This undoubtedly means that the ionised gas tail is formed after the interaction of the galaxy interstellar medium (ISM) with the ICM. The cometary shape of the tails and the lack of any associated diffuse tidal structure rule out any gravitational perturbation, which would affect the gaseous and the stellar component at the same time. The most convincing evidence comes from the recent, very deep observations of nearby clusters using narrow-band filters centred around the H α Balmer recombination line, which revealed the presence of extended (up to ≃100 kpc) low surface brightness (Σ(H α)≃10 −18 erg s −1 cm −1 arcsec −2) tails of ionised gas. In the local Universe, where high sensitivity multi-frequency observations are available, allowing the detection of the different stellar components and gas phases (cold, ionised, hot) with a spectacular angular resolution, it is becoming evident that within structures as massive as M 200 ≥ 10 14 M ⊙, ram-pressure stripping is the dominant process responsible for the quenching of the star-formation activity of disc galaxies ( Boselli & Gavazzi 2006). 1980) whose contribution might change at different cosmic epochs, is at the origin of these observed differences ( Boselli & Gavazzi 2006, 2014). Sarazin 1986) intracluster medium, (ICM Gunn & Gott 1972 Cowie & Songaila 1977 Nulsen 1982 Larson et al. 1998) or hydrodynamic interactions with the hot ( T ≃ 10 7 − 10 8 K) and dense ( ρ ICM ≃ 10 −3 cm −3, e.g. It is still unclear which, among the different mechanisms proposed in the literature – gravitational interactions ( Merritt 1983 Byrd & Valtonen 1990 Moore et al. In these dense regions, the fraction of quiescent galaxies increases with decreasing redshift (e.g. High-density regions such as rich clusters of galaxies are mainly composed of early-type objects, (ellipticals and lenticulars) while the field is dominated by late-type, star-forming systems. Since the seminal work of Dressler (1980), it has become evident that the environment plays a major role in shaping galaxy evolution.
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The density of the gas derived using the observed λ3729/ λ3726 line ratio implies a very short recombination time, suggesting that a source of ionisation is necessary to keep the gas ionised within the tail. This observation is thus the first evidence that dynamical interactions with the intracluster medium were active when the Universe was only half its present age. All this observational evidence suggests that the gas was removed during a ram-pressure stripping event. The tails, which have a cometary shape with a typical surface brightness of a few 10 −18 erg s −1 cm −2 arcsec −2, extend up to ≃100 kpc (projected distance) from the galaxy discs, and are not associated with any stellar component. Multi-Unit Spectroscopic Explorer (MUSE) observations of the cluster of galaxies CGr32 ( M 200 ≃ 2 × 10 14 M ⊙) at z = 0.73 reveal the presence of two massive star-forming galaxies with extended tails of diffuse gas detected in the λ λ3727–3729 Å emission-line doublet. Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany Stockholm University, Department of Astronomy and Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, University Centre, 10691 Stockholm, Sweden Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands Weilbacher 7Īix Marseille Univ., CNRS, CNES, LAM, Marseille, FranceĮ-mail: Université de Toulouse, CNRS, CNES, UPS, Toulouse, France
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