from Alexov et al. paper
from Alexov et al. paper
The dramatic differences between present-epoch cluster and field galaxy populations indicate that environment plays a strong role in galaxy evolution. Here we present a brief overview of some of the recent observations of moderate to high redshift clusters. A consistent picture of galaxy evolution in clusters appears to be emerging, which includes a population of galaxies which formed early in the cluster history, as well field galaxies which have had their star formation truncated upon falling into the cluster potential. Galaxy interactions probably play an important role in exhausting star formation in some of these galaxies. However, there is significant variation in the populations of different cluster samples, with substantial evidence that some galaxies have their star formation terminated more gradually. This suggests that different mechanisms may dominate, perhaps because of differences in the recent merging history of the clusters. We also present a recent analysis of population gradients in clusters which suggests that the observed evolution in cluster populations is consistent with a scenario where changing infall rates drive the fraction of star forming galaxies in clusters, rather than a changing physical mechanism within the cluster. Thus, galaxy populations may provide a fundamental measure of the growth of large scale structure.
S. ANDREON, J.-C. CUILLANDRE and R. PELLO .ps .pdf
The Coma cluster luminosity function (LF) from ultraviolet (2000 AA ) to the near--infrared (H band) is summarized. In the UV the LF is very steep, much steeper than in the optical. The steep Coma UV LF implies that faint and bright galaxies give similar contributions to the total UV flux and to the total metal production rate. The ComaUV LF is dominated in number and luminosity by blue galaxies, which are often faint in the optical. Therefore the Coma UV LF is dominated by star forming galaxies, not by massive and large galaxies. The optical Coma LF is relatively steep (alpha=-1.4) over the 11 magnitudes sampled, but its slope and shape depend on considered filter and magnitude. We found a clear steeping of the FL going from B to R bands, indicative of the presence of a large number of red dwarfs, as faint as three bright globular clusters. Furthermore, using Hubble Space Telescope images, we discover that blends of globular clusters, not resolved in individual components due to seeing, look like dwarf galaxies when observed from the ground and are numerous and bright. The existence of these fake extended sources increases the steepness of the LF at faint magnitudes, if not deal on. This concern affects previous deep probing of the luminosity function, but not the present work. The near--infrared LF wa s computed on a near--infrared selected sample of galaxies which photometry is complete down to the typical dwarf (M^* +5) luminosity. The Coma LF can be described by a Schechter function with intermediate slope (alpha sim-1.3), plus a dip at M_Hsim-22 mag. The shape of the Coma LF in H band is quite similar to th e one found in the B band. The similarity of the LF in the optical and H bands implies that in the central region of Coma there is no new population of galaxies which is too faint to be observed in the optical band (because dust enshrouded, for instance), down to the magnitudes of dwarfs. The exponential cut of the LF at the bright end is in good agreement with the one derived from shallower near--infrared samples o f galaxies, both in clusters and in the field. The faint end of the LF, reaching M_Hsim-19 mag (roughly M_Bsim -15), is steep, but less than previously suggested from shallower near--infrared observations of an adjacent region in the Coma cluster.
ISABEL. MARQUEZ et al. .ps .pdf
We show from theoretical calculations that elliptical galaxies obey a scaling relation between potential energy and mass. We have also shown that their specific entropy is quasi-constant. These two laws define two 2-manifolds in the space defined by the three Sersic law parameters (intensity Sigma_0, scale a and shape nu). Elliptical galaxies are distributed on a thin line, which is the intersection of these two 2-manifolds. The scaling relation between potential energy and mass allows to naturally explain the origin of the observed correlation between absolute magnitude and mean surface brightness. From the theoretical location of elliptical galaxies in the thin line traced by the two laws, the photometrical plane can be naturally derived. These two relations are indeed observed in 142 galaxies belonging to three nearby clusters (with the assumption that light traces mass) and in simulations of dark halos. They have many consequences on galaxy formation, evolution and cosmology, and should allow to derive a new distance indicator.
