Cluster Luminosity Function Research Articles

Imaging of star clusters in unperturbed spiral galaxies with the Advanced Camera for Surveys

We present results from ACS and WFPC observations in the low luminosity galaxy NGC 45. We identified 28 young star cluster candidates. While the exact values of age, mass, and extinction depend somewhat on the choice of SSP models, we find no young clusters with masses higher than a few 1000 Msun for any model choice. We derive the luminosity function of young star clusters and find a slope of alpha=-1.94+-0.28. We also identified 19 old globular clusters and we estimate a specific frequency of globular clusters of S_N=1.4-1.9 which is significantly higher than observed for other late-type galaxies (e.g. SMC, LMC, M33). Most of these globular clusters appear to belong to a metal-poor population, although they coincide spatially with the location of the bulge of NGC 45.

ACS Photometry of Extended, Luminous Globular Clusters in the Outskirts of M31

A new population of extended, luminous globular clusters has recently been discovered in the outskirts of M31. These objects have luminosities typical of classical globular clusters, but much larger half-light radii. We report the first results from deep ACS imaging of four such clusters, one of which is a newly-discovered example lying at a projected distance of ~60 kpc from M31. Our F606W, F814W colour-magnitude diagrams extend ~3 magnitudes below the horizontal branch level, and clearly demonstrate, for the first time, that all four clusters are composed of >10 Gyr old, metal-poor stellar populations. No evidence for multiple populations is observed. From a comparison with Galactic globular cluster fiducials we estimate metallicities in the range -2.2 < [Fe/H] < -1.8. The observed horizontal branch morphologies show a clear second parameter effect between the clusters. Preliminary radial luminosity profiles suggest integrated magnitudes in the range -6.6 < M_V < -7.7, near the median value of the globular cluster luminosity function. Our results confirm that these four objects are bona fide old, metal-poor globular clusters, albeit with combined structures and luminosities unlike those observed for any other globular clusters in the Local Group or beyond.

Structural Parameters for Globular Clusters in NGC 5128. II.Hubble Space TelescopeACS Imaging and New Clusters

We report the first results from an imaging program with the ACS on the Hubble Space Telescope designed to measure the structural characteristics of a wide range of globular clusters in NGC 5128, the nearest giant elliptical galaxy. From 12 ACS WFC fields we have measured a total of 62 previously known globular clusters and have discovered 69 new high-probability cluster candidates not found in any previous work. We present magnitudes and color indices for all of these, along with rough measurements of their effective diameters and ellipticities. The luminosity distribution of this nearly uncontaminated sample of clusters matches well with the normal globular cluster luminosity function for giant elliptical galaxies, and the cluster scale size and ellipticity distributions are similar to those in the Milky Way system. The indication from this survey is that many hundreds of individual clusters remain to be found with carefully designed search techniques in the future. A very rough estimate of the total cluster population from our data suggests NGC ≃ 1500 in NGC 5128 over all magnitudes and within a projected radius R = 25' from the galaxy center.

Trends in the Globular Cluster Luminosity Function of Early-Type Galaxies

We present results from a study of the globular cluster luminosity function (GCLF) in a sample of 89 early-type galaxies observed as part of the ACS Virgo Cluster Survey. Using a Gaussian parametrization of the GCLF, we find a highly significant correlation between the GCLF dispersion, sigma, and the galaxy luminosity, M_B, in the sense that the GC systems in fainter galaxies have narrower luminosity functions. The GCLF dispersions in the Milky Way and M31 are fully consistent with this trend, implying that the correlation between sigma and galaxy luminosity is more fundamental than older suggestions that GCLF shape is a function of galaxy Hubble type. We show that the sigma - M_B relation results from a bonafide narrowing of the distribution of (logarithmic) cluster masses in fainter galaxies. We further show that this behavior is mirrored by a steepening of the GC mass function for relatively high masses, M >~ 3 x 10^5 M_sun, a mass regime in which the shape of the GCLF is not strongly affected by dynamical evolution over a Hubble time. We argue that this trend arises from variations in initial conditions and requires explanation by theories of cluster formation. Finally, we confirm that in bright galaxies, the GCLF "turns over" at the canonical mass scale of M_TO ~ 2 x 10^5 M_sun. However, we find that M_TO scatters to lower values (~1-2 x 10^5 M_sun) in galaxies fainter than M_B >~ -18.5, an important consideration if the GCLF is to be used as a distance indicator for dwarf ellipticals.

