The influence of bars on nuclear activity

Gravitational torques induced by a stellar bar on the interstellar medium of a disk galaxy instigate radial inflow of gas toward the central regions of the galaxy. Accordingly, the presence of a bar should reinforce nuclear star formation activity or the fueling of an active galactic nucleus (AGN). We test this hypothesis by comparing the detection rate and intensity of nuclear H II regions and AGNs among barred and unbarred galaxies in a sample of over 300 spirals selected from our recent optical spectroscopic survey of nearby galaxies. The AGN group includes Seyfert nuclei as well as low-ionization nuclear emission-line regions (LINERs). Among late-type spirals (Sc-Sm), as opposed to early-type spirals (SO/a-Sbc), we observe in the barred group a very marginal increase in the detection rate of H II nuclei and a corresponding decrease in the incidence of AGNs. The minor differences in detection rates, however, are statistically insignificant, most likely stemming from selection effects and not from a genuine influence of the bar. The presence of a bar seems to have no noticeable impact on the likelihood of a galaxy for hosting either nuclear star formation or an AGN. The nuclei of early-type barred spirals (SO/a-Sbc) do exhibit measurably higher star formation rates than do their unbarred counterparts, as indicated by either the luminosity or the equivalent width of H alpha emission. By contrast, late-type spirals do not show such an effect. These results agree with previous studies and can be explained most easily in terms of structural differences between bars in early-type and late-type spirals. Nuclear H II regions spanning a wide range of intensities are found regardless of the presence of a bar, suggesting that a bar is neither a necessary nor a sufficient condition for star formation to occur in galactic nuclei. Other factors, such as the availablity of gas, must be equally important. Bars, on the other hand, have a negligible effect on the strength of the AGNs in our sample, regardless of the Hubble type of the host galaxy. This result confirms conclusions reached by other studies based on much smaller samples. Assuming that AGNs are fueled by gas from the interstellar medium of the host galaxy, some inferences concerning the fueling process can be made. We speculate that inner Lindblad resonances, particularly common in barred galaxies with large bulge-to-disk ratios, prevent gas which has been radially transported from large scales from reaching the nucleus. We discuss the feasibility of sustaining the power output of nearby AGNs with debris from tidal disruption of stars by a supermassive black hole as well as with mass loss from evolved stars. We conclude that such processes should be sufficient for fueling the low-luminosity nuclear sources found in many nearby galaxies.