A two-arm gaseous spiral in the inner 200 pc of the early-type galaxy NGC 2974: signature of an inner bar

Abstract

TIGER integral-field spectrography and HST/WFPC2 imaging of the E3 galaxy NGC 2974 are used to derive the kinematics of the stellar and ionized gas components in its central 500 pc. We derive a numerical two-integral distribution function from a MGE mass model using the HQ formalism. The TIGER as well as published long-slit stellar kinematics are well fitted with this self-consistent model, requiring neither the addition of a significant mass contribution from a hidden disc structure, nor the presence of a central dark mass. The data reveal the presence of a striking, highly contrasted, two-arm gaseous spiral structure within a radius of ~200 pc, corresponding to a total mass of 6.8x10^4 Msun of ionized gas. We use a deconvolved TIGER datacube to probe its kinematics at a resolution of about 0.35 arcsec. Strong departures from circular motions are observed, as well as high velocity dispersion values on the inner side of the arms. We interpret the observed gas morphology and kinematics as the signature of streaming gas flows driven by a ~540 pc diameter bar with Ome=700+/-100 km/s/kpc. This hypothesis is strongly supported by the predictions of a density wave model. This model predicts that the bar should lie at about 35 degree from the line of nodes, and implies gas inflow towards the central ~50 pc. The quadrupole pertubation due to this bar is estimated to represent less than 2% of the underlying gravitational potential. Despite its weakness, the inner bar of NGC 2974 may be able to drive some gas within a 10 pc radius. We suggest that the presence of such inner bars might be more common among early-type disk galaxies than is generally thought, and that deep high-resolution emission-line imagery may be the best way to detect such structures.