Luminosity functions of YSO clusters in Sh-2 255, W3 Main and NGC 7538 star forming regions

AbstractWe have conducted deep near-infrared surveys of the Sh-2 255, W3 Main and NGC 7538 massive star forming regions using simultaneous observations of the JHKs-band with the near-infrared camera SIRIUS on the UH 88-inch telescope and with SUBARU. The near-infrared surveys cover a total area of ~ 72 arcmin2 of three regions with 10-σ limiting magnitudes of ~ 19.5, 18.4 and 17.3 in J, H and Ks-band, respectively. Based on the color-color and color-magnitude diagrams and their clustering properties, the candidate young stellar objects are identified and their luminosity functions are constructed in Sh-2 255, W3 Main and NGC 7538 star forming regions. A large number of previously unreported red sources (H-K &gt; 2) have also been detected around these regions. We argue that these red stars are most probably pre-main-sequence stars with intrinsic color excesses. The detected young stellar objects show a clear clustering pattern in each region: the Class I-like sources are mostly clustered in molecular cloud region, while the Class II-like sources are in or around more evolved optical HII regions. We find that the slopes of the Ks-band luminosity functions of Sh-2 255, W3 Main and NGC 7538 are lower than the typical values reported for the young embedded clusters, and their stellar populations are primarily composed of low mass pre-main-sequence stars. From the slopes of the Ks-band luminosity functions, we infer that Sh-2 255, W3 Main and NGC 7538 star forming regions are rather young (age ≤ 1 Myr).

The Early-Type Dwarf Galaxy Population of the Fornax Cluster

AbstractAIMS: We analyse the photometric properties of the early-type Fornax cluster dwarf galaxy population (MV&gt; − 17 mag), based on a wide field imaging study of the central cluster area in V and I bandpasses. We used the instrument/telescope combination IMACS/Magellan at Las Campanas Observatory, providing much larger light collecting area and better image resolution than previous wide field imaging surveys.METHODS: We create a fiducial sample of Fornax cluster dwarf ellipticals (dEs) in the following three steps: (1) To verify cluster membership, we measured I-band surface brightness fluctuations (SBF) distances to candidate dEs known from previous surveys; (2) We re-assessed morphological classifications for those candidate dEs that are too faint for SBF detection; and (3) We searched for new candidate dEs in the size-luminosity regime close to the resolution limit of previous surveys.RESULTS: (1) We confirm cluster membership for 28 candidate dEs in the range −16.6 &lt; MV &lt; −10.1 mag by means of SBF measurement. We find no SBF background galaxy. (2) Of 51 further candidate dEs in the range −13.2 &lt; MV &lt; −8.6 mag, 2/3 are confirmed as probable cluster members by morphological re-assessment, while 1/3 are re-classified as probable background objects. (3) We find 12 new dE candidates in the range −12.3 &lt; MV &lt; −8.8 mag, two of which are directly confirmed via SBF measurement. The resulting fiducial dE sample follows a well-defined surface brightness – magnitude relation, showing that Fornax dEs are about 40% larger than Local Group dEs. The sample also defines a colour-magnitude relation that appears slightly shallower than that of Local Group dEs. The early-type dwarf galaxy luminosity function in Fornax has a very flat faint end slope α ≃ −1.1 ± 0.1. We discuss these findings in the context of structure formation theories.CONCLUSIONS: The SBF method is a very powerful tool to help constrain the faint end of the galaxy luminosity function in nearby galaxy clusters. For the Fornax cluster, morphological cluster memberships – if performed at sufficient resolution – are very reliable.This contribution is based on Mieske et al. (2006).

