Tidally induced bar-like galaxies in simulated clusters

TL;DR

Using the IllustrisTNG cosmological simulations, the paper analyzes tidally induced bar-like galaxies in cluster environments, focusing on 77 class-A objects among a larger sample of 277 bar-like systems. It shows that bar formation times track pericenter passages around massive cluster hosts, with $t_{bar}$ typically equal to or slightly later than $t_{peri}$, and that all systems undergo strong tidal stripping of dark matter and gas, accompanied by a decline in rotation. The final bar strength $A_{2,\max}$ shows little dependence on the remaining dark matter fraction, indicating that initial progenitor properties and orbital geometry dominate post-formation evolution. The results corroborate earlier controlled simulations of tidal bars, extend them to a cosmological setting, and imply that such bars would appear as S0-like, prolate systems observationally, potentially misclassified as ellipticals in some projections.

Abstract

One of the scenarios for bar formation in galaxies involves their interaction with a more massive companion. The stellar component is then transformed from a disk into a bar-like prolate spheroid. I investigated a subsample of 77 bar-like galaxies tidally induced in the cluster environment, selected among the previously studied sample of bar-like galaxies from the IllustrisTNG simulations. I present six clear, convincing examples of bar-like galaxies formed after an interaction with a progenitor of a massive brightest cluster galaxy (BCG) and describe the properties of their bars. For the whole sample, the time of bar formation is strongly correlated with and typically slightly greater than the time of the pericenter passage. All galaxies are strongly stripped of dark matter and gas, and their rotation is similarly diminished. A larger pericenter distance typically requires a higher host mass in order to transform the galaxy, but the interactions show no preference for prograde configurations. The final strength of the bars does not correlate with the amount of tidal stripping experienced because of the variety of initial properties of the progenitors and the subsequent evolution over the next pericenter passages. In spite of difficulties in the interpretation of some cases involving mergers and multiple interactions, the results confirm in the cosmological context the reality of tidal bar formation in cluster environments previously studied using controlled simulations.

Figures (8)

• Figure 1: Formation of bar-like galaxies in clusters. The columns present the results for different galaxies, ID41623, ID41628, and ID41639. Upper row: Evolution of the total dark, stellar, and gas mass shown with the blue, red, and green lines, respectively. Second row: Same masses, but the vertical scale is adjusted to the stellar and gas mass. Third row: Evolution of three structural properties of the galaxies, the axis ratios $b/a$ (blue line) and $c/a$ (red), and the triaxiality parameter $T$ (green). Fourth row: Rotation measure in terms of the fractional mass of stars on circular orbits $f$ (blue) and the bar strength $A_2$ (red). Fifth row: Distance of the galaxy from the cluster BCG. The vertical dashed lines indicate the pericenter at which the bar was induced.
• Figure 2: Same as Fig. \ref{['evolution1']}, but for other examples of bar-like galaxies.
• Figure 3: Surface density distribution of the stellar components of the six selected bar-like galaxies in the face-on view at the present time. The surface density, $\Sigma,$ is normalized to the central maximum value in each case, and the contours are equally spaced in $\log \Sigma$.
• Figure 4: Profiles of bar mode, $A_2 (R)$, for the six selected galaxies, at their respective last apocenters on the orbit around the cluster. Measurements were carried out in bins of $\Delta R = 0.5$ kpc.
• Figure 5: Evolution of the profile of the bar mode, $A_2 (R)$, over time for galaxy ID177727.