TIDAL ARMS ARE UBIQUITOUS IN SPIRAL GALAXIES

Using a self-gravitating disk galaxy tidal perturbation survey, we find that close passages of unexpectedly small mass perturbers can excite grand design spiral patterns. Conservatively, (perturber mass/galaxy mass)/(perturber distance/galaxy disk radius)3 must be at least 0.01 to create a grand design pattern for companions passing in the same sense as the disk turns. For example, a direct disk edge grazer only 0.01 of a galaxy's mass will excite grand design arms. We find using the survey by Holmberg of companions around disk galaxies that about one in three of the disk galaxies in the survey are currently being perturbed at or above the critical level, strongly supporting the tidal hypothesis. A companion getting closer than the critical distance will continue to perturb the galaxy more strongly than the critical level as it approaches the galaxy and as its orbit decays via gravitational drag. Considering both the companion decay time and the arm lifetime, a galaxy in Holmberg's sample with an average number of companions will have a 99% chance of having tidal arms. However, variation in the number of companions per galaxy in the sample implies that the actual percentage is somewhere between 99% and 80%. Tidal arms should thus be ubiquitous (but not universal) in grand design spirals regardless of any internal mechanisms creating spiral arms. These results are consistent with Kormandy and Norman's observational correlation of companions and grand design spiral patterns in differentially rotating galaxies. Our simulation results and examination of Holmberg's survey indicate that the typical encounter which generates a grand design arm pattern will be very close by a low-mass companion. In such encounters, tidal arms form quickly in the outer disk of our simulations of direct encounters then wind up to create a pitch-versus-radius relation like a material arm as a "signature" of recent tidal passage. After the encounter, tidal arms excite "epicyclic spur" arms which extend inward from the tidal arm. Ultimately, the long-term result of a tidal perturbation is a "classical" density wave in the inner disk. If multiple close passages occur, a given galaxy may show each of these two stages of excitation, respectively, in its outer and inner disks. Observational evidence of this sequence is seen in M51 including the "material arm" signature. Close perturbers which are insufficient to create grand design patterns in our simulations create filamentary patterns similar to those actually seen in apparently isolated differentially rotating spiral galaxies by Kormendy and Norman.