Jose Alfonso López Aguerri, Rafael Barrena Delgado, Casiana Muñoz Tuñón, Cristina Zurita Espinosa, Claudio Dalla Vecchia, Walter Boschin, Alejandro Lumbreras Calle
E.M. Corsini, L. Morelli (Univ. Padova); J.M. Vílchez, J. Iglesias (IAA, Granada); C. del Burgo, E. Jiménez Bailon, S. Sánchez (UNAM, Mexico); N. Napolitano (Obs. Capodimonte); M. Girardi, S. Borgani (Univ. Trieste); A. Biviano (Obs. Astronómico de Trieste); V. Debattista (Univ. Central Lancashire); E. D'Onghia (Univ. Wisconsin-Madison); M. Moles (Centro de Estudios de Física de Aragón); M. de Santos Lleo (ESA); M. Arnaboldi (ESO); O. Gerhard (MPIA); R. Sánchez Janssen (ATC, UK); M. Huertas-Company (Obs. Paris); A. Diaferio (Univ. Turin), V. Wild (Univ. St Andrews); S. Zarattini (Osservatorio Astronomico di Trieste); A. Aragón-Salamanca (Univ. Nottingham), R. Peletier (Kapteyn Institute); S. Trager (Kapteyn Institute, Netherlands); G. Dalton (Oxford University)
Galaxies in the universe can be located in different environments, some of them are isolated or in low density regions, they are usually called field galaxies. The others can be located in galaxy associations, going from loose groups to clusters or superclusters of galaxies. One of the foremost challenges of the modern Astrophysics is to achieve a complete theory about galaxy evolution. This theory should explain the relation between the environment and the galaxy evolution. Galaxy clusters are high density environments where galaxies interact one to each other and with the intracluster material (ICM). In addition, the cluster dynamics is drove by the high density and quantity of dark matter present in them. Therefore, galaxy clusters are complex systems with multiple components (galaxies, ICM, dark matter) which are tightly bounded. The mix of all these components, as well as their interactions, makes galaxy clusters ideal laboratories to study the different mechanisms which cause the different evolution of galaxies in this high density environments with respect to field galaxies.
It is well known that the observational properties of galaxies present in the field and in high density environments are different. The morphology-density relation probably represents one of the most important hints which demonstrate these differences (Hubble & Humason 1931, ApJ, 74, 43; Dressler 1980, ApJ, 236, 351). It establish that early type galaxies (ellipticals and S0s) are more frequent in clusters than late-type ones. In addition, early-types are preferentially located in the central regions of the clusters while late-type galaxies are present mostly in their outer regions. Other observational properties which differs from clusters with respect to the field are the presence of the brightest cluster galaxies (BCG; Rines et al. 2007, ApJ, 665, L9) and the intracluster diffuse light (ver Aguerri et al. 2005, AJ, 129, 2585). BCGs properties are correlated with the cluster properties, for instance, they are located in the center of the potential well and they have luminosities that correlates with the total luminosity and mass of the cluster (Lin & Mohr 2004, ApJ, 617, 879). Observational evidences suggest that BCGs formation is mainly dominated by the merger of smaller galaxies, during this process, an important amount of stars could be unbounded from the galaxy giving rise to the intracluster light (De Lucia & Blaizot 2007, MNRAS, 375, 2; Murante et al. 2007, MNRAS, 377,2). On the other hand, the amount of neutral hydrogen present in disk galaxies represent also a hint of the different evolution between cluster and field galaxies. In fact, it is found that galaxies in clusters are deficient in HI with respect to the field and that this deficiency is greater when galaxies are located close to the cluster center (Solanes et al. 2001, ApJ, 548, 97).
These observational differences between cluster and field galaxies are believed to be caused by the different processes that take place in the high density environments. These mechanisms can be divided in two categories: gravitational interactions between galaxies and the cluster potential, and hydrodynamics interactions between the interstelar medium and the ICM. The main mechanisms proposed until now are: dynamical friction (Chandrasekhar 1943, ApJ, 97, 255; Binney & Tremaine 1987, Galactic Dynamics), mergers and tidal interactions between galaxy pairs (Binney & Tremaine 1987), tidal interactions between galaxies and the cluster potential, harassment (Moore et al. 1996, Nature, 379, 613; Moore et al. 1998, ApJ, 495, 139; Moore et al. 1999, MNRAS, 304, 465), ram-pressure stripping (Gun & Gott 1972, ApJ, 176, 1; Quilis et al. 2000, Science, 288, 1617), and starvation (Larson et al. 1980, ApJ, 237, 692).
