Figure 1. Contour map for the luminosity-weighted mean line-of-sight velocity of red neighbors, aligned for the central rotations of the CALIFA galaxies. Color codes indicate the line-of-sight velocities in units of km/s. [From Figure 12 of Lee et al. 2019b]

More and more studies have been showing that various clues to trace the formation history of a galaxy are hidden in its kinematics. Particularly, galaxy rotation provides simple but strong constraints on the past events of galaxy assembly. Since angular momentum is always conserved in an isolated system, the angular momentum of a rotating proto-galactic gas cloud or a binary system with two merging galaxies must remain in the galaxy formed through such processes. Hence, tracing the variations in angular momenta of galaxies is very helpful to reconstruct the whole picture of galaxy formation and evolution. It is based on simple physics, but in reality the detailed origins of galaxy rotation were not well understood, before the advent of integral field spectroscopy (IFS).

In the last decade, owing to various IFS surveys and related studies, we have learned various respects about how galaxy rotation is influenced by environment. Now it is well known that even early-type galaxies mostly rotate and such rotation is tightly related to environmental density, as well as that direct interactions or mergers between galaxies significantly affect the position angle of galaxy rotation axis in various ways. One of the recent and interesting findings is the discovery of dynamical coherence between galaxy rotation and neighbor motion in several-hundred-kpc scales, which implies that direct fly-by interactions with massive neighbors may significantly change the rotational direction of a galaxy (Lee et al. 2019, ApJ, 872, 78, hereafter L19a ).

Figure 2. Luminosity-weighted mean velocity profiles of the red neighbors. Upper panel: in 1-Mpc bins, Middle panel: in accumulation along distance, and Lower panel: in 3 selected large bins, with statistical uncertainties. [From Figures 10 and 11 of Lee et al. 2019b]

In this study (Lee et al. 2019, ApJ, 884, 104; L19b ), we extended the work in L19a out to 15 Mpc, in order to examine if such dynamical coherence is established even in large scales. The Calar Alto Legacy Integral Field Area (CALIFA) survey data and the NASA-Sloan Atlas (NSA) catalog are used in this work. By combining the IFS information of the 445 CALIFA galaxies and the huge spectroscopic database of about 145,000 NSA galaxies, we built a composite map of the velocity distribution of neighbor galaxies within 15 Mpc from the CALIFA galaxies (Figure 1). Unexpectedly, we found that the dynamical coherence is even detected at least out to 6 Mpc (with 2.8σ statistical significance), as shown in Figure 2. It is an amazing and very mysterious phenomenon, which has never been reported before L19b.

The interpretation of this result is not easy. It is clear that 6 Mpc is too large distance for galaxies to directly interact with each other in such separation. Then, what causes the dynamical coherence in the large scales? Very cautiously, we suggest a scenario, in which the large-scale coherence results from a possible relationship between the long-term motion of a large-scale structure and the rotations of the galaxies in it. If such a motion of a large-scale structure drives the coherent angular momenta of the galaxy-forming proto-clouds in it, the angular momenta will be conserved even after the proto-clouds form galaxies, until they suffer some disturbances from outside like galaxy interactions or merging events. However, further studies both in observations and simulations are required to check possible scenarios and to better understand the origin of this mysterious coherence in large scales.

In the Data page, the basic information and the average dynamics of neighbors are listed for each CALIFA galaxy in our sample.

There is a short news article that highlights our results.