The Nuclear Gasdynamics and Star Formation of NGC 7469

Abstract

We report interferometric radio CO 2-1 and HCN 1-0 observations at resolutions of 07 and 20, respectively, and 0085 resolution adaptive optics K-band spectroscopy, including H2 1-0 S(1) line emission and CO 2-0 stellar absorption, of the inner few arcseconds of NGC 7469. The CO 2-1 map shows a ring of molecular clouds (which in general lie outside the compact knots seen in K-band images) and a bright extended nucleus, with a bar or pair of spiral arms between them. The dynamical structure of both the radio CO 2-1 and the K-band H2 1-0 S(1) lines at their different resolutions can be reproduced using a single axisymmetric mass model comprising three components: a broad disk, a ring 4''-5'' across, and an extended nucleus, which we interpret as an inner nuclear ring about 05 across. The velocity residuals between the model and the data have a standard deviation of 25 km s-1, and no noncircular motions faster than this are seen, although this may be because in some cases a secondary bar is not expected to cause gas inflow. From the dynamical mass and estimates of the stellar mass we find that the CO-to-H2 conversion is 0.4-0.8 times that for the Milky Way, following the trend to small factors that has been reported for intense star-forming environments. The central H2 1-0 S(1) morphology has a strong peak at the nucleus, but this does not trace the mass distribution; the rotation curves indicate that there is no strong nuclear mass concentration. The origins of the 1-0 S(1) emission are instead likely to lie in X-ray and UV irradiation of gas by the active galactic nucleus rather than via processes associated with star formation. Using the 2.3 ?m stellar CO 2-0 band head absorption and the slope of the continuum, we have directly resolved the nuclear star cluster to be 015-020 across and find that it is asymmetric. This cluster has an age of less than about 60 Myr and contributes 20%-30% of the nuclear K-band light and about 10% of the nuclear bolometric luminosity. Within a radius of ~4'' gas contributes more than half the total mass, but in the nucleus, within a radius of 01, it is likely that most of the mass is due instead to stars.