Luminosity Function of Faint Globular Clusters in M87

We present the luminosity function to very faint magnitudes for the globular clusters in M87, based on a 30 orbit \textit{Hubble Space Telescope (HST)} WFPC2 imaging program. The very deep images and corresponding improved false source rejection allow us to probe the mass function further beyond the turnover than has been done before. We compare our luminosity function to those that have been observed in the past, and confirm the similarity of the turnover luminosity between M87 and the Milky Way. We also find with high statistical significance that the M87 luminosity function is broader than that of the Milky Way. We discuss how determining the mass function of the cluster system to low masses can constrain theoretical models of the dynamical evolution of globular cluster systems. Our mass function is consistent with the dependence of mass loss on the initial cluster mass given by classical evaporation, and somewhat inconsistent with newer proposals that have a shallower mass dependence. In addition, the rate of mass loss is consistent with standard evaporation models, and not with the much higher rates proposed by some recent studies of very young cluster systems. We also find that the mass-size relation has very little slope, indicating that there is almost no increase in the size of a cluster with increasing mass.

The Luminosity Function of Nearby Galaxy Clusters. II. Redshifts and Luminosity Function for Galaxies in the Region of the Centaurus Cluster

We acquired spectra for a random sample of galaxies within a 0.83 deg2 region centered on the core of the Centaurus Cluster. Radial velocities were obtained for 225 galaxies to limiting magnitudes of V -17.0 are considered dwarfs. We constructed the cluster galaxy luminosity function (LF) by using these spectroscopic results to calculate the expected fraction of cluster members in each magnitude bin. The faint-end slope of the LF using this method is shallower than the one obtained using a statistical method to correct for background galaxy contamination. We also use the spectroscopy results to define surface brightness criteria to establish membership for the full sample. Using these criteria, we find a LF very similar to the one constructed with the statistical background correction. For both, we find a faint-end slope α ~ -1.4. The error in faint-end slope for the statistically corrected LF is ~±0.2. Adjusting the surface brightness membership criteria, we find that the data are consistent with a faint-end slope as shallow as -1.22 or as steep as -1.50. We describe in this paper some of the limitations of using these methods for constructing the galaxy LF. This is Paper II in our investigation of the cluster galaxy LF.

The old globular cluster system of the dIrr galaxy NGC 1427A in the Fornax cluster

We present a study of the old globular cluster (GC) population of the dwarf irregular galaxy NGC 1427A using multi-wavelength VLT observations in U, B, V, I, H_alpha, J, H, and Ks bands under excellent observing conditions. We applied color and size selection criteria to select old GC candidates and made use of archival ACS images taken with the Hubble Space Telescope to reject contaminating background sources and blended objects from the GC candidates' list. The H_alpha observations were used to check for contamination due to compact, highly reddened young star clusters whose colors and sizes could mimic those of old GCs. After accounting for contamination we obtain a total number of 38+/-8 GC candidates with colors consistent with an old (~10 Gyr) and metal-poor (Z < 0.4xZ_solar) population as judged by simple stellar population models. Our contamination analysis indicates that the density distribution of GCs in the outskirts of the Fornax central cD galaxy NGC1399 may not be spherically symmetric. We derive a present-day specific frequency S_N of 1.6+/-0.23 for NGC 1427A, a value significantly larger than what is observed in the Local Group dwarf irregular galaxies and comparable with the values found for the same galaxy types in the Virgo and Fornax clusters. Assuming a universal globular cluster luminosity function turnover magnitude, we derive a distance modulus to NGC 1427A of 31.01+/-0.21 mag which places it 3.2+/-2.5(statistic)+/-1.6(systematic) Mpc in front of the Fornax central cD galaxy NGC 1399. The implications of this result for the relationship between NGC 1427A and the cluster environment are briefly discussed.