All these mechanisms induce morphological transformations in the galaxies. The main aim of this project is to understand the importance of every mechanisms as a function of the environment and how they dominates the evolution of the different galaxy types (giants and dwarfs) in clusters. Quantifying the efficiency of these mechanisms observationally is not easy since some of them are in act at the same time, take different timescales to be important, and they efficiency depends on the position in the cluster. However, as explained above, a series of observational evidences can be directly measured: i) the morphological distribution of galaxies in the cluster (type vs radius), ii) the luminosity functions, iii) the diffuse light (amount and distribution) and iv) the presence of substructures. In this project we plan to measure these quantities for nearby galaxy clusters.
The build-up of the red sequence in the Hercules clusters
We present the study of the colour-magnitude diagram of the cluster Abell 2151 (A 2151), with a particular focus on the low-mass end. The deep spectroscopy with AF2/WYFFOS@WHT and the caustic method enable us to obtain 360 members within 1.3 R200 and absolute magnitude Mr ≲ M*r+6. This nearby cluster shows a well defined red sequence up to Mr = -18.5; at fainter magnitudes only 36 per cent of the galaxies lie on the extrapolation of the red sequence. We compare the red sequences of A 2151 and Abell 85, which is another nearby cluster with similar spectroscopic data, but with different mass and dynamical state. Both clusters show similar red sequences at the bright end (Mr ≤ -19.5), whereas large differences appear at the faint end. This result suggests that the reddening of bright galaxies is independent of environment, unlike the dwarf population (Mr ≥ -18.0).
Figure 1: Lower panel: colour–magnitude diagram of the galaxies in the direction of A 2151. Dark grey dots are the galaxies excluded from target with the colour-cut. Light grey dots are the target galaxies and black points are the velocities obtained. Red and blue symbols show red and blue cluster members, respectively. The solid line represents the red sequence of the cluster. Upper panel: spectroscopic completeness (C, green diamonds), red (Cred, red dots) and blue (Cblue, blue squares), and cluster member fraction (fm, black triangles) as a function of r-band magnitude. Note that the red sequence is not well defined for magnitudes fainter than -18.5
Formation of dwarf galaxies by interactions in galaxy clusters
The surface brightness distribution of ~30-40% of the early-type dwarf galaxies with - 18 ≤ MB ≤ -15 in the Virgo and the Coma clusters is fitted by models that include two structural components (Sérsic + exponential) as for bright disc galaxies. Are these low-luminosity galaxies copies of bright disc galaxies? or are the remnants of bright galaxies strongly transformed by cluster environmental effects?. We have analysed the location of bright disc galaxies and early-type dwarfs in the rb,e/h- n plane. The location in this plane of the two-component dwarf galaxies was compared with the remnants of tidally disrupted disc galaxies reported by numerical simulations. Bright unbarred disc galaxies show a strong correlation in the rb,e/h-n plane. Galaxies with larger Sérsic shape parameters show a higher rb,e/h ratio. In contrast, two-component early-type dwarf galaxies do not follow the same correlation. A fraction (~55%) of them are located outside the locus defined in this plane by having 95% of bright disc galaxies. This distribution indicates that they are not a low-mass replica of bright disc galaxies. The different location in the rb,e/h- n plane of two-component early-type dwarfs and bright galaxies can be qualitatively explain whether the former are remnants of disc galaxies strongly transformed by tidal processes. This implies that the progenitors of ~20-25% of early-type dwarf galaxies with - 18 ≤ MB ≤ -15 in the Virgo and Coma clusters could be bright disc galaxies transformed by effects of the environment. These tidally transformed galaxies can be selected according to their location in the rb,e/h-n plane.
Figure 2: Distribution of bright disc galaxies (grey region) and dS0 (red dots and green squares) in the rb,e/h–n plane. The grey region represents the locus of 95% of the bright disc galaxies in this plane. The solid line shows the median of the distribution of the bright disc galaxies. The upper right corner of the rb,e/h–n relation for bright disc galaxies is mainly made by bulge-dominated systems. In contrast, disc dominated systems are mainly located in the lower left corner of the relation. The black stars represent fast rotators according to the classification given by Toloba et al. (2014)