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

TL;DR

This work investigates whether galaxy environment affects bar kinematics by measuring the bar rotation rate $\mathcal{R}$ via the Tremaine–Weinberg method on IFU data from MaNGA and CALIFA for 334 local-universe galaxies. By cross-matching with environmental density $\log\Sigma$, the authors find that bars in high-density environments are significantly slower (median $\mathcal{R}\approx1.67$) than those in low-density environments (median $\mathcal{R}\approx1.37$), with $p_{\mathrm{AD}}=0.002$ and a weak positive correlation $\beta=0.39\pm0.13$, $r=0.20$. The mass dependence is not significant, suggesting environment plays a larger role in bar dynamics. These results support simulations where tidal interactions in dense regions slow bar pattern speeds and highlight the need for larger IFU samples (e.g., Hector) to robustly quantify environmental effects on bar formation and evolution.

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 to 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, $R$, which is used to classify bars as either fast ($1\leq R \leq1.4$) or slow ($R \gt 1.4$). We use estimates of $R$ obtained with the Tremaine-Weinberg method applied to Integral Field Unit spectroscopy from MaNGA and CALIFA. After cross-matching these with the projected neighbour density, $\logΣ$, we retain 286 galaxies. The analysis reveals that bars in high-density environments are significantly slower (median $R = 1.67^{+0.72}_{-0.42}$) compared to bars in low-density environments (median $R = 1.37^{+0.51}_{-0.34}$); Anderson-Darling $\textit{p}$-value of $p_{\mathrm{AD}}= 0.002$ ($3.1\,σ$). 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 $\textit{N}$-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).

Figures (2)

• Figure 1: Distributions of the bar rotation rate, $\mathcal{R}$, for 286 galaxies. The distributions for the low, intermediate and high-density samples fitted with Gaussian kernel estimates are blue, red and black, respectively. The dashed vertical lines indicate the median values of $\mathcal{R}$ in the samples. The Anderson--Darling p-value with the null hypothesis being that the $\mathcal{R}$ values in the low and high-density samples were drawn from the same population is $p_{\mathrm{AD}} = 0.002$, a statistically significant result ($3.1\,\sigma$). This means that galaxies in low-density environments preferentially host faster bars (with lower $\mathcal{R}$) than galaxies in high-density environments, despite a significant overlap between the two distributions.
• Figure 2: Scatter plot of the bar rotation rate, $\mathcal{R}$, against projected neighbour density, $\log~\Sigma$, for 286 galaxies. The error bar represents median errors in $\mathcal{R}$ and $\log~\Sigma$. Low, intermediate and high-mass galaxies are coloured blue, red and black, respectively (the same colouring has been used for the histograms). 11 galaxies with unknown mass are coloured grey. The black dashed line shows a robust linear regression obtained with the BCES $y|x$ method with the grey shaded region representing $1\,\sigma$ confidence. The best-fitting slope to the data obtained with this method is $\beta = 0.39 \pm 0.13$. The Pearson correlation coefficient between $\mathcal{R}$ and $\log~\Sigma$ is $r~=~0.20$ with $3.3\,\sigma$ significance, indicating a statistically significant weakly positive correlation. The histograms show low, intermediate and high-mass galaxy samples fitted with Gaussian kernel estimates. The Anderson--Darling p-value with the null hypothesis being that the $\mathcal{R}$ values in the low and high-mass samples were drawn from the same population is $p_{\mathrm{AD}} = 0.17$ ($1.4\,\sigma$), so the distributions are not statistically significantly different.