Gas streaming motions towards the nucleus of M81

Abstract

We present two-dimensional stellar and gaseous kinematics of the inner 120×250 pc2 of the LINER/Seyfert 1 galaxy M81, from optical spectra obtained with the Gemini Multi-Object Spectrograph (GMOS) integral field spectrograph on the Gemini-North telescope at a spatial resolution of ≈10 pc. The stellar velocity field shows circular rotation and, overall, is very similar to the published large-scale velocity field, but deviations are observed close to the minor axis which can be attributed to stellar motions possibly associated with a nuclear bar. The stellar velocity dispersion of the bulge is 162±15 km s-ˡ, in good agreement with previous measurements and leading to a black hole mass of MBH =5.5+3.6 −2.0 × 10 7Mʘ based on the MBH–σ relationship. The gas kinematics is dominated by non-circular motions and the subtraction of the stellar velocity field reveals blueshifts of ≈−100 km s-ˡ on the far side of the galaxy and a few redshifts on the near side. These characteristics can be interpreted in terms of streaming towards the centre if the gas is in the plane. On the basis of the observed velocities and geometry of the flow, we estimate a mass inflow rate in ionized gas of ≈4.0×10−3Mʘ yr-ˡ, which is of the order of the accretion rate necessary to power the LINER nucleus of M81. We have also applied the technique of principal component analysis (PCA) to our data, which reveals the presence of a rotating nuclear gas disc within ≈50 pc from the nucleus and a compact outflow, approximately perpendicular to the disc. The PCA combined with the observed gas velocity field shows that the nuclear disc is being fed by gas circulating in the galaxy plane. The presence of the outflow is supported by a compact jet seen in radio observations at a similar orientation, as well as by an enhancement of the [OI]/Hα line ratio, probably resulting from shock excitation of the circumnuclear gas by the radio jet.With these observations we are thus resolving both the feeding – via the nuclear disc and observed gas inflow, and the feedback – via the outflow, around the low-luminosity active nucleus of M81.