Mapping of molecular gas inflow towards the Seyfert nucleus of NGC 4051 using Gemini NIFS

Abstract

We present 2D stellar and gaseous kinematics of the inner ~ 130×180 pc² of the Narrow-Line Seyfert 1 galaxy NGC4051 at a sampling of 4.5 pc, from near-infrared K-band spectroscopic observations obtained with the Gemini’s Near-infrared Integral Field Spectrograph (NIFS) operating with the ALTAIR adaptive optics module. We have used the CO absorption band heads around 2.3 μm to obtain the stellar kinematics which show the turnover of the rotation curve at only ≈ 55 pc from the nucleus, revealing a highly concentrated gravitational potential. The stellar velocity dispersion of the bulge is ≈ 60 km s-ˡ – implying on a nuclear black hole mass of ≈ 10 6 Mʘ – within which patches of lower velocity dispersion suggest the presence of regions of more recent star formation. From measurements of the emission-line profiles we have constructed 2D maps for the flux distributions, line ratios, radial velocities and gas velocity dispersions for the H2, HII and [Ca VIII] emitting gas. Each emission-line samples a distinct kinematics. The Brγ emission-line shows no rotation as well as no blueshifts or redshifts in excess of 30 km s-ˡ, and is thus not restricted to the galaxy plane. The [Ca VIII] coronal region is compact but resolved, extending over the inner 75 pc. It shows the highest blueshifts – of up to −250 km s-ˡ, and the highest velocity dispersions, interpreted as due to outflows from the active nucleus, supporting an origin close to the nucleus. Subtraction of the stellar velocity field from the gaseous velocity field has allowed us to isolate non-circular motions observed in the H2 emitting gas. The most conspicuous kinematic structures are two nuclear spiral arms – one observed in blueshift in the far side of the galaxy (to the north-east), and the other observed in redshift in the near side of the galaxy (to the south-west).We interpret these structures as inflows towards the nucleus, a result similar to those of previous studies in which we have found streaming motions along nuclear spirals in ionized gas using optical Integral Filed Unit (IFU) observations. We have calculated the mass inflow rate along the nuclear spiral arms, obtaining ˙MH2 ≈ 4×10−5 Mʘ yr-ˡ, a value ~ 100 times smaller than the accretion rate necessary to power the active nucleus. This can be understood as due to the fact that we are only seeing the hot ‘skin’ (the H2 emitting gas) of the total mass inflow rate, which is probably dominated by cold molecular gas. From the H2 emission-line ratios we conclude that X-ray heating can account for the observed emission, but the H2 λ2.1218 μm/Br γ line ratio suggests some contribution from shocks in localized regions close to the compact radio jet.