Stellar kinematics in double-barred galaxies: the sigma-hollows

TL;DR

This study uses integral-field spectroscopy to analyze stellar kinematics in four double-barred galaxies, revealing sigma-hollows at the ends of inner bars caused by contrast between bulge and bar dynamics.

Contribution

It identifies and explains sigma-hollows in velocity dispersion maps as a result of the contrast between bulge and inner bar kinematics in double-barred galaxies.

Findings

Sigma-hollows are observed at the ends of inner bars.

The hollows are caused by contrast between bulge and bar kinematics.

Numerical simulations support the interpretation of sigma-hollows.

Abstract

We present SAURON integral-field stellar velocity and velocity dispersion maps for four double-barred early-type galaxies: NGC2859, NGC3941, NGC4725 and NGC5850. The presence of the inner bar does not produce major changes in the line-of-sight velocity, but it appears to have an important effect in the stellar velocity dispersion maps: we find two sigma-hollows of amplitudes between 10 and 40 km/s on either side of the center, at the ends of the inner bars. We have performed numerical simulations to explain these features. Ruling out other possibilities, we conclude that the sigma-hollows are an effect of the contrast between two kinematically different components: the high velocity dispersion of the bulge and the more ordered motion (low velocity dispersion) of the inner bar.