			Home About us Contact

Galaxy Clusters (galaxy + cluster)

Distribution by Scientific Domains

Physics and Astronomy	100%

Selected Abstracts

Cluster temperature profiles and Sunyaev,Zeldovich observations

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2004
Steen H. Hansen
ABSTRACT Galaxy clusters are not isothermal, and the radial temperature dependence will affect the cluster parameters derived through the observation of the Sunyaev,Zeldovich (SZ) effect. We show that the derived peculiar velocity will be systematically shifted by 10,20 per cent. For future all-sky surveys one cannot rely on the observationally expensive X-ray observations to remove this systematic error, but should instead reach for sufficient angular resolution to perform a deprojection in the SZ spectra. The Compton-weighted electron temperature is accurately derived through SZ observations. [source]

Swimming against the current: simulations of central AGN evolution in dynamic galaxy clusters

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2010
Brian J. Morsony
ABSTRACT We present a series of three-dimensional hydrodynamical simulations of central active galactic nuclei (AGN)-driven jets in a dynamic, cosmologically evolved galaxy cluster. Extending previous work, we study jet powers ranging from Ljet= 1044 erg s,1 to Ljet= 1046 erg s,1 and in duration from 30 to 200 Myr. We find that large-scale motions of cluster gas disrupt the AGN jets, causing energy to be distributed throughout the centre of the cluster, rather than confined to a narrow angle around the jet axis. Disruption of the jet also leads to the appearance of multiple disconnected X-ray bubbles from a long-duration AGN with a constant luminosity. This implies that observations of multiple bubbles in a cluster are not necessarily an expression of the AGN duty cycle. We find that the ,sphere of influence' of the AGN, the radial scale within which the cluster is strongly affected by the jet, scales as R,L1/3jet. Increasing the duration of AGN activity does not increase the radius affected by the AGN significantly, but does change the magnitude of the AGN's effects. How an AGN delivers energy to a cluster will determine where that energy is deposited: a high luminosity is needed to heat material outside the core of the cluster, while a low-luminosity, long-duration AGN is more efficient at heating the inner few tens of kpc. [source]

The potential of X-ray cluster surveys to constrain primordial non-Gaussianity

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010
B. Sartoris
ABSTRACT We present forecasts for constraints on deviations from Gaussian distribution of primordial density perturbations from future high-sensitivity X-ray surveys of galaxy clusters. Our analysis is based on computing the Fisher matrix for number counts and large-scale power spectrum of clusters. The surveys that we consider have high sensitivity and wide area to detect about 2.5 × 105 extended sources, and to provide reliable measurements of robust mass proxies for about 2 × 104 clusters. Based on the so-called self-calibration approach, and including Planck priors in our analysis, we constrain at once nine cosmological parameters and four nuisance parameters, which define the relation between cluster mass and X-ray flux. Because of the scale dependence of large-scale bias induced by local-shape non-Gaussianity, we find that the power spectrum provides strong constraints on the non-Gaussianity fNL parameter, which complement the stringent constraints on the power spectrum normalization, ,8, from the number counts. To quantify the joint constraints on the two parameters, ,8 and fNL, that specify the timing of structure formation for a fixed background expansion, we define the figure of merit . We find that our surveys constrain deviations from Gaussianity with a precision of ,fNL, 10 at 1, confidence level, with FoMSFT, 39. We point out that constraints on fNL are weakly sensitive to the uncertainties in the knowledge of the nuisance parameters. As an application of non-Gaussian constraints from available data, we analyse the impact of positive skewness on the occurrence of XMMU-J2235, a massive distant cluster recently discovered at z, 1.4. We confirm that in a WMAP -7 Gaussian ,CDM cosmology, within the survey volume, , 5 × 10,3 objects like this are expected to be found. To increase the probability of finding such a cluster by a factor of at least 10, one needs to evade either the available constraints on fNL or on the power-spectrum normalization ,8. [source]

Swimming against the current: simulations of central AGN evolution in dynamic galaxy clusters

Abundances, masses and weak-lensing mass profiles of galaxy clusters as a function of richness and luminosity in ,CDM cosmologies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2010
Stefan Hilbert
ABSTRACT We test the concordance , cold dark matter (,CDM) cosmology by comparing predictions for the mean properties of galaxy clusters to observations. We use high-resolution N -body simulations of cosmic structure formation and semi-analytic models of galaxy formation to compute the abundance, mean density profile and mass of galaxy clusters as a function of richness and luminosity, and we compare these predictions to observations of clusters in the Sloan Digital Sky Survey (SDSS) maxBCG catalogue. We discuss the scatter in the mass,richness relation, the reconstruction of the cluster mass function from the mass,richness relation and fits to the weak-lensing cluster mass profiles. The impact of cosmological parameters on the predictions is investigated by comparing results from galaxy models based on the Millennium Simulation (MS) and the WMAP1 simulation to those from the WMAP3 simulation. We find that the simulated weak-lensing mass profiles and the observed profiles of the SDSS maxBCG clusters agree well in shape and amplitude. The mass,richness relations in the simulations are close to the observed relation, with differences ,30 per cent. The MS and WMAP1 simulations yield cluster abundances similar to those observed, whereas abundances in the WMAP3 simulation are two to three times lower. The differences in cluster abundance, mass and density amplitude between the simulations and the observations can be attributed to differences in the underlying cosmological parameters, in particular the power spectrum normalization ,8. Better agreement between predictions and observations should be reached with a normalization 0.722 < ,8 < 0.9 (probably closer to the upper value), i.e. between the values underlying the two simulation sets. [source]

