Ragusa, Rossella (2023) The Intra Cluster Light from loose groups to rich clusters: the relationship between its physical properties and the environment. [Tesi di dottorato]

Anteprima

Testo Ragusa_Rossella_35.pdf Download (372MB) | Anteprima

Abstract

Since proposed and then observed by Zwicky [1937, 1957] in the Coma cluster, we know that there is an additional component in the light distribution of the galaxy clusters, the ICL. The ICL is made of baryons (stars, globular clusters and planetary nebulae) that are bound to no specific galaxy member of the group (cluster), but only to the potential well of the system. This light is observed as a diffuse and very faint emission (μV ≥ 26.5 mag/arcsec2 [e.g. Mihos et al., 2005]) extended out to several hundred of kpc from the group or cluster centre. The scenario of hierarchical formation of structure, within the λCDM paradigm, predicts that the ICL is the product of the gravitational interactions (accretions and mergings) involved in the formation of the most massive galaxies at the centre of groups and clusters, named as the brightest group (BGG) or cluster galaxy (BCG), respectively. Hence, the ICL represents the fossil record of the mass assembly in galaxies and, as a consequence, the physical properties of the ICL, total luminosity, color, stellar population and amount (parametrized by the fraction fICL = LICL/LSystem, in a specific filter) allow us to get knowledge about the formation channels that contributed to the ICL and the dynamical and evolutionary state of the system. A way to constrain the physical processes that form the ICL is to see how its fraction correlates with the virial mass (Mvir) of the host environment. Theoretical predictions, based on the prescriptions of the different simulations, report contradictory results. Due to its low surface brightness nature, the detection and study of the ICL is a challenging task, which requires very deep images covering large areas around the center of the groups or clusters. So far, the few available measurements of the fICL have prevented any conclusion on the correlation with the Mvir. So, at the beginning of my Ph.D., neither from a theoretical side nor from an observational side, there were concrete results about the relationship between the physical properties of the ICL and that of the environment. In particular, since the ICL can be estimated using several observational methodologies and, in addition, it depends on the depth of the data and on the adopted observational strategy, the main requirements needed, to address the dependencies of the fICL with Mvir and the evolutionary state of the environments, are a homogeneous and statistically significant sample of groups and clusters of galaxies, spanning the whole halo mass range covered by the theoretical predictions. No less important, the data should be deep and wide-field enough to be able to detect and study the faint and very extended (i.e. μg ≥ 27mag/arcsec2 and out to hundreds kpc) components of groups and clusters, such as the ICL. Taking advantage of the long integration time and the large covered area of the VST Early Type GAlaxy Survey (VEGAS), we are able to map the galaxies’ outskirts and the ICL component down to μg ∼ 29 - 30 mag/arcsec2 and out to hundreds kpc. Satisfying all the requirements described above, we have given sight to this project to address our limited knowledge about the physical properties of the Intra- Cluster Light (ICL) and their relationship with those of the environments in which it resides, e.g. the Mvir and Early-Type Galaxy ratio, fETG (ETGs/(ETGs+LTGs)). The first step was to build a robust methodology, implementing the one already used for the previous studies of deep photometry inside the VEGAS series, that would be suitable for the study of the ICL. This method lead to consistent results in different environments: we chose Hickson Compact Group 86 (HCG 86) as the first target of this project since compact groups, being very dense environments, represent the best laboratory for analyzing the gravitational interactions between galaxies. We analyzed the photometric properties of the ICL in HCG 86 and illustrated our methodology. These first results, consistent with the theoretical predictions, were published in a first paper [Ragusa et al., 2021]. Then, we tested the method also on a loose group (the "Leo I pair" ), to verify that it performed well even in such kinds of environments. In this work [Ragusa et al., 2022], we obtained not only important achievements on the physical properties of the ICL in this group, but also another solid result about the robustness of our methodology: the photometric transition radius (Rtr) estimated in our work (i.e. the transition between the inner and brighter part of the BCG and the outskirts, e.g. bounded stellar envelope plus unbounded ICL), and the kinematic one found by Hartke et al. [2020] coincide. In fact, estimating the Rtr, with photometric data alone, is really complicated, since the transition between the two components, described above, occurs very smoothly. Once I built a solid methodology, I analyzed 17 new targets inside the VEGAS survey, in a wide range of virial masses (1012.5 ≤ Mvir ≤ 1015). Overall, I revisit the relationship between the fICL and some environmental properties of the systems under investigation, i.e. Mvir and fETG. Using a statistically significant and homogeneous sample of 22 groups and clusters of galaxies ( 17 new targets analyzed by me [Ragusa et al., 2023], and five previously analyzed by others VEGAS members [Iodice et al., 2020b, Raj et al., 2020, Spavone et al., 2018, 2020]) in the local Universe (z ≤ 0.05). With this data set, I have expanded the ICL sample, doubling the previous estimates available from the literature for z ≤ 0.05. This allows us to provide stringent constraints on a possible link between the formation of the ICL with the host environment, and a more robust comparison with the theoretical predictions. Disproving several theoretical studies, and in agreement with some others, the main result of my Ph.D. project (published in an A&A Letter [Ragusa et al., 2023]) is the absence of any significant trend between the fraction of ICL with respect to the virial mass of the host environment. Since the new data points are all derived with the same methodology and from the same observational setup, and all have comparable depth, we concluded that the large observed scatter points towards an intrinsic property of the ICL, and, therefore, does not depend on data processing. On the other hand, a mild relationship between the fraction of ICL and ETGs/(ETGs+LTGs) has been found, where a large fraction of ICL is found in groups and clusters of galaxies dominated by earlier morphological types. Other collateral results of my analysis have been published in the references below. I have also applied the same ICL analysis both to the simulated data predicted for future facilities such as the LSST telescope and to the data from the Hubble Space Telescope, in order to study the ICL evolution with the redshift, which is predicted by the theoretical simulations. These will be the subjects of the future perspectives of this project. Incidentally, the color composite of HCG 86 published in this work (Fig. 2.4) was awarded by the ESO as a picture of the week, in July 2021.

Downloads

Actions (login required)

Modifica documento