The luminosity function of young star clusters: implications for the maximum mass and luminosity of clusters

We introduce a method to relate a possible truncation of the star cluster mass function at the high mass end to the shape of the cluster luminosity function (LF). We compare the observed LFs of five galaxies containing young star clusters with synthetic cluster population models with varying initial conditions. The LF of the SMC, the LMC and NGC 5236 are characterized by a power-law behavior , with a mean exponent of . This can be explained by a cluster population formed with a constant cluster formation rate, in which the maximum cluster mass per logarithmic age bin is determined by the size-of-sample effect and therefore increases with log (age/yr). The LFs of NGC 6946 and M 51 are better described by a double power-law distribution or a Schechter function. When a cluster population has a mass function that is truncated below the limit given by the size-of-sample effect, the total LF shows a bend at the magnitude of the maximum mass, with the age of the oldest cluster in the population, typically a few Gyr due to disruption. For NGC 6946 and M 51 this suggests a maximum mass of , although the bend is only a 1–2 σ detection. Faint-ward of the bend the LF has the same slope as the underlying initial cluster mass function and bright-ward of the bend it is steeper. This behavior can be well explained by our population model. We compare our results with the only other galaxy for which a bend in the LF has been observed, the “Antennae” galaxies (NGC 4038/4039). There the bend occurs brighter than in NGC 6946 and M 51, corresponding to a maximum cluster mass of . Hence, if the maximum cluster mass has a physical limit, then it can vary between different galaxies. The fact that we only observe this bend in the LF in the “Antennae” galaxies, NGC 6946 and M 51 is because there are enough clusters available to reach the limit. In other galaxies there might be a physical limit as well, but the number of clusters formed or observed is so low, that the LF is not sampled up to the luminosity of the bend. The LF can then be approximated with a single power-law distribution, with an index similar to the initial mass function index.

NGC 2401: a template of the young population of the Norma–Cygnus arm in the Third Galactic Quadrant

Abstract Based on a deep optical CCD [UBV(RI)C] photometric survey and on the Two-Micron All Sky Survey (2MASS) data, we derived the main parameters of the open cluster NGC 2401. We found that this cluster is placed at 6.3 ± 0.5 kpc (VO−MV= 14.0 ± 0.2) from the Sun and is 25 Myr old, which allows us to identify NGC 2401 as a member of the young population belonging to the innermost side of the extension of the Norma–Cygnus spiral arm in the Third Galactic Quadrant. A spectroscopic study of the emission star LSS 440 that lies in the cluster area revealed that it is a B0Ve star; however, we could not confirm that it is a cluster member. We also constructed the cluster luminosity function (LF) down to V∼ 22 and the cluster initial mass function (IMF) for all stars with masses above M∼ 1–2 M⊙. It was found that the slope of the cluster IMF is x≈ 1.8 ± 0.2. The presence of a probable pre-main-sequence (PMS) star population associated with the cluster is weakly revealed.

Nearby Spiral Globular Cluster Systems. I. Luminosity Functions

We compare the near-infrared (JHK) globular cluster luminosity functions (GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies. We obtained near-infrared photometry with the Persson's Auxiliary Nasmyth Infrared Camera on the Baade Telescope for 38 objects (mostly globular cluster candidates) in the Sculptor Group. We also have near-infrared photometry from the Two Micron All Sky Survey (2MASS)-6X database for 360 M31 globular cluster candidates and aperture photometry for 96 Milky Way globular cluster candidates from the 2MASS All-Sky and Second Incremental Release databases. The M31 6X GCLFs peak at absolute reddening-corrected magnitudes of M = -9.18, M = -9.73, and M = -9.98. The mean brightness of the Milky Way objects is consistent with that of M31 after accounting for incompleteness. The average Sculptor absolute magnitudes (correcting for relative distance from the literature and foreground reddening) are M = -9.18, M = -9.70, and M = -9.80. NGC 300 alone has absolute foreground-dereddened magnitudes M = -8.87, M = -9.39, and M = -9.46 using the newest Gieren et al. distance. This implies either that the NGC 300 GCLF may be intrinsically fainter than that of the larger galaxy M31 or that NGC 300 may be slightly farther away than previously thought. Straightforward application of our M31 GCLF results as a calibrator gives NGC 300 distance moduli of 26.68 ± 0.14 using J, 26.71 ± 0.14 using H, and 26.89 ± 0.14 using K.

