Molecular gas in NUclei of GAlaxies (NUGA) -: II.: The ringed LINER NGC 7217

We present CO(1-0) and CO(2-1) maps of the LINER galaxy NGC7217, obtained with the IRAM interferometer, at 2.4" x 1.9" and 1.2" x 0.8" resolution respectively. The nuclear ring (at r = 12" = 0.8 kpc) dominates the CO maps, and has a remarkable sharp surface density gradient at its inner edge. The latter is the site of the stellar/Halpha ring, while the CO emission ring extends farther or is broader (500-600 pc). This means that the star formation has been more intense toward the inner edge of the CO ring, in a thin layer, just at the location of the high gas density gradient. The CO(2-1)/CO(1-0) ratio is close to 1, typical of warm optically thick gas with high density. The overall morphology of the ring is quite circular, with no evidence of non-circular velocities. In the CO(2-1) map, a central concentration might be associated with the circumnuclear ionized gas detected inside r = 3" and interpreted as a polar ring in the literature. The CO(2-1) emission inside 3" coincides with a spiral dust lane, clearly seen in the HST V - I color image. N-body simulations including gas dissipation and star formation are performed to better understand the nature of the nuclear ring observed. The observed rotation curve of NGC 7217 allows two possibilities, according to the adopted mass for the disk: (1) either the disk is massive, allowing a strong bar to develop, or (2) it is dominated in mass by an extended bulge/stellar halo, and supports only a mild oval distortion. The amount of gas also plays an important role in the disk stability, and therefore the initial gas fraction was varied, with star formation reducing the total gas fraction to the observed value. The present observations support only the bulge-dominated model, which is able to account for the nuclear ring in CO and its position relative to the stellar and Ha ring. In this model, the gas content was higher in the recent past (having been consumed via star formation), and the structures formed were more self-gravitating. Only a mild bar formed, which has now vanished, but the stars formed in the highest gas density peaks toward the inner edge of the nuclear ring, which corresponds to the observed thin stellar ring. We see no evidence for an ongoing fueling of the nucleus; instead, gas inside the ring is presently experiencing an outward flow. To account for the nuclear activity, some gas infall and fueling must have occured in the recent past (a few Myr ago), since some, albeit very small, CO emission is detected at the very center. These observations have been made in the context of the NUclei of GAlaxies (NUGA) project, aimed at the study of the different mechanisms for gas fueling of AGN.