A Bridge Between Giants: Tracing the Past of NGC 4709 Through Its Star Clusters

In galaxy clusters, cosmic collisions and mergers are common, and the evidence often lingers in the form of star clusters orbiting their host galaxies. Sara Federle and collaborators explore the elliptical galaxy NGC 4709, the brightest member of a galaxy group known as Cen 45, to understand its past interactions, especially with its massive neighbor, NGC 4696.

Observations and Identifying Globular Clusters

The study begins by highlighting the power of globular clusters (GCs), dense, spherical groups of ancient stars, as tools to study galaxy history. These clusters are resilient to disruption, and their color and spatial distribution reveal signs of mergers and interactions long after the events occurred. Using data from the Magellan 6.5-meter telescope and its MegaCam camera, the team imaged four fields in the Centaurus cluster, which includes both NGC 4709 and NGC 4696. After carefully removing the bright light of the galaxies from the images, they detected tens of thousands of point-like sources and, through shape and color filtering, identified 556 reliable GC candidates orbiting NGC 4709.

Color Patterns and Clues to Galactic History

To distinguish globular clusters from stars and galaxies in the background, the authors applied color and brightness filters based on known GC properties. These clusters came in two distinct color groups, commonly interpreted as metal-poor (blue) and metal-rich (red), with peaks at specific color values. This bimodal pattern is typical for massive galaxies and suggests a mix of GCs formed in the galaxy itself and others acquired during past mergers. Intriguingly, the orientation of the clusters wasn't random. Their distribution showed two main angles, with one pointing directly toward NGC 4696. This alignment supports the idea that past interactions between the two galaxies affected not just their gas and stars, but also their GC systems.

Measuring Distance Through Luminosity

Using the brightness of the clusters, the researchers also estimated how far away NGC 4709 is, about 30 million parsecs (roughly 98 million light-years). This is surprisingly closer than NGC 4696, which lies about 8.5 million parsecs farther. The mismatch in distance suggests that NGC 4709 may have recently passed by NGC 4696 in a close, fast encounter. Such a flyby could strip away star clusters from both galaxies, leaving behind a "bridge" of GCs between them. Indeed, the authors found a distinct population of clusters between the two galaxies with an intermediate distance, strengthening this idea.

Mapping the Star Cluster Landscape

The paper also explores how GCs are distributed across the whole field. Blue clusters, which are more likely to be acquired from other galaxies, are spread more widely and even trace connections to smaller nearby galaxies. In contrast, red clusters are tightly packed around NGC 4709’s center, implying they likely formed within the galaxy itself. This contrast provides a snapshot of how galaxy interactions reshape not only stars and gas but also the surrounding cluster population.

Connecting the Dots Between NGC 4709 and NGC 4696

In addition to their differing distances, the globular cluster systems of NGC 4709 and NGC 4696 show distinct patterns in shape and color distribution. However, the population of clusters located between them shares characteristics with both, especially the red population in NGC 4709. This bridge of clusters is interpreted as a remnant of their past encounter, supporting previous X-ray studies that hinted at shock-heated gas and metallicity changes in the same region.

Conclusion and Future Work

In conclusion, the authors paint a picture of NGC 4709 as an outlier within the Centaurus cluster, having experienced a dramatic past encounter with NGC 4696. Their globular cluster system holds the memory of that event, in their colors, locations, and orientations. The study opens the door for future comparisons with computer simulations, such as E-MOSAICS, to reconstruct the three-dimensional motions and interactions of these galactic giants.

Source: Federle