Intracluster light: a luminous tracer for dark matter in clusters of galaxies (1807.11488v2)

Abstract: The bulk of stars in galaxy clusters are confined within their constituent galaxies. Those stars do not trace the extended distribution of dark matter well as they are located in the central regions of the cluster's dark matter sub-halos. A small fraction of stars is expected, however, to follow the global dark matter shape of the cluster. These are the stars whose extended spatial distribution results from the merging activity of galaxies and form the intracluster light (ICL). In this work, we compare the bi-dimensional distribution of dark matter in massive galaxy clusters (as traced by gravitational lensing models) with the distribution of the ICL. To do that, we use the superb data from the Hubble Frontier Fields Initiative. Using the Modified Hausdorff distance (MHD) as a way of quantifying the similarities between the mass and ICL distributions, we find an excellent agreement (MHD~25 kpc) between the two components. This result shows that the ICL exquisitely follows the global dark matter distribution, providing an accurate luminous tracer of dark matter. This finding opens up the possibility of exploring the distribution of dark matter in galaxy clusters in detail using only deep imaging observations.

Intracluster Light as a Luminous Tracer for Dark Matter in Galaxy Clusters

This paper presents a detailed examination of intracluster light (ICL) as a robust tracer of dark matter (DM) within galaxy clusters, challenging some conventional methods such as X-ray emission in identifying DM distributions. The authors utilize deep imaging data from the Hubble Frontier Fields (HFF) to assess the spatial correlation between ICL and DM distributions derived from gravitational lensing models. The paper applies the Modified Hausdorff Distance (MHD) metric to compare the similarity between the ICL distribution and mass maps, addressing a key observational challenge—how to accurately map the DM spread in galaxy clusters without relying heavily on gravitational lensing reconstructions, which can be observationally demanding.

Key Insights and Findings

1. ICL and Dark Matter Correlation: The paper demonstrates a high agreement between ICL and DM spatial distributions, with a typical MHD distance of approximately 25 kpc indicating their close alignment. This suggests that the ICL is a reliable luminous proxy for tracing DM distributions, conforming well to theoretical expectations from cosmological simulations.
2. Advantages Over X-ray Emission: The paper highlights that especially in non-relaxed galaxy clusters undergoing mergers, the ICL remains a superior indicator of DM compared to X-ray mapping. X-ray emissions often show significant offsets due to gas dynamics, failing to align closely with DM distributions in turbulent environments. The ICL, being composed of stars stripped from galaxies, behaves as a collisionless component alongside DM, thus accurately following the gravitational potential.
3. Practical Implications: Utilizing ICL as a dark matter tracer offers practical advantages. Its detection through deep imaging is less complex than spectroscopy or the intricate observational setups necessary for gravitational lensing reconstructions. This could simplify future observational strategies, enabling more efficient mapping of DM across expansive regions in clusters.
4. Future Directions and Challenges: The results open pathways for exploring larger scales and alternative clusters to further validate the efficacy of ICL as a DM tracer. As telescope technology advances with upcoming projects like LSST and WFIRST, broader datasets might reinforce or challenge these findings, pushing the precision limits of current methodologies.

Implications and Theoretical Context

The findings align with hierarchical models of structure formation in the Universe, particularly in the $\Lambda$CDM paradigm. As clusters build up through mergers, the ICL forms from stripped galaxies providing clues about the assembly and mass distribution patterns within clusters. The paper underscores ICL’s potential in refining cosmological models and simulations by offering comparatively straightforward observational access to intricate DM maps, which traditionally demands substantial computational resources and observational data.

Conclusion

By providing convincing evidence of ICL's utility as a tracer for the undetected DM in galaxy clusters, this paper advances our understanding of cosmic mass distributions, proposing a practical observational method that might guide future astrophysical research in looking beyond gravitational lensing complexities. The paper sets a promising precedent for employing visible light in tracing DM in diverse cosmic environments, paving the way for enhanced cosmological exploration with fewer observational constraints.