Observational evidence for a truncation of the star cluster initial mass function at the high mass end

We present the luminosity function (LF) of star clusters in M51 based on HST/ACS observations taken as part of the Hubble Heritage project. The clusters are selected based on their size and with the resulting 5990 clusters we present one of the largest cluster samples of a single galaxy. We find that the LF can be approximated with a double power-law distribution with a break around M_V = -8.9. On the bright side the index of the power-law distribution is steeper (a = 2.75) than on the faint-side (a = 1.93), similar to what was found earlier for the ``Antennae'' galaxies. The location of the bend, however, occurs about 1.6 mag fainter in M51. We confront the observed LF with the model for the evolution of integrated properties of cluster populations of Gieles et al., which predicts that a truncated cluster initial mass function would result in a bend in, and a double power-law behaviour of, the integrated LF. The combination of the large field-of view and the high star cluster formation rate of M51 make it possible to detect such a bend in the LF. Hence, we conclude that there exists a fundamental upper limit to the mass of star clusters in M51. Assuming a power-law cluster initial mass function with exponentional cut-off of the form NdM ~ M^-b * exp(-M/M_C)dM, we find that M_C = 10^5 M_sun. A direct comparison with the LF of the ``Antennae'' suggests that there M_C = 4*10^5 M_sun.

An analytic investigation of the scatter in the integrated X-ray properties of galaxy groups and clusters

We revisit the scaling relationships between the dark matter mass and observed X-ray luminosity and temperature of galaxy clusters and groups in the local Universe. Specifically, we compare recent observations with analytic models of the intracluster medium in which the gas entropy distribution has been shifted by a variable amount, K o , to investigate the origin of the scatter in these scaling relations, and its influence on the luminosity and temperature functions. We find that variations in halo concentration or formation epoch (which might determine the time available for low-entropy gas to cool out) are insufficient to explain the amount of scatter in the mass and X-ray luminosity (M-L) relation. Instead, a range of entropy floors at a fixed halo mass, spanning approximately ∼50-700 keV cm 2 , is required to match the data. This range is likely related to the variance in heating and/or cooling efficiency from halo to halo. We demonstrate that these models are consistent with the observed temperature and luminosity functions of clusters, with a normalization of σ 8 ∼ 0.8 in agreement with Wilkinson Microwave Anisotropy Probe (WMAP) measurements (for h = 0.7 and Ω m = 0.3); in particular, the scatter in the M-L relation has an important influence on the shape of the luminosity function, and must be accounted for to provide a consistent result. Finally, we present predictions for the redshift evolution of these scaling relations and luminosity/temperature functions. Comparison with recent data at z < 0.7 shows reasonable agreement with a model that assumes a median entropy floor of K = 200 keV cm 2 . When observations are extended to group scales (kT ≤ 1 keV). this evolution will have the potential to discriminate between an entropy floor that is independent of redshift (e.g. in a preheating scenario) and one that depends on the cooling time of the halo.

