Sticky-Particle Simulations of Barred Galaxies

The most fundamental property of a bar is its pattern speed. Whereas observations seem to favor fast bars, theoretical models and the most sophisticated live halo N-body models suggest a significant slowdown of bars via angular momentum exchange with the other galactic components, in particular with massive halos. Nevertheless, reliable direct measurements (Tremaine-Weinberg method using stellar absorption line kinematics) are limited to early type spirals, mainly to S0's: it would therefore be very interesting to address how bars rotate in intermediate and later type spirals. We are using sticky-particle simulations to estimate the pattern speeds of galaxies over a wide range of Hubble types. Our simple approach - following inelastically colliding test particles orbiting in a rigidly rotating fixed gravity field - does not address the pattern speed evolution or the possibility of multiple pattern speeds, but instead gives an estimate for the instantaneous single pattern speed consistent with the observed morphology of a given galaxy. In particular, if kinematical data are also available the best matching pattern speed should be accurate within about 10 percent (Salo et al. 1999, AJ 117, 792). We have recently (Rautiainen, Salo & Laurikainen 2005, ApJ 631, L129) applied this method to modeling of 38 barred galaxies from the Ohio State University Bright Galaxy Survey, using the gravitational potentials derived from the H-band survey images (Laurikainen et al. 2004 MNRAS 355, 1251, Buta et al. 2004 AJ 127, 279), and using the B-band images to trace the bar-induced ring and spiral structures. Our simulations, comprising the largest uniform set of indirect pattern speed estimates, agree well with the direct measurements of fast bars in early types. However, they also indicate a tendency of slower bars in later types: if confirmed, such a dependence will provide an important constraint for the theoretical models of bar-halo interaction. Similar modeling is now in progress using the publicly available Spitzer 3.6 micron (used for gravity potential) and 8 micron data (warm dust as a morphological tracer). This work has been supported by the Academy of Finland, and by the Magnus Ehrnrooth Foundation.