 4418 and VV 655, respectively. Systemic HI velocities of 2202 ± 20 km s^-1 (emission) and 2105.4 ± 10 km s^-1 (absorption) are measured for VV 655 and NGC 4418, respectively. Redshifted HI absorption is seen (v_c = 2194.0 ± 4.4 km s^-1) towards NGC 4418, suggesting gas infall. North-west of NGC 4418, we detect HI in emission, blueshifted (v_c = 2061.9 ± 5.1 km s^-1) with respect to NGC 4418, 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_⊙ yr^-1.

Conclusions. The morphology and velocity structure seen in HI is consistent with an interaction scenario where gas was transferred from VV 655 to NGC 4418. We argue that the galaxies have passed each other once, about 190 Myr ago, and that this interaction has caused the tidal HI bridge and HI tail seen today. Some gas is falling towards NGC 4418, and may fuel the activity in the centre. We interpret blueshifted HI-emission north-west of NGC 4418 as a continuation of the outflow previously reported on smaller scales, powered by star formation and/or black hole accretion in the centre.