Galaxies in Progress

Research project P 14278 Galaxies in Progess Karl RAKOS 06.03.2000 Photometrical and Morphological Evolution of Galaxies in Clusters Since the discovery of the Butcher-Oemler effect 1978, ApJ.219, 18, numerous photometric and spectroscopic studies have concentrated on this high fraction of blue, star forming galaxies in distant rich clusters, which seems to be absent in their nearby, present-day counterparts. We have overcome the k-correction problem using rest-frame Str6mgren photometry to map the evolution of galaxies in clusters between z = 0.2 and z = 0.9. and shown the fraction of blue galaxies to rise from an average of 20% at z = 0.4 to 80% at z = 0.9 (Rakos & Schombert 1995, Ap.J.439. 47) However, the fate of the blue population seen in high redshift clusters still remains unclear, in spite of the considerable effort put into this area of research over the last 10 years. One of the reasons for this might be the intrinsic color-morphology degeneracy. Indeed, the overlap between morphological data of high-z cluster galaxies and multicolor or spectral information is rather poor due to the different telescope-apertures used for either type of observations. Where this overlap exist, it has revealed, that most luminous blue cluster galaxies are normal late- type systems. supplemented by starbursting and / or post-starburst interacting or merging galaxies. Our data contain some clues pointing to a possible solution: We obtained the luminosity functions of blue galaxies in A2317, A2283, Al 15 and A2218 which display a clear excess at faint magnitudes. The colors of many of these faint, blue galaxies are consistent with a starburst phenomenon. They are in fact bursting dwarf galaxies, which would also be consistent with the large number of dwarf elliptical and nucleated dwarf galaxies seen in nearby clusters like Virgo and Fornax. If the dwarfs in nearby clusters are indeed the present-day counterparts of the faint blue, high-z cluster galaxies, then one would expect them to disply starburst and post-starbudtt signatures in their spectral distribution. We want to verify this by imaging the Fornax Cluster with Str6mgren filters. As we have shown in ApJ. 466, 122, Str6ringren photometry can identify post-starburst object and even attribute ages, i.e. time elapsed since last burst, by comparing the observations to synthetic colors derived from population synthesis models. During the October 1998 1 and my c6investigator S. Steindling from Tel-Aviv University, have used CTIO Schmidt telescope to observe Fornax cluster. In 9 clear nights we have recorded about 700 frames 2k x 2k, covering about 10 square degrees of the cluster. We have been able to observe magnitude and colors (u,v.b.y) of large number cluster dwarfs. If the nucleated dwarfs in nearby Fornax cluster are indeed the present day counterparts of the faint, blue, high-z cluster galaxies, then one would expect them to display starburst and post-starburst signatures. The number density of blue and red galaxies is a strong function of cluster radius. The red population dominates the core of the cluster with the blue population having a marked decrease inside 0.3 Mpc. This is in agreement with one of the predictions of "galaxy herassment" models, where the blue population is due to tidally induced star formation from the cluster and the most massive members. We have found that the fraction of EISO galaxies depends strongly from the fraction of Starbursts in the clusters and also on the fraction of blue galaxies and further it is corelated with the radius of the maximum number density of blue galaxies from the cluster center. The blue fraction is smaller in rich clusters with larger central velocity dispersion. To avoid statistics of small numbers, we would like to observe a few very rich clusters using 4m and 6m class telescopes in imaging and spectrographic mode to include galaxies as faint as possible and as far as z = 0.5 We would also like to use globular cluster systems around selected galaxies in clusters as probes of the evolution of galaxies. Observing Globular clusters in the Galaxy using Stroemgren system filters, we have resolved age- metallicity degeneracy wich inhibit broadband colors. Globular cluster systems are window into the star formation history of a galaxy. The globulars around many bright ellipticals have a bimodal color distribution which may signal 1) a past merger event, 2) a second epoch of star formation or 3) a discontinuity in the chemical evolution of a galaxy. Using our new age-metalicity calibration in Stroerngren photometry we will be able to determine if the second globular population is due to past star formation events, or the accretion of systems around dwarf galaxies.

Since the discovery of the Butcher - Oemler effect ( increased number of blue galaxies with the distance ) , numerous photometric and spectroscopic studies have concentrated on this high fraction of blue, star forming galaxies in distant rich cluster of galaxies, which seems to be absent in their nearby, present - day counterparts. To overcome the so called k-correction ( one effect of expanding universe ) we have used rest -frame Strömgren photometry ( adapting filters for any given redshift ) to map the evolution of galaxies in clusters between redshift of 0.2 and 0.9 and shown the fraction of blue galaxies to rise from average 20% at the redshift of 0.4 to 80% at the redshift of 0.9. However, the fate of the blue population seen in high redshift clusters still remains unclear, in spite of the considerable effort put into this area of research over the last 15 years. One of the reasons for this might be the intrinsic color-morfology degeneracy. Indeed, the overlap between morphological data of high redshift galaxies and multicolor or spectral information is rather poor due to the different telescope-apertures used for either type of observations. The most luminous blue cluster galaxies are normal late-type systems, supplemented by starbursting and / or post-starburst interacting or merging galaxies. We have focused our studies on the determination of age and metallicity through the use of Strömgren photometry. In numerous publications we have shown that the color indices are, especially in objects composed of single stellar population, correlated to the age and metallicity. The age-metallicity degeneracy can be broken with the use of these photometry. Our observation of dwarf galaxies in Fornax galaxy cluster shows that dwarf galaxies can be divided in two groups, normal (dE) and nucleated (dE,N) . We have shown that dE,N are direct continuation of globular clusters toward larger masses, in terms of their color indices. Whereas dEs are similar to their more massive cousins, the giant ellipticals. According to the high metallicity content the dEs must be fragments of the original protogalaxies. Fragmentation of galaxies in clusters is therefore a real process during their living. In addition to the Fornax cluster we have observed Coma, A2218 ( one of the richest Abell clusters) and A2125 (using 4m KPNO telescope) . We see that A2218, a very dense cluster, has 3 times more dEs galaxies than in Fornax, while both have equal number of dE,Ns (observed under identical conditions). The number of dEs are evidently a product of the cluster density and therefore of a large number of galaxy interactions with time. Cluster A2125 is about 2 Gyrs younger ( redshift of 0.24 ) and has dE,Ns with about ten times lower metallicity from Fornax dE,Ns. We have for first time established this corelation between age and metallicity. Our future goal is to observe dwarf elliptical galaxies in galaxy clusters with higher redshift, to follow the behaviour of these very numerous galaxies closer to the time of their formation. For the first time these galaxies will be observed at redshift greater than 0.4 which corresponds to 4 Gyrs younger than today. We will be able to test the correlation between density of galaxy clusters and number of dE galaxies ( residuals of destroyed galaxies ), the correlation between age of dE,N galaxies and their metallicity and the correlation between the density of galxy clusters and the metallicity of dE,N galaxies in clusters.