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Tramp stars in the intergalactic space within galaxy clusters: records of a violent history

Not all the stars in a galaxy cluster are found in galaxies, but many of them just float in the intergalactic space, being unbound to any particular galaxy. Where do they come from? And how is this population of stars created? By studying a sample of nearly 700 clusters of galaxies from the Sloan Digital Sky Survey, scientists at the Max Planck Institute for Astrophysics have measured for the first time the systematic properties of this mysterious component. These results reveal important clues on the violent processes by which galaxy clusters are shaped, and represent a valuable benchmark for theoretical models.

Fig. 1: A rich cluster of galaxies that has been utilised for the present study. The fluffy white-yellowish sources are galaxies belonging to the cluster. All the rest are foreground and background sources, including two bright stars. The intracluster light is not visible in images like this, but combining several hundreds of them it can be realiably measured, as explained in the text.

Fig. 2: The creation of intergalactic stars caught in act in the nearby cluster Abell 1367: two trails of gas and stars depart from the two galaxies on the left and in the bottom right corner, probably due to a tidal event occurred 50 million years ago. Courtesy of Peppo Gavazzi, Universita' di Milano Bicocca

Galaxy clusters are large collections of tens to thousends of galaxies, bound together by their mutual gravitational force and extended over several millions of light years. In the optical light a typical cluster of galaxies looks like the one in Fig. 1. Apparently all the light that we see is coming from stars that belong to some galaxy. However, since long time it has been recognized that the intergalactic space is filled in with a diffuse starlight. In 1951 Fritz Zwicky reported that "vast and often very irregular swarms of stars and other matter exist in the spaces between the conventional spiral, elliptical and irregular galaxies" that make up one of the largest known cluster in the constellation of Coma Berenices. Even though this claim probably reflected more the profound conviction of the Swiss astronomer rather than a real detection on the relatively poor photographic material available at the time, the presence of diffuse starlight has been confirmed by several observations in more recent years. In fact, such observations are severely hampered by the intrinsic low surface brightness of this intracluster light (ICL): the luminous flux that we receive per unit area is roughly a factor 1,000 less than what we receive from normal galaxies and roughly the same factor less than the glow of a "dark" night sky. Successful detections have been obtained so far only for a small number of galaxy clusters (less than 40) by means of very long exposures and painstaking data reductions. Yet this is insufficient in order to understand the origin and the role that the ICL has in the context of cluster formation. A systematic description of the ICL is badly needed.

At the Max-Planck-Institut fuer Astrophysik we have recently developed a novel technique (see also linkPfeil.gif"The outermost stellar haloes of galaxies") that allows us to combine images of several hundreds of clusters, so that the resulting sensitivity is strongly enhanced. Images are stacked after masking out all the galaxies in the clusters and the contaminant sources which happen to lie in the field of view (such as foreground stars and other galaxies). In this way, we obtain an image where the diffuse ICL alone emerges from an extremely flat and uniformly illuminated background, and can be measured with unprecedented accuracy over four orders of magnitude in surface brightness. Furthermore, what we measure is the luminous emission averaged over a statistically representative sample, thus making it possible to draw very general conclusions.

The huge imaging database for this study has been provided by the linkPfeilExtern.gifSloan Digital Sky Survey. From roughly 1,500 square degrees in the sky, we have identified 683 clusters of galaxies between z=0.2 and z=0.3, whose images in three different optical bands (i.e. wavelengths) have been stacked.

From our analysis we can assess that a significant fraction (17.5%) of the optical luminosity of a cluster of galaxies is contributed by intergalactic stars. This fraction is apparently independent of the global properties of clusters. This implies, for example, that clusters having more or more luminous galaxies have also more intergalactic stars. Their distribution, however, does not reflect the distribution of the galaxies: intergalactic stars are relatively more abundant in the central regions of clusters than in the outskirts. On the other hand, our data clearly indicate that these stars share very similar properties with those in galaxies, as far as their chemical composition and age are concerned. These and other more complex analyses indicate that intergalactic stars are in fact born within galaxies. The reason why they eventually abandon their birthplace for a tramp life most probably resides in the violent interactions that galaxies undergo in dense cluster environments. Galaxy mergers, repeated galactic encounters, strong tidal fields, that stretch and disrupt galaxies, can produce a large number of these unbound stars, with increasingly higher efficiency the closer one goes to the cluster center.

Our results provide important clues on the relevant physical mechanisms of interaction in galaxy clusters. For the first time, we have produced strong quantitative observations of the amount of "damage" deriving from gravitational interactions, that will be possible to confront with models of cluster formation and evolution.

Stefano Zibetti


Zibetti, S., White, S.D.M., Schneider, D.P., Brinkmann, J., Intergalactic stars in z ~ 0.25 galaxy clusters: systematic properties from stacking of Sloan Digital Sky Survey imaging data, MNRAS submitted

Zibetti, S, and White, S.D.M., Intracluster light at z ~ 0.25 from SDSS imaging data, in the proceedings of "IAU Colloquium 195 - Outskirts of galaxy clusters: intense life in the suburbs - Torino, Italy - March 12-16, 2004", astro-ph/0404326

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