An Observational Estimate for the Mean Secular Evolution Rate in Spiral Galaxies

TL;DR

This paper observationally quantifies how gravitational torques within stellar disks of spiral galaxies drive angular momentum redistribution, with a typical timescale of about 4 billion years, offering new empirical insights into galaxy evolution.

Contribution

It provides the first observational measurement of torque-driven angular momentum flow in spiral galaxies, constraining secular evolution processes.

Findings

Stellar gravitational torques transport angular momentum outward in disks.

The timescale for angular momentum redistribution is approximately 4 Gyr.

This process predominantly occurs within three disk scale lengths.

Abstract

We have observationally quantified the effect of gravitational torques on stars in disk 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 disk act to transport angular momentum outward over much of the disk (within 3 disk scale lengths). The strength of the ensemble-averaged gravitational torques within one disk scale length have a timescale of ~ 4 Gyr for angular momentum redistribution. 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. (abridged)