X-ray groups and clusters of galaxies in the Subaru,XMM Deep Field

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010
A. Finoguenov
Abstract We present the results of a search for galaxy clusters in the Subaru,XMM Deep Field (SXDF). We reach a depth for a total cluster flux in the 0.5,2 keV band of 2 × 10,15 erg cm,2 s,1 over one of the widest XMM,Newton contiguous raster surveys, covering an area of 1.3 deg2. Cluster candidates are identified through a wavelet detection of extended X-ray emission. The red-sequence technique allows us to identify 57 cluster candidates. We report on the progress with the cluster spectroscopic follow-up and derive their properties based on the X-ray luminosity and cluster scaling relations. In addition, three sources are identified as X-ray counterparts of radio lobes, and in three further sources, an X-ray counterpart of the radio lobes provides a significant fraction of the total flux of the source. In the area covered by near-infrared data, our identification success rate achieves 86 per cent. We detect a number of radio galaxies within our groups, and for a luminosity-limited sample of radio galaxies we compute halo occupation statistics using a marked cluster mass function. We compare the cluster detection statistics in the SXDF with that in the literature and provide the modelling using the concordance cosmology combined with current knowledge of the X-ray cluster properties. The joint cluster log(N) , log(S) is overpredicted by the model, and an agreement can be achieved through a reduction of the concordance ,8 value by 5 per cent. Having considered the dn/dz and the X-ray luminosity function of clusters, we conclude that to pin down the origin of disagreement a much wider (50 deg2) survey is needed. [source]

Cosmology and cluster halo scaling relations

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009
Pablo A. Araya-Melo
ABSTRACT We explore the effects of dark matter and dark energy on the dynamical scaling properties of galaxy clusters. We investigate the cluster Faber,Jackson (FJ), Kormendy and Fundamental Plane (FP) relations between the mass, radius and velocity dispersion of cluster-sized haloes in cosmological N -body simulations. The simulations span a wide range of cosmological parameters, representing open, flat and closed Universes. Independently of the cosmology, we find that the simulated clusters are close to a perfect virial state and do indeed define an FP. The fitted parameters of the FJ, Kormendy and FP relationships do not show any significant dependence on ,m and/or ,,. One outstanding effect is the influence of ,m on the thickness of the FP. Following the time evolution of our models, we find slight changes of FJ and Kormendy parameters in high-,m universe, along with a slight decrease of FP fitting parameters. We also see an initial increase of the FP thickness followed by a convergence to a nearly constant value. The epoch of convergence is later for higher values of ,m, while the thickness remains constant in the low- ,m , models. We also find a continuous increase of the FP thickness in the standard cold dark matter cosmology. There is no evidence that these differences are due to the different power spectrum slopes at cluster scales. From the point of view of the FP, there is little difference between clusters that quietly accreted their mass and those that underwent massive mergers. The principal effect of strong mergers is to significantly change the ratio of the half-mass radius rhalf to the harmonic mean radius rh. [source]

Constraints on modified gravity from the observed X-ray luminosity function of galaxy clusters

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009
David Rapetti
ABSTRACT We use measurements of the growth of cosmic structure, as inferred from the observed evolution of the X-ray luminosity function (XLF) of galaxy clusters, to constrain departures from general relativity (GR) on cosmological scales. We employ the popular growth rate parameterization, ,m(z),, for which GR predicts a growth index ,, 0.55. We use observations of the cosmic microwave background (CMB), type Ia supernovae (SNIa) and X-ray cluster gas mass fractions (fgas), to simultaneously constrain the expansion history and energy content of the Universe, as described by the background model parameters: ,m, w and ,k, i.e. the mean matter density, the dark energy equation of state parameter and the mean curvature, respectively. Using conservative allowances for systematic uncertainties, in particular for the evolution of the mass,luminosity scaling relation in the XLF analysis, we find ,= 0.51+0.16,0.15 and ,m= 0.27 ± 0.02 (68.3 per cent confidence limits), for a flat cosmological constant, cold dark matter (,CDM) background model. Allowing w to be a free parameter, we find ,= 0.44+0.17,0.15. Relaxing the flatness prior in the ,CDM model, we obtain ,= 0.51+0.19,0.16. When in addition to the XLF data we use the CMB data to constrain , through the ISW effect, we obtain a combined constraint of ,= 0.45+0.14,0.12 for the flat ,CDM model. Our analysis provides the tightest constraints to date on the growth index. We find no evidence for departures from GR on cosmