Kinematics and Excitation of the Nuclear Spiral in the Active Galaxy Arp 102B

Abstract

We present a two-dimensional analysis of the gaseous excitation and kinematics of the inner 2.5 × 1.7 kpc2 of the low-ionization nuclear emission-line region (LINER)/Seyfert 1 galaxy Arp 102B, from optical spectra obtained with the Gemini Multi-Object Spectrograph Integral Field Unit on the Gemini North telescope at a spatial resolution of ≈250 pc. Emission-line flux maps show the same two-armed nuclear spiral we have discovered in previous observations with the Hubble Space Telescope Advanced Camera for Surveys. One arm reaches 1 kpc to the east and the other 500 pc to the west, with an 8.4 GHz Very Large Array bent radio jet correlating with the former. Gas excitation along the arms is low, with line ratios typical of LINERs, and which rule out gas ionization by stars. The gas density is highest (≈500–900 cm−3) at the nucleus and in the northern border of the east arm, at a region where the radio jet seems to be deflected. Centroid velocity maps suggest that most gas is in rotation in an inclined disc with line of nodes along position angle ≈88°, redshifts to the west and blueshifts to the east, with lower blueshifts correlated with the eastern arm and radio jet. This correlation suggests that the jet is interacting with gas in the disc. This interaction is supported by the gas excitation as a function of distance from the nucleus, which requires the contribution from shocks. Channel maps show blueshifts but also some redshifts at the eastern arm and jet location which can be interpreted as originated in the front and back walls of an outflow pushed by the radio jet, suggesting also that the outflow is launched close to the plane of the sky. Principal Component Analysis applied to our data supports this interpretation. We estimate a mass outflow rate along the east arm of 0.26-0.32 M⊙ yr− 1 (depending on the assumed outflow geometry), which is between one and two orders of magnitude higher than the mass accretion rate to the active nucleus, implying that there is mass-loading of the nuclear outflow from circumnuclear gas. The power of this outflow is 0.06–0.3 per cent Lbol. We propose a scenario in which gas has been recently captured by Arp 102B in an interaction with Arp 102A, settling in a disc rotating around the nucleus of Arp 102B and triggering its nuclear activity. A nuclear jet is pushing the circumnuclear gas, giving origin to the nuclear arms. A blueshifted emitting gas knot is observed at ≈300 pc southeast from the nucleus and can be interpreted as another (more compact) outflow, with a possible counterpart to the northwest.