Nuclear spirals as signatures of supermassive black holes

Recent high-resolution images of spiral galaxies show a wide variety of features, including grand-design nuclear spirals in the central region. The morphology of grand-design spirals can be further divided by the openness of the arms, tightly wound spirals with winding angles of around 3pi radians and open spirals with winding angles of around p radians. Based on hydrodynamical simulations, we have investigated the mechanism responsible for the openness of nuclear spirals. Since the gas flow in the nuclear region is mainly governed by the central mass concentration near the nuclei and by the sound speed of the gas, we have examined various models with different mass concentrations, represented by the mass of the central black hole, and with different sound speeds. We found that the tightly wound spirals can be formed when the mass of the black hole is large enough to remove the inner-inner Lindblad resonances and when the sound speeds lie between 15 and 20 km s(-1). Thus, the presence of tightly wound nuclear spiral arms could imply the presence of a relatively massive black hole in the center.