On the Dynamical Relevance of Galaxy Spiral Arm Evolution. I. Arm Density Structure

We explore the spiral arm structural properties in a large variety of simulated galaxy systems. We study spiral arms arising from isolated barred and unbarred galaxies, as well as from interactions with small satellites. In all these first models, galactic systems are all embedded in a spherical dark matter halo. We also study spiral arms arising from a galactic system embedded in a triaxial dark matter halo. Simulations used in this work have been obtained by using different N-body codes and initial conditions techniques. Our strategy is to study the 3D arm structure through the analysis of pitch angle, along/transverse/vertical density laws and their corresponding scale lengths, and spiral lifetime. Our main results are as follows. First, the radial density profile of all spiral arms analyzed in this work is exponential. This profile resembles the one of the disk but with a scale length that is systematically larger (5%–40%). This result suggests that spiral arm gravitational influence is important beyond the scale radius of the disk. Second, the vertical and transversal density laws of the spiral arms follow a sech2. The vertical scale length is compatible with the one of the disk; this is observed in all spiral arms analyzed here, independently of their origin, i.e., bar, high-order disk perturbation, tidal interaction with satellites, or halo triaxiality. Third, in the triaxial and satellite simulations, spiral arms follow a logarithmic locus all through their lifetime; the remaining models develop transient, recurrent, and short-lived spirals with a nondefined locus. In all cases, spiral arms wind up in their lifetime with a small pitch angle reduction. It is common that newborn spirals inherit the pitch angle of the previous ones; this result challenges the dynamical relevance of arm evolution. Finally, from the analysis of public photometric observations of NGC 2543, we state that the properties of observed spiral arm structure can be consistent with our conclusions. Further and systematic comparisons with observations are needed in order to confirm our results.