Robert J. J. Grand, Daisuke Kawata, Mark Cropper
Spiral galaxies are observed to exhibit a range of morphologies, in particular in the shape of spiral arms. A key diagnostic parameter is the pitch angle, which describes how tightly wound the spiral arms are. Observationally and analytically, a correlation between pitch angle and galactic shear rate has been detected. For the first time, we perform a suite of N-body simulations to calculate and compare the pitch angles of both individual density waves and overall spiral structure by use of two independent techniques. We find that higher galactic shear rates produce more tightly wound spiral arms, both in individual mode patterns (density waves) and in the overall density enhancement. Although the mode pattern pitch angles by construction remain constant with time, the overall logarithmic spiral arm winds over time, which is consistent with both the observational scatter in pitch angle versus shear seen from observations, and the recent idea that multiple mode patterns may interfere with each other to create apparently winding spiral arm structures.
View original:
http://arxiv.org/abs/1209.4083
No comments:
Post a Comment