RASS-SDSS Galaxy cluster survey

We analyze the Luminosity Functions (LFs) of a subsample of 69 clusters from the RASS-SDSS galaxy cluster catalog. When calculated within the cluster physical sizes, given by r 200 or r 500 , all the cluster LFs appear to have the same shape, well fitted by a composite of two Schechter functions with a marked upturn and a steepening at the faint-end. Previously reported cluster-to-cluster variations of the LF faint-end slope are due to the use of a metric cluster aperture for computing the LF of clusters of different masses. We determine the composite LF for early- and late-type galaxies, where the typing is based on the galaxy u - r colors. The late-type LF is well fitted by a single Schechter function with a steep slope (α = -2.0 in the r band, within r 200 ). The early-type LF instead cannot be fitted by a single Schechter function, and a composite of two Schechter functions is needed. The faint-end upturn of the global cluster LF is due to the early-type cluster galaxies. The shape of the bright-end tail of the early-type LF does not seem to depend upon the local galaxy density or the distance from ihe cluster center. The late-type LF shows a significant variation only very near the cluster center. On the other hand, the faint-end tail of the early-type LF shows a significant and continuous variation with the environment. We provide evidence that the process responsible for creating the excess population of dwarf early type galaxies in clusters is a threshold process that occurs when the density exceeds ∼500 times the critical density of the Universe. We interpret our results in the context of the harassment scenario where faint early-type cluster galaxies are predicted to be the descendants of tidally-stripped late-type galaxies.

RR Lyrae-based calibration of the Globular Cluster Luminosity Function

We test whether the peak absolute magnitude Mv(TO) of the Globular Cluster Luminosity Function (GCLF) can be used for reliable extragalactic distance determinations. Starting with the luminosity function of the Galactic Globular Clusters listed in Harris catalog, we determine Mv(TO) either using current calibrations of the absolute magnitude Mv(RR) of RR Lyrae stars as a function of the cluster metal content [Fe/H] and adopting selected cluster samples. We show that the peak magnitude is slightly affected by the adopted Mv(RR)-[Fe/H] relation, while it depends on the criteria to select the cluster sample. As for the GCLFs in other external galaxies, using Surface Brightness Fluctuations (SBF) measurements we give evidence that the luminosity functions of the blue (metal-poor) Globular Clusters peak at the same luminosity within ~0.2 mag, whereas for the red (metal-rich) samples the agreement is within ~0.5 mag even accounting for the theoretical metallicity correction expected for clusters with similar ages and mass distributions. Then, using the SBF absolute magnitudes provided by a Cepheid distance scale calibrated on a fiducial distance to LMC (m(LMC)=18.50 mag), we show that the Mv(TO) value of the metal-poor clusters in external galaxies(-7.67+/-0.23 mag) is in excellent agreement with the value of both Galactic (-7.66+/-0.11 mag) and M31 (-7.65+/-0.19 mag)ones.

Measurement of Galaxy Cluster Sizes, Radial Profiles, and Luminosity Functions from SDSS Photometric Data

Imaging data from the Sloan Digital Sky Survey is used to measure the empirical size-richness relation for a large sample of galaxy clusters. Using population subtraction methods, we determine the radius at which the cluster galaxy number density is 200/Omega_m times the mean galaxy density, without assuming a model for the radial distribution of galaxies in clusters. If these galaxies are unbiased on Mpc scales, this galaxy-density-based R_200 reflects the characteristic radii of clusters. We measure the scaling of this characteristic radius with richness over an order of magnitude in cluster richness, from rich clusters to poor groups. We use this information to examine the radial profiles of galaxies in clusters as a function of cluster richness, finding that the concentration of the galaxy distribution decreases with richness and is systematically lower than the concentrations measured for dark matter profiles in N-body simulations. Using these scaled radii, we investigate the behavior of the cluster luminosity function, and find that it is well matched by a Schechter function for galaxies brighter than M_r = -18 only after the central galaxy has been removed. We find that the luminosity function varies with richness and with distance from the cluster center, underscoring the importance of using an aperture that scales with cluster mass to compare physically equivalent regions of these different systems. We note that the lowest richness systems in our catalog have properties consistent with those expected of the earliest-forming halos; our cluster-finding algorithm, in addition to reliably finding clusters, may be efficient at finding fossil groups.

