MUSE Kinematics Show NGC 3957 Bar Formed at Least 7 Gyr Ago

Researchers applied dust-corrected multi-Gaussian expansion and population-orbit superposition modeling to MUSE spectra of NGC 3957. They decomposed the galaxy into a cold main disc with vertical velocity dispersion of 20-30 km/s, a compact nuclear stellar disc, and a hot component. The nuclear disc's age and metallicity indicate the bar has persisted since at least 7 Gyr ago while the outer disc shows a positive age gradient beyond the bar radius. The main disc's low dispersion rules out significant mergers in the last 8 Gyr, pointing to secular evolution inside a small-to-medium group rather than cluster stripping. This outside-in fading pattern arises from mild gas starvation combined with bar-driven central star formation, producing signatures that can be mistaken for dense-environment effects seen in Fornax S0s. Comparison with cluster lenticulars shows internal processes alone can generate the observed stellar population gradients. Future wide-field IFS surveys targeting group S0s at similar resolution will test whether this faded-spiral pathway dominates outside rich clusters.