Density waves inside the inner Lindblad resonance: Nuclear spirals in disk galaxies

We analyze the formation of grand-design two-arm spiral structure in the nuclear regions of disk galaxies. Such morphology has been recently detected in a number of objects using high-resolution near-infrared observations. Motivated by (1) the observed continuity between the nuclear and kiloparsec-scale spiral structures and (2) low arm-interarm contrast, we apply the density wave theory to explain the basic properties of the spiral nuclear morphology. In particular, we address the mechanism for the formation, maintenance, and detailed shape of nuclear spirals. We find that the latter depends mostly on the share of the underlying gravitational potential and the sound speed in the gas. Detection of nuclear spiral arms provides diagnostics of mass distribution within the central kiloparsec of disk galaxies. Our results are supported by two-dimensional numerical simulations of gas response to the background gravitational potential of a barred stellar disk. We investigate the parameter space allowed for the formation of nuclear spirals using a new method for constructing a gravitational potential in a barred galaxy, where positions of resonances are prescribed.