The combination of the exquisite spatial resolution offered by the adaptive optics system at the CFHT with sensitive state-of-the-art near-infrared cameras, allows us to study galaxy structure at unprecedented scales, and in great detail. We have embarked on a program where high resolution near-infrared and optical imaging of the nuclear structures of starburst and active galaxies are a fundamental tool in constraining our understanding of the dynamical processes governing bars, inflow, and (circum)nuclear activity. Here we report the discovery of trailing nuclear spiral structure at scales of less than a hundred parsecs in the core region of the galaxy NGC 5248. We also show that interconnecting spiral structure is present in this galaxy at four different spatial scales, ranging from a hundred parsecs to 15 kpc. This discovery of nuclear spiral armlets demonstrates, once again, the power of adaptive optics imaging, and the quality of the CFHT AO system.
Little is known of the structures that govern the dynamics of disk galaxies at scales smaller than a few hundred parsecs. Circumnuclear starburst regions are often found in barred galaxies at scales of a few hundred parsecs to a few kpc. These starburst "rings" are usually associated with dynamical Inner Lindblad Resonances, where the epicyclic motions of the stars are in resonance with the overall pattern (bar, oval distortion or spiral arms). Beyond the nuclear "rings" the dominant structural features are bars and spiral arms, often seen in combination with ring-like features which are thought to be associated with dynamical resonances.
We have imaged a small sample of nuclear starburst and Seyfert galaxies with the CFHT in the J, H and K bands. The aims of our project include detecting leading spiral arms, similar to those observed in M100 (Knapen et al. 1995a,b), as well as resolving individual star forming regions around the nucleus (Ryder et al. 1998). Images in different NIR bands also allow us to study the dust lane morphology in the nuclear region, which places important constraints on our numerical modelling of how gas dynamics in barred galaxies influences the distribution of gas and star formation in the circumnuclear region (see e.g. Knapen et al. 1995b).
Nuclear spiral structure, such as we have now discovered thanks to AO in NGC 5248, has hardly been seen at such small scales before. Zaritsky, Rix & Rieke (1993) report nuclear spiral structure in M51, but this structure exists outside a 400 pc radius, whereas "our" nuclear spiral is inside of this radius. Grillmair et al. (1997) have reported HST V-band images of the nuclear region of M51. They see small-scale dust lanes, but these dust lanes have a chaotic appearance, although some linear features are also seen. Recently, Regan & Mulchaey (1997) have reported nuclear spiral structure in a sample of Seyfert galaxies, based on a combination of HST NICMOS and WFPC2 images. They speculate that the nuclear spirals could be responsible for helping to feed the active nucleus.
Here we report on the circumnuclear starburst (but not AGN!) galaxy NGC 5248. The starburst "ring", at an average galactocentric radius of about 5 arcsec, is clearly visible in Hubble Space Telescope (HST) images in the ultraviolet (Maoz et al. 1996). We used the Adaptive Optics Bonnette (PUEO) in combination with the Montreal NIR Camera MONICA in March 1997 to take short exposures in J, H and K of the central 9 arcsec × 9 arcsec region of NGC 5248. We used a nearby bright star as the adaptive optics guide star. The total integration time was 180 seconds in each band.
Figure 1 (upper left) shows our adaptive optics J-K colour index image of the central 9 arcsec × 9 arcsec region. We estimate that the resolution of this image is close to the diffraction limit at J, or about 0.2 - 0.3 arcsec. The nuclear starburst region is partly visible near the edges of the image. The most interesting feature in the image is the red (dark in the figure) spiral structure connecting from the star forming circumnuclear region inward. The spiral structure is trailing and fairly tightly wound. Its red colour suggests that it is actually a dust spiral. This mini-spiral, at scales of a few hundreds of parsecs, has not been observed before in this galaxy, not even in the UV HST images.
Figure 1 also shows how the nuclear spiral structures connects outward into the disk of the galaxy. The top right panel of Figure 1 shows an older CFHT MONICA J-K colour index image, taken in 1995, before the AO system was introduced. The image covers the central 1 arcminute in NGC 5248, of which only the central part is shown here, and has subarcsec resolution. Again, red (dusty) spiral structure can be seen, this time outside the nuclear starburst ring, which is located very close to the edge of the AO image shown in the top left panel of Figure 1. Finally, the lower panel of Figure 1 shows a false colour B-band image taken with the Isaac Newton on La Palma, and obtained by us from the ING archive. The main spiral structure in the disk of NGC 5248, at radii of several kpc, can now be seen. Outside of this 2-3 arcmin region, a set of weaker spiral arms can be distinguished, extending out to at least 15 kpc. The details of the morphological and dynamical interconnection of the different spirals will be described in forthcoming papers.
It thus appears that the trailing "grand-design" spiral structure at small scales in NGC 5248, as reported here, is the first of its kind to be unravelled. We are now planning to image a larger sample of galaxies with circumnuclear starbursts to study whether nuclear spirals are common in this type of galaxies, and how they relate to the (circum)nuclear activity of the host galaxy. During our observing nights scheduled for September 1998 we will exploit the larger field of view offered by the combination of PUEO and the KIR camera on the CFHT, which should yield significant progress in our understanding of galaxy dynamics.