Bars in low-density environments rotate faster than bars in dense regions

ABSTRACT Does the environment of a galaxy directly influence the kinematics of its bar? We present observational evidence that bars in high-density environments exhibit significantly slower rotation rates than bars in low-density environments. Galactic bars are central, extended structures composed of stars, dust and gas, present in approximately 30–70 per cent of luminous spiral galaxies in the local Universe. Recent simulation studies have suggested that the environment can influence the bar rotation rate, $\mathcal {R}$, which is used to classify bars as either fast ($1\le \mathcal {R}\le 1.4$) or slow ($\mathcal {R}>1.4$). We use estimates of $\mathcal {R}$ obtained with the Tremaine–Weinberg method applied to Integral Field Unit spectroscopy from Mapping Nearby Galaxies at Apache Point Observatory and Calar Alto Legacy Integral Field Area. After cross-matching these with the projected neighbour density, $\log \Sigma$, we retain 286 galaxies. The analysis reveals that bars in high-density environments are significantly slower (median $\mathcal {R} = 1.65^{+0.13}_{-0.11}$) compared to bars in low-density environments (median $\mathcal {R} =1.39^{+0.09}_{-0.08}$); Anderson–Darling p-value of $p_{\mathrm{AD}}= 0.002$ ($3.1\, \sigma$). This study marks the first empirical test of the hypothesis that fast bars are formed by global instabilities in isolated galaxies, while slow bars are triggered by tidal interactions in dense environments, in agreement with predictions from numerous N-body simulations. Future studies would benefit from a larger sample of galaxies with reliable Integral Field Unit data, required to measure bar rotation rates. Specifically, more data are necessary to study the environmental influence on bar formation within dense settings (i.e. groups, clusters and filaments).