2010MNRAS.407..163F

We have observationally quantified the effect of gravitational torques on stars in disc galaxies due to the stellar distribution itself and explored whether these torques are efficient at transporting angular momentum within a Hubble time. We derive instantaneous torque maps for a sample of 24 spiral galaxies, based on stellar mass maps that were derived using the pixel-by-pixel mass-to-light estimator by Zibetti, Rix and Charlot. In conjunction with an estimate of the rotation velocity, the mass maps allow us to determine the torque-induced instantaneous angular momentum flow across different radii, resulting from the overall stellar distributions for each galaxy in the sample. By stacking the sample, which effectively replaces a time average by an ensemble average, we find that the torques due to the stellar disc act to transport angular momentum outwards over much of the disc (within three disc scalelengths). The strength of the ensemble-averaged gravitational torques within one disc scalelength has a time-scale of 4 Gyr for angular momentum redistribution.

The individual torque profiles show that only a third of our sample exhibit torques strong enough to redistribute angular momentum within a Hubble time, mostly those with strong bars. However, advective angular momentum transport is another source of angular momentum redistribution, especially in the presence of long-lived spiral arms, but is not accessible to direct observations. The torque-driven angular momentum redistribution is thus observed to be effective, either in one-third of disc galaxies or in most disc galaxies one third of the time and should lead to either changes in the mass density profile or the orbital shapes.

We use a set of self-consistent disc, bulge and halo simulated isolated disc galaxies with realistic cold gas fractions to verify that the torques exerted by the stellar distribution, such as spiral arms or a bar, exceed those of the gas and halo, as assumed in the analysis of the observations.

This study is the first to observationally determine the strength of torque-driven angular momentum flow of stars for a sample of spiral galaxies, providing an important empirical constraint on secular evolution.