The 2dF Galaxy Redshift Survey: correlation with the ROSAT-ESO flux-limited X-ray galaxy cluster survey

The ROSAT-European Southern Observatory (ESO) flux-limited X-ray (REFLEX) galaxy cluster survey and the Two-degree Field Galaxy Redshift Survey (2dFGRS), respectively, comprise the largest, homogeneous X-ray selected cluster catalogue and completed galaxy redshift survey. In this work, we combine these two outstanding data sets in order to study the effect of the large-scale cluster environment, as traced by X-ray luminosity, on the properties of the cluster member galaxies. We measure the LX−σr relation from the correlated data set and find it to be consistent with recent results found in the literature. Using a sample of 19 clusters with LX≥ 0.36 × 1044 erg s−1 in the 0.1–2.4 keV band, and 49 clusters with lower X-ray luminosity, we find that the fraction of early spectral type (η=−1.4), passively evolving galaxies is significantly higher in the high-LX sample within R200. We extend the investigation to include composite bJ cluster luminosity functions, and find that the characteristic magnitude of the Schechter-function fit to the early-type luminosity function is fainter for the high-LX sample compared to the low-LX sample (ΔM*= 0.58 ± 0.14). This seems to be driven by a deficit of such galaxies with MbJ∼−21. In contrast, we find no significant differences between the luminosity functions of star-forming, late-type galaxies. We believe these results are consistent with a scenario in which the high-LX clusters are more dynamically evolved systems than the low-LX clusters.

Galics- VI. Modelling hierarchical galaxy formation in clusters

High-resolution N-body re-simulations of 15 massive (10 14 10 15 M⊙) dark matter haloes have been combined with the hybrid galaxy formation model GalICS (Hatton et al. 2003), to study the formation and evolution of galaxies in clusters, within the framework of the hierarchical merging scenario. This paper describes the high-resolution resimulation technique used to build the dark matter halo sample, and discusses its reliability. New features incorporated in GalICS include a better description of galaxy positioning after dark matter halo merger events, a more reliable computation of the temperature of the inter-galactic medium as a function of redshift, that also takes into account the re ionisation history of the Universe, and a semi-analytic description of the ram pressure strippi ng of cold gas from galactic discs, suffered by galaxies during their motion through the diffuse hot intra-cluster medium. Within the multitude of available model results, we choose to focus here on the luminosity functions, morphological fractions and colour distributions of galax ies in clusters and in cluster outskirts, at z = 0. No systematic dependency on cluster richness is found either for the galaxy luminosity functions, morphological mixes, or colour distributions. Moving from higher density (cluster cores), to lower density environments (clust er outskirts), we detect a progressive flattening of the luminosity functions, an increase of the fr action of spirals and a decrease of that of ellipticals and S0s, and the progressive emergence of a bluer tail in the distributions of galaxy colours, especially for spirals. As compared to cluster spirals, early-type galaxies show a flatter luminosity function, and more homogeneous and redder colours. An overall good agreement is found between our results and the observations, particularly in terms of the cluster luminosity functions and morphological mixes. However, some discrepancies are also apparent, with too faint magnitudes of the brightest cl uster members, especially in the B band, and galaxy colours tendentially too red (or not blue enough) in the model, with respect to the observations. Finally, ram pressure stripping appea rs to affect very little our results.

Toward understanding environmental effects in SDSS clusters

We find clusters and superclusters of galaxies using the Data Release 1 of the Sloan Digital Sky Survey. We determine the luminosity function of clusters and find that clusters in a high-density environment have a luminosity a factor of {approx}5 higher than in a low-density environment. We also study clusters and superclusters in numerical simulations. Simulated clusters in a high-density environment are also more massive than those in a low-density environment. Comparison of the density distribution at various epochs in simulations shows that in large low-density regions (voids) dynamical evolution is very slow and stops at an early epoch. In contrast, in large regions of higher density (superclusters) dynamical evolution starts early and continues until the present; here particles cluster early, and by merging of smaller groups very rich systems of galaxies form.