ENVIRONMENTAL EFFECTS ON GALACTIC AND EXTRAGALACTIC STELLAR CLUSTERS ----------------- Project for the Megaprime Survey ----------------- G. Bergond, A. Gomez, J. Guibert, F. Leeuwin, S. Leon, G. Meylan 1999 1) Galactic clusters The study of the structure of Galactic Star Clusters provides many clues on the dynamical evolution of these systems, as well as valuable constraints for models of the Galactic gravitational field. Indeed, the internal dynamics of star clusters, as well as various external effects (Galactic potential, giant molecular clouds...) give rise to several well known phenomena like mass segregation and evaporation. This implies a constant evolution of the shape of open clusters (OCs) and globular clusters (GCs). For some years, realistic simulations (N-body, Fokker-Planck...) of star clusters have been performed (e.g., Terlevich 1987 for OCs or Combes et al. 1999 for GCs). They predict some remarkable effects on the shape of the clusters: significant compression of central regions due to disk or bulge shocking (Leon et al. 1997), tidal tails due to the gravitational harassment by the Galactic field... It is essential to compare these theoretical results to observations. Recently, tidal tails have been detected by us around GCs (Leon et al. 1999a,b) and OCs (Bergond et al. 1999) using star-counts on Schmidt plates. The shapes observed are in very good agreement with predictions of simulations, and it is important to extend these first investigations to more accurate studies based on CCD images. A wide-field detector is necessary, as tidal tails extend very far, up to several tidal radii. The Megaprime giant CCD camera corresponds exactly to our needs, namely, to wide-field multi-color photometric studies of areas around OCs and GCs. The multi-color UBVRI approach is necessary in order to correctly distinguish the cluster members from field stars in the Color-Magnitudes Diagrams (CMDs) we will obtain. After the selection of members, the star-counts will be treated using a wavelet algorithm described in Leon et al. (1999a,b). This method allows to efficiently emphasize extended structures like tidal tails. All these observations will be compared with results predicted by the simulations we are performing in parallel. The large sample of star clusters we propose to observe will provide a complete snapshot of the dynamical evolutionary status of OCs and GCs in the Galaxy. The features detected in each individual cluster will give access to the more general problem of the global evolution of the Galactic open and globular cluster systems, which has fundamental consequences for the formation and evolution of the Galaxy itself. 2) Extragalactic cluster systems We also propose to extend this study to external galaxies. In particular, the observation of stellar cluster systems in Local Group members (M31, M33...), nearby individual galaxies (M81, NGC 2403...), nearby groups (Leo group, NGC 4278, as well as some Hickson groups like H44 or H68), and finally in the Virgo cluster will provide new informations on various cluster systems. GCs were certainly the first stellar systems to have formed, before their host galaxy. Their observation hence provides many clues on chemical and dynamical processes at the epoch of galaxy formation (Searle & Zinn 1978). GC systems probably contain a wealth of information in relation with the history of their host galaxy. Thus, a major goal in this field is to understand the variations observed for the value of the specific frequency Sn of GCs in a galaxy, as well as correlations between Sn and the Hubble type of the galaxy, its environment, etc. An important recent clue, therefore, is given by the detection of two (or more) subpopulations in the GC systems of many early-type galaxies. The redder of the subpopulations is probably of non-primordial origin, and may have been created during mergers (Ashman & Zepf 1992). The young "super-clusters" that have been observed in pairs (cf. Schweizer & Seitzer 1998) could well be the progenitors of those GCs. Our observations of GC systems around a sample of galaxies, in variably dense regions, will allow to better understand the effect of galaxy environments on GC systems (Blakeslee 1999). We plan to do these observations using 5 colors, to sharpen the detection of the GCs, and also the determination of the colours distribution. We hope to detect enough inter-galactic GCs (which should be numerous, if we extrapolate on recent estimates for the intergalactic PN population, cf. Freeman 1999) so that we can initiate their study, and that of their possible relation to the bluer subpopulation of GCs. We also want to compare our observations to numerical simulations in progress, of clusters of galaxies, each galaxy having its own GC system. The numerical models aim at understanding the dynamical effects than can affect the GC systems within a cluster of galaxies, more specifically the tidal stripping creating the population of inter-galactic GCs. A model for the different colour populations is also included, in order to check different scenarii for the origin of GC subpopulations. Needless to say, the use of Megacam is invaluable in order to perform this UBVRI photometric survey of GC and OC systems on selected wide fields around external galaxies. The multi-color approach is still required in order to separate candidate stellar clusters mainly from background compact elliptical galaxies, and also from foreground stars. We can give the following estimates for the exposure times: (All the proposed targets can be observed at a low air-mass from the Mauna Kea and have very different right ascensions so that there are observable targets all over the year.) -> We plan to observe a carefull selection of 20 OCs and 20 GCs, showing a wide range of properties (age, [Fe/H], etc.). To reach medium deep magnitudes (B = 24) we propose to use 5 minute exposures, each field being observed with 5 offsets (dithering pattern in order to remove cosmic rays and obtain a constant illumination on the whole 1 deg^2 field, corrected for the gaps between the CCDs and cosmetic defects). Taking into account a realistic mean overall efficiency of 85% (pointing and guiding, readout time), this implies a total exposure time per cluster of ~140 minutes. That is, we will need 12 nights to perform the whole Galactic survey. Let us point out that this study of OCs and GCs could be particularly interesting for the more general problem of galactic structure and stellar populations, as our targets are located at very different galactic latitudes and longitudes. -> We also propose to observe about 40 fields encompassing Local Group galaxies, nearby groups, and selected areas in the Virgo cluster. With exposures of 10 minutes (in order to detect easily most of the stellar clusters up to the distance of Virgo, that is, m-M=31.2), we hence need about 24 additional nights. ---> A total of 36 nights should thus be devoted to our project. These figures could be revised according to eventual new specifications of the camera at different wavelengths. Finally, would our selection of OCs and GCs, as well as extragalactic fields, be of interest for other groups (e.g., the stellar counts survey), some kind of "joint proposal" could be established in order to avoid redundancy in the observations. --------------33EB22A491B Content-Type: text/plain; charset="us-ascii"; name="working_group.txt" Content-Disposition: inline; filename="working_group.txt" X-Sun-Content-Length: 1676 --------------------------------------------------------------------- | Brief declaration of interest to participate in the working group | --------------------------------------------------------------------- Megaprime Survey Proposal: Environmental Effects on Galactic and Extragalactic Stellar Clusters ____________________________________________________________________ G. Bergond, A. Gomez, J. Guibert, F. Leeuwin, S. Leon & G. Meylan Since we have acquired some expertise in the treatment of wide-field images, in particular at the Image Analysis Center (CAI-MAMA) of the Paris Observatory, we think that our team could delegate one of us to take part in the Megaprime working group. Indeed, our project implies handling large amounts of wide-field, deep, and multi-color data for which we have already developped software, going from Schmidt plates to wide-field CCDs images. We have indeed observed with large CCDs (CFH12K and WFI at La Silla), and we also plan to observe central parts of the Virgo cluster with the CFH12K camera (submission of proposal #F328 on GC systems in Virgo, semester 2000A). Some publications of our team on the subject have been accepted or are in preparation (Leon, Bergond & Vallenari 1999a, A&A 344, 450; Leon, Combes & Meylan 1999b, A&A in press; Bergond, Leon & Guibert 1999, in prep.). In particular, a wavelet method (Leon et al. 1999a,b) has been developed and is well suited to the analysis of extended structures on wide-field images. We are also developing a set of tools aimed at numerical simulations of individuals and systems of star clusters in parallel with our Megacam observations.