Finding diffuse light in galaxy clusters

In rich galaxy clusters, which contain thousands of Milky-Way sized galaxies along with hot gas and dark matter, there is faint starlight called intra-cluster light (ICL) and here it forms an important component of the overall stellar light budget.  Recent research has established that the diffuse ICL can constitute ~30% of the entire stellar emission from clusters and even dominate the light from the giant central galaxies ubiquitous to large clusters. Hence understanding the properties and origin of the ICL is essential for a full theoretical understanding of the evolution of galaxies, along with the baryonic matter in dense clusters environments.

Observationally there is a big problem measuring the ICL due to its diffuse nature. Typically the ICL has a surface brightness (a measure of diffuseness) which is a million times fainter than the emission caused by the Earth’s atmosphere on a moonless night. This makes it extremely difficult to detect with current astronomical telescopes. Furthermore state-of-the-art image processing and automated detection software of astronomical data is usually tuned to find high surface brightness objects such as bright galaxies and stars and will often miss diffuse emitted light such as ICL. To illustrate this the figure below shows 3 images of the same cluster from the Hyper Suprime Cam Subaru Strategic Program survey (taken from Furnell et al, 2019, MNRAS, submitted). The left hand image shows the original data from the survey. The image clearly shows a dearth (called “divot”) in the background immediately surrounding the central cluster galaxy, arising out of heavy data processing. Such effects prevent accurate recovery of low surface brightness emission. The central image shows the “divot” correction model we apply to account for this systematic over subtraction of the background. The corrected image is given on the right of this figure and clearly shows a vast improvement in the uniformity of the background.

Figure 1 (from Furnell et al, 2019): left - the original HSC image; centre - the "divots" identified by the software; right - the corrected image. Credit: Chris Collins, Kate Furnell (LJMU); Sugata Kaviraj (Herts).

LSST will deliver frequent periodic imaging of large areas to unprecedented depths providing a unique opportunity to study the faint ICL in clusters at cosmological distances for the first time.  Without careful post-processing of the data, we estimate that at least 20% of the ICL will be missed and therefore we are currently in the process of developing algorithms to optimally extract the ICL based on the “divot” method described above. With accurate ICL measurements we will answer key unsolved questions, such as: how quickly does the ICL assemble ; how does the ICL relate to properties of the host clusters?; can we use the ICL as a proxy for the dark matter potential in clusters as suggested by some authors? Armed with answers to these questions we can finally piece together the history of the largest gravitationally bound structures in the universe.