Dusty spirals versus gas kinematics in the inner kiloparsec of four low-luminosity active galactic nuclei

Abstract

We used the Gemini Multi-Object Spectrograph Integral Field Unit tomap the gas distribution, excitation and kinematics within the inner kiloparsec of four nearby low-luminosity active galaxies: NGC 3982, NGC 4501, NGC 2787 and NGC 4450. The observations cover the spectral range 5600–7000 Å at a velocity resolution of 120 km s−1 and spatial resolution ranging from 50 to 70 pc at the galaxies. Extended emission in Hα, [N II]λλ6548, 6583, [S II] λλ6716, 6730 over most of the field of view is observed for all galaxies, while only NGC 3982 shows [O I] λ6300 extended emission. The Hα equivalent widths (WHα) combined with the [N II]/H α line ratios reveal that NGC 3982 and NGC 4450 harbour Seyfert nuclei surrounded by regions with low-ionization nuclear emission-line region (LINER) excitation, while NGC 2787 and NGC 4501 harbour LINER nuclei. NGC 3982 shows a partial ring of recent star formation at 500 pc from the nucleus, while in NGC 4501 a region at 500 pc west of the nucleus shows LINER excitation but has been interpreted as an aging HII region with the gas excitation dominated by shocks from supernovae The line-of-sight velocity field of the gas shows a rotation pattern for all galaxies, with deviations from pure disc rotation observed in NGC 3982, NGC 4501 and NGC 4450. For NGC 4501 and NGC 4450, many of these deviations are spatially coincident with dust structures seen in optical continuum images, leading to the interpretation that the deviations are due to shocks in the gas traced by the dust. A speculation is that these shocks lead to loss of angular momentum, allowing the gas to be transferred inwards to feed the active galactic nucleus. In the case of NGC 2787, instead of deviations in the rotation field, we see a misalignment of 40◦ between the orientation of the line of nodes of the gas rotation and the photometric major axis of the galaxy. Evidence of compact nuclear outflows is seen in NGC 4501 and NGC 4450.