Galaxy Systems

Andrea Biviano, Mario Nonino, Massimo Ramella, Paolo Tozzi

Galaxy systems (groups and clusters of galaxies) are the largest objects of the Universe in dynamical equilibrium. They are large overdensities of galaxies, from 10 to 10000 times larger than the average field galaxy density. A notable example of a large galaxy cluster (1E0657-55) observed with the VLT is shown in the Figure above. Despite the visual appearance, only 5% of the total mass of a galaxy system is actually contained in stars. X-ray band observations reveal in fact that all galaxy clusters contain very large quantities of hot diffuse gas (composed of hydrogen, helium, and traces of heavier elements, at temperatures of hundreds millions degrees). Such X-ray emission is observed even in the most distant clusters known, such as RDCS1252.9-2927, at a redshift z=1.23. The optical image of this distant cluster is shown in the Figure below (courtesy of P. Rosati) together with the cluster diffuse X-ray emission.

Dynamical analyses indicate that the hot gas component together with the stellar component only amount to the 20% of the total mass of a galaxy system. The rest is in the form of a mass component that cannot be detected through the electromagnetic radiation, and is therefore called 'dark matter'. Dark matter is most likely non-baryonic.

The analysis of galaxy systems therefore concerns several aspects of the extragalactic astronomy: the mass content of the Universe and the formation and evolution of large scale structures, the evolution of the galaxies members of groups and clusters, and the interplay between these galaxies and the intracluster diffuse hot gas. At the Astronomical Observatory of Trieste, we deal with these topics through the following research programs:

Wide field studies of Local Group galaxies - The dwarf spheroidals are the most common galaxies in the Local Group of galaxies, and possibly in the whole universe. The star formation history of these galaxies is characterized by several recurrent episodes of star formation. The Carina galaxy is a typical example of a dwarf spheroidal with a very complex star formation history. In order to investigate the origin of such a complex star formation history, at the Trieste Observatory we have obtained wide-field optical images of Carina using WFI@ESO2.2 and the MOSAIC-II camera at the prime focus of the 4m telescope of CTIO. We aim at studying the stellar component of Carina, and investigating its star formation history characterized by at least three very strong starburst episodes. Currently, we are performing spectroscopic observations with VIMOS of stars selected by their location in the colour-magnitude diagram. These observations will allow us to analyse the kinematics of stars born in the three different starburst events (the Figure below show a field in Carina).

Groups of galaxies - Groups are systems of few galaxies, considerably less than usually contained in galaxy clusters. The properties and identification of galaxy groups must be obtained from the observations of a quite limited number of galaxies per group. Therefore, the study of galaxy groups is a very difficult task. Despite this, it is very important to study galaxy groups for several reasons, such as: a) according to current evolutionary theories of the universe, galaxy clusters form via the aggregation of groups; b) there are far many more groups than clusters, so galaxies usually reside in groups; c) groups are generally characterized by younger dynamical age than clusters, so they represent a different environment where galaxy evolution can be studied.

In our research we deal with many aspects of the group of galaxies problematic: from their identification, to the estimate of galaxy groups properties, from the analysis of the group dynamical status, to the comparison with cosmological models predictions. The identification of groups in the optical bands is obtained through the analysis of the projected ditribution of galaxies on the sky. The properties of the identified groups may depend on the identification algorithm. At the Trieste Observatory we have developed an algorithm that performs significantly better than traditional ones (see progetto VORONOI ).

In order to gain a full understanding of the observational properties of galaxy groups, we use a multiwavelength approach, from the X-ray band to the infrared, although most of our analyses are based on optical observations. Observations in the X-ray band allows to detect the overall electromagnetic emission of a galaxy group as a whole, not as the sum of the independent emission of the individual galaxies (see the X-ray emission from the group RXJ0916+1736 in the Figure above, credit: MPE). Detection of the diffuse X-ray emission from a group is therefore a clear indication that the group itself is a physical entity rather than a mere apparent concentration of unrelated galaxies seen in projection on the sky. On the other hand, observations in the infrared sample the emission of the oldest stars in the group galaxies. These old stars are much more long lived than those stars emitting most of their radiation in the blue band. These young blue stars can be created by sudden starburst events stimulated by, e.g., interactions with neighboring galaxies. The infrared emission is therefore a more reliable tracer of the group luminosity averaged over a long time interval, than the optical blue emission. In the Figure below, the group of galaxies USGCC 215 is shown (the image is taken from the Digital Sky Survey). Confirmed group members are identified by yellow circles.

Structure and internal dynamics of galaxy clusters - Determining the mass, the mass distribution and the internal structure of galaxy clusters is very important for Cosmology. Such studies allow to set observational constraints on theories of cosmic structure formation and evolution, as well as on the nature of the dark matter. At the Trieste Observatory this research topic is addressed by the analysis of nearby and intermediate-distance clusters (out to a distance of 5 billions light years), for which the data allow us to preform detailed dynamical analyses. We investigate in particular the relative distribution of dark and baryonic matter (galaxies and the diffuse intracluster gas), the presence of substructures, i.e. remnants of groups gravitationally accreted by the cluster. In the Figure below, we show an optical band image of the clusterAbell 1445; galaxy number density isocontours indicate the presence of a significant substructure (at the lower right-hand part of the image).

The evolution of cluster galaxies - The study of galaxy evolution is fundamental to our understanding of the cosmological theories. Clusters only contain a very small fraction of all the galaxies in the universe, yet they are a very important laboratory for the analysis of galaxy evolutionary processes. In fact, clusters are characterized by their high mass density and by the presence of a very hot diffuse gas, and both can significantly affect the properties of cluster galaxies. As a matter of fact, the properties of cluster galaxies are quite different from those of field galaxies. Groups, on the other hand, are the most common environment of galaxies, an environment with intermediate characteristics between the cluster and the field environments. At the Trieste Observatory we analyse the properties of cluster and group galaxies via multiwavelength observations, focussing in particular on the infrared emission of galaxies (observed with the Infrared Space Observatory) and on optical spectral features characterizing the evolutionary age of the galaxies. We are particularly interested in determining how galaxy properties are affected by the very energetic cluster-group collisions. We also study the evolution of field galaxies as they infall into the cluster gravitational potential well.

The property of the intracluster diffuse hot gas - The extraordinary performances of the Chandra and XMM satellites now allow to analyse the X-ray emission of very distant clusters of galaxies, out to redshifts larger than z=1 (corresponding to a lookback time of approximately 10 billion years). The analyses of the clusters X-ray spectra clearly show the presence of heavy elements ("metals") in clusters, Iron and Oxygen in particular. The density of these heavy elements indicates that the diffuse intracluster gas (which is responsible of the cluster X-ray emission) has been enriched by elements produced by the star formation processes occurred in the cluster galaxies. The relations which hold among all X-ray observables (total luminosity, gas temperature, spatial distribution), are also indicators of complex evolutionary processes which are dependent on the cosmological model and on the energetic processes on galactic scales. At the Trieste Observatory we are currently investigating a sample of distant galaxy clusters, mostly in the X-ray (with multiwavelength observations of the most interesting clusters of our sample). We want to understand which is the link between the history of cosmic baryons and the star formation processes. This is currently a major unsolved problem of the observational and theoretical cosmology, entering any model for the formation of cosmological structures. In the Figures below, we show an X-ray image of the cluster MS1054 (at a redshift z=0.83) and its spectrum, as observed with Chandra.