The ICL is a very faint and diffuse light enveloping the cluster galaxies. Its detection requires deep and high-quality images, and its study is challenging because it is very complex to disentangle the ICL from the light that galaxies emit. In this project, I applied an algorithm of my design to analyze the ICL, called CHEFs Intracluster Light Estimator (CICLE). Using a sample of 41 clusters observed by the Hubble Space Telescope (HST) within the frame of the Reionization Lensing Cluster Survey (RELICS) and other programs, such as the Frontier Fields initiative, simulations, and fossil groups (FGs), I studied the ICL in systems with different masses: from the massive clusters of the RELICS sample to the lower-mass groups. Additionally, FGs are a particular type of galaxy association, where the brightest cluster galaxy is much more luminous than the other close galaxy members. It is believed that these systems must be ancient to reach this particular configuration and have remained isolated for an extended period of time.
The main result of my project is that the ICL can be used as a highly reliable indicator of the dynamical state of the systems. I observed that the ICL in merging clusters (active) is more abundant than in relaxed (passive) systems. Its color is also different: the ICL in passive clusters is similar to that of the massive galaxy members. In merging systems, it is a mixture of redder and bluer stellar populations. I also measured a parameter called ICL fraction, which compares the light emitted by the ICL with the light emitted by the whole cluster; this is, ICL plus galaxies. I found that active clusters show an excess in the ICL fraction measured at specific wavelengths. In contrast, relaxed clusters have approximately the same ICL fraction at all wavelengths. Moreover, this behavior does not depend on the cluster's distance from us (redshift), but this excess seems ubiquitous. Only the location of this excess (or wavelength where it appears) differs depending on the cluster's redshift, indicating that the stars thrown into the ICL by the violent merger come from galaxies with different properties.
I also discovered that this excess is non-existent for FGs, indicating that these systems are indeed ancient and relaxed, which supports the hypothesis that they have remained undisturbed and isolated for an extended period of time. Additionally, I found that the ICL fractions and the system's mass (calculated from its hot gas distribution, visible in X-rays) can estimate the time elapsed since their last external interaction. All the evidence confirms that FGs are relics from the past. However, not all the systems that satisfy the definition are real fossils since their ICL shows they must have recently undergone a merger. Thus, I can use the ICL fractions to identify bona fide FGs from regular clusters that transitory reached the typical FG configuration.
I also used the ICL for other purposes, such as the discovery of Earendel, the most distant star known so far. Analyzing the cluster WHL0137-08, which is a powerful gravitational lens, we found a highly magnified galaxy forming an arc, dubbed the Sunrise Arc, at redshift 6.2. We observed a star (or a binary system) called Earendel in a region of maximum magnification. The star emitted light 900 million years after the Big Bang taking 12.9 billion years to reach us. I estimated the mass of the ICL in the region surrounding Earendel to predict if the flux of the star should vary in the following years. Although it was a rare possibility, my ICL measurements and microlensing simulations predicted that Earendel's flux should not change. Now, we have HST and JWST observations that confirmed that the flux of the star, surprisingly, has remained almost constant for the last eight years, making this the most successful and astonishing application of the ICL to date.
All the work done during these two years of project have been presented in 12 articles published in peer-reviewed, top-ranked scientifical journals (one in Nature), plus one accepted and another submitted,five international conferences and meetings, four outreach talks (two in the European Researcher's Night), a press release, several interviews for TV, radio, and newspapers, and an article for the bulletin of the Spanish Astronomical Society.
