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Longobardi, Alessia (2015): Where stellar halos coexist with intracluster light: a case study of the giant Virgo-central galaxy M87. Dissertation, LMU München: Faculty of Physics



The work presented in this thesis studies the role that accretion events play in the evolution of galaxies in dense environments, such as galaxy clusters. Cosmological simulations allow us to study in detail the evolution of galaxies' halos in cluster environments and have shown that the formation of extended halos around central cluster galaxies and intracluster light (ICL) is closely correlated to the morphological transformation of galaxies in clusters. However, the extremely low surface brightness of these components makes it difficult to gather observational constraints. This thesis studies the light and stellar motion in the halo of the giant elliptical galaxy M87 and its surrounding IC component at the centre of the Virgo cluster. Virgo is the nearest ($\sim 15$~Mpc away) large scale structure, a young cluster characterised by both spatial and kinematic substructures. M87 has one of the oldest stellar populations in the local Universe and a stellar halo that contains $\sim$ 70\% of the galaxy light down to $\rm{\mu_{V}=27\, mag\, arcsec^{-2}}$. Moreover, deep images of the Virgo cluster core have revealed an extended network of tidal features suggesting that accretion events characterise its mass assembly. Thus, M87 and its host environment are prime targets to shed light on the hierarchical assembly of structure in the Universe. This work uses new Suprime-Cam@Subaru photometry and FLAMES@VLT spectroscopy to study a $\sim 0.5\, \rm{deg^2}$ area around M87, in the transition region between galaxy halo and ICL. We use planetary nebulas (PNs) as tracers, whose strong [OIII] $\lambda$5007 \AA\ emission line makes them excellent photometric and kinematic probes, also at large distances from the galaxy's centre. The photometric analysis of the PN sample shows the superposition of two stellar populations, both halo and ICL. This is confirmed by spectroscopically detected PNs, whose velocity phase-space also reveals that halo and ICL split into two different kinematic components. They have very different spatial distributions and parent stars, as indicated by the properties of the PN populations they are associated with, such as the $\alpha$-parameter and the slope of the planetary nebula luminosity function (PNLF). In this thesis I give the observational proof that in Virgo the central galaxy and the ICL both evolve through the ongoing accretion of smaller systems. However, stellar halo and ICL are dynamically distinct components with different velocity and density distributions, and parent stellar populations. Whether or not these conclusions are true for different galaxies in different Virgo subclusters is still an open question and the topic of one of my planned future studies.