VERA. E. MARGONINER et al. .ps .pdf
We present the study of the Butcher-Oemler effect in a sample of 295 Abell clusters. gri CCD photometry allowed the determination of the fraction of blue galaxies, and was also used to estimate photometric redshifts with an accuracy of 0.04, for those clusters without spectroscopic measurements. The resulting redshift range for the sample is 0 less 0.4. Our observations show a strong Butcher-Oemler effect, with an increase in the fraction of blue galaxies, fb, with redshift that seems to indicate that strong evolution of the cluster galaxy population can already be observed at low redshifts. We find a strong link between cluster richness and the fraction of blue galaxies, fb. The slope of the fB(z) relation is the same for all richnesses, but at a given redshift, fb~ is systematically higher for poor clusters.
T.J.PONMAN and S.F.HELSDON .ps .pdf
Galaxy groups have potentials deep enough to bring galaxies into strong interaction, and to retain much of any material detached or ejected from them. However, unlike rich clusters, groups have virial temperatures similar to those of individual galaxies, so that energetic events within galaxies can have a significant impact on the properties of the system as a whole. Groups therefore provide a powerful probe of galaxy evolution. Here we concentrate on the effects of the group environment on the X-ray properties of galaxies. We find that late-type galaxies in groups have X-ray properties very similar to those in the field, but that early-type galaxies can have strikingly higher X-ray luminosities if they are located in the centre of a group potential. Failure to recognise this last effect has introduced a good deal of confusion into past work on the X-ray properties of early-type galaxies.
A. ALEXOV, D. SILVA and M. PIERCE .ps .pdf
A wide-field imaging study of intermediate-redshift (z = 0.39), Butcher-Oemler cluster CL0024+1654 has been initiated. Previous studies of the galaxy population of this cluster have concentrated only on the most centrally located galaxies. To improve this situation, wide-field (20 x 20 arcmin) UBVI images have been obtained. In this poster, a preliminary analysis of this dataset is presented. In particular, the color and spatial distributions of the galaxies are used to discuss the nature of the intra-cluster ``faint blue'' galaxy population and extent to which CL0024+1654 is embedded in a larger super-cluster complex. An analysis of optical band ``drop-out'' objects, candidates for high-redshift galaxies, is also presented. These data are deep enough (S/N = 5 at B = 24.5) that the luminosity function of the elliptical, S0, and spiral galaxies is sampled well enough to investigate population differences within a class as a function of luminosity/mass. This part of the analysis has not been completed and it is not discussed further here.
M.J. DRINKWATER et al. .ps .pdf
We have discovered a population of low luminosity compact objects in the Fornax Cluster which are unlike any known type of galaxy and appear to represent an entirely new component of galaxy clusters. These objects were discovered during our Fornax Spectroscopic Survey which is using the Two degree Field spectrograph of the Anglo-Australian Telescope to obtain the first complete spectroscopic sample of the Fornax cluster. Our survey is unique in that all objects (both stars and galaxies) in our magnitude limits (16.5 < B_J< 19.7) are measured, to eliminate morphological selection biases. The largest of the compact objects is just resolved in our photographic data, giving a scale size of about 80 pc; the others are all unresolved. They have optical spectra consistent with old stellar systems, but are more luminous (-13< M_B< -11) than any known globular clusters. The luminosity distribution of these objects overlaps that of the nuclei of nucleated dwarf elliptical galaxies so they may be the remains of dwarf galaxies broken apart in the cluster environment.
IGNACIO FERRERAS and JOSEPH SILK. .ps .pdf
AJIT KEMBHAVI, YOGESH WADADEKAR and HABIB KHOSROSHAHI. .ps .pdf
We report the existence of a single plane in the space of global photometric parameters describing elliptical galaxies and the bulges of early type spiral galaxies. The three parameters which define the plane are obtained by fitting the Sersic form to the brightness distribution obtained from near-infrared K band images. Known correlations like the Kormendy relation are projections of this photometric plane. The existence of the plane may help constrain bulge formation models.
We present a new deep wide angle (40'X 1deg) multicolor U, B, V, R and I survey of the Coma cluster core. It is one of the deepest multicolor survey and one of the largest CCD field available to date for the Coma cluster. Although this cluster is probably the most studied rich cluster, open questions, as the slope of the faint end of the luminosity function, subsist. Our purpose is to use this new deep and large sample to determine the most realistic luminosity function, constrain the very faint population behavior and study environmental effects in this cluster.