Diffuse Light Bridge in Abell 3667 Signals Early Stages of Major Merger
The space between galaxies in a cluster is not entirely dark. Scattered across these vast regions is a faint, diffuse glow known as intracluster light (ICL). The source of this light is believed to be groups of stars that have been stripped from their parent galaxies. At these distances, individual stars are far too faint and closely packed to be resolved, but their combined light appears as a haze permeating the cluster. These are able to uniquely trace the cluster’s gravitational potential and can retain a record of past interactions long after other signs have faded, providing insights into a cluster’s dynamical history and the distribution of its dark matter.
The upcoming Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) is expected to capture the entire southern sky to unprecedented surface brightness limits, which would enable highly detailed analysis of the ICL in this region.
Now, a team led Anthony Englert present their use of openly accessible pipelines provided by the LSST to process deep archival data from the Dark Energy Camera (DECam), the same instrument used for the Dark Energy Survey (DES) between 2013 and 2019. In their dataset, the team managed to uncover a bridge of ICL connecting the two brightest galaxies within the Abell 3667 cluster.
Their study aimed to produce the deepest possible ground-based optical imaging of the cluster to date. This required assembling ~30 hours of exposures in multiple filters captured over the last ten years, then passing them through a modified version of the LSST Data Release Pipeline. Key adaptations included correcting for subtle instrumental effects and applying an advanced background-modelling algorithm called ‘skycorr’, which aims to preserve low brightness features.
Once processed, the data revealed a network of highly diffuse structure built around two specific galaxies, BGC1 and BCG2. In particular, they discovered an almost rectangular region of ICL connecting the two. This bridge is visible within optical bandwidths, and spans the ~400 kpc space between these targets.
Further analysis of the light surrounding these targets revealed that both BCG1 and BCG2 exhibited steep, negative colour gradients in their inner regions, although in BCG2, the gradient briefly flattens between 50 and 100 kpc, matching the uniformly flat colour of the bridge itself. This match suggests that the bridge’s stars were stripped from BCG2 during a close encounter with BCG1. The team also examined an intermediate galaxy, (LEDA 64218) along the bridge’s path and found no evidence that it had contributed significantly to the feature.
The authors propose that Abell 3667 is in the early stages of a major merger. The two brightest galaxies may have recently passed each other, with tidal forces disrupting BCG2 and drawing out the stellar bridge. This picture is consistent with existing radio and X-ray evidence.
Englert et al. note that future LSST observations will reveal structures one magnitude dimmer, add four more photometric bands, and cover the entire cluster environment. Such detail would enable precise age and metallicity mapping of most features. The team also intend to conduct a full multi-wavelength analysis of Abell 3667, combining optical, X-ray and radio data with simulations to better constrain its dynamical history.
--
Journal Source: A. M. Englert et al., The Intracluster Light of Abell 3667: Unveiling an Optical Bridge in LSST Precursor Data, The Astrophysical Journal Letters, Vol. 989, No. 1, (2025), DOI: 10.3847/2041-8213/ade8f1
Cover Image Credit: Abell et al. 1989