The Hosts of Active Galaxies

An outstanding problem in current active galactic nuclei research is what role, if any, the host galaxy plays in the fueling of the central engine. For the observed luminosities of Seyfert galaxies, mass accretion rates of approximately one-solar mass per year are required. Although the interstellar medium of the host galaxy contains enough mass to be a possible fuel source, it is not clear how matter on kiloparsec scales is transported to the small scales of the nucleus. From a theoretical standpoint, galactic bars are perhaps the most viable candidate for facilitating the transfer of mass from large to small scales. There have been many attempts to determine the importance of such bars in Seyfert galaxies, but in general the studies find little or no evidence that Seyfert nuclei occur more frequently in barred systems than in normal spiral galaxies.

The failure to find evidence for bars in most Seyfert galaxies may be due to the fact that early studies concentrated on optical wavelengths where the presence of extinction or a young stellar population might mask any bar structures. For example, the prototypical Seyfert 2 galaxy NGC 1068 is classified as an unbarred spiral at optical wavelengths, but a strong bar is clearly seen in near-infrared images. The near-infrared is expected to be a good place to study the host galaxy of Seyferts because the galaxy energy distribution peaks at approximately 1 micron, while the energy distribution of the active nucleus is at a minimum here. Furthermore, since neither dust nor young luminous stars strongly affect the observed emission at K band, the light at this wavelength is a good tracer of the mass distribution, and thus bars.

To study the role bars play in the fueling of active galaxies, Michael Regan (STScI) and I embarked on a large near-infrared imaging survey of a sample of normal and Seyfert galaxies. This sample offers several advantages over the previous studies of bars in Seyferts. First, our Seyfert galaxies have been selected from the Revised-Shapely Ames (RSA) Catalog of Bright Galaxies. These Seyferts are relatively nearby which reduces the dilution of their nuclear spectra by the host galaxy light. Thus, this sample is less biased against low-luminosity galaxies than other Seyfert samples. The nearness of these galaxies also means we can resolve bars on the several hundred parsec scale. Finally, we have also imaged a comparison sample of normal galaxies under the same conditions, so a direct comparison between the hosts of active and normal galaxies can be made.

Not surprisingly, we find evidence for bars in many galaxies classified as unbarred in the optical. However, approximately 20% of the Seyfert galaxies show no evidence for bars, even in the near-infrared. Furthermore, the percentage of barred galaxies is comparable in the normal and active galaxy samples. While some of the unbarred Seyferts have nearby companions that might be responsible for inducing the nuclear activity, others appear relatively isolated.

While our study would seem to eliminate large-scale bars as a universal mechanism for fueling nuclear activity, there may still be differences between the normal and active galaxies on much smaller scales. In fact, theoretical models indicate that gas inflow generated by a rotating bar extends down to approximately the inner kiloparsec of the galaxy, not close enough to the central engine for viscosity or cloud-cloud collisions to take over and directly feed the central engine. However, these same simulations suggest that a bars-within-bars mechanism may develop and continue to drive material towards the nucleus. In this model, gas driven inward from the large-scale stellar bar accumulates in the central few hundred parsecs in a rapidly rotating disk. If the mass of the gas in this disk is an appreciable fraction (> 20%) of the dynamical mass at that radius, the disk may become unstable and a gaseous bar can form. A dynamically unstable series of such bars may exist, driving inflow until the point where the central supermassive blackhole dominates the gravitational potential (inner 50 pc).

The simulations described above imply that our ground-based images may be inadequate to probe the small scales where differences between the normal and active galaxies are taking place. To look for these differences we proposed and have been awarded HST time with NICMOS to study the central regions of normal and active galaxies. Our sample includes 182 objects, so we expect to be able to make statistical statements about the properties of these galaxies on the scales of 10-30 parsec. The NICMOS images should provide a direct test of the bars-within-bars scenario and hopefully provide clues into what role if any the host galaxy plays in the fueling of nuclear activity.

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