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

TIGER integral-field spectrography and Hubble Space Telescope Wide Field and Planetary Camera 2 (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 multi-Gaussian expansion (MGE) mass model using the Hunter & Qian formalism. The TIGER and published long-slit stellar kinematics, including higher-order moments, 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 (at that spatial resolution). The data reveal the presence of a striking, highly contrasted, two-arm gaseous spiral structure within a radius of similar to200 pc, corresponding to a total mass of 6.8x10(4) solar masses of ionized gas. We use a deconvolved TIGER data cube to probe its kinematics at a resolution of about 0.35 arcsec FWHM. 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 similar to540-pc diameter bar with Omega(p)=700+/-100 km s(-1) kpc(-1). This hypothesis is strongly supported by the predictions of a density wave model. This model predicts that the bar should lie at about 35degrees from the line of nodes, and implies gas inflow towards the central similar to50 pc. The quadrupole perturbation due to this bar is estimated to represent less than 2 per cent of the underlying gravitational potential (a maximum torque of about 10 per cent), explaining the lack of a direct detection via broad-band photometry in the visible. 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 disc galaxies than is generally thought, and that deep high-resolution emission-line imagery may be the best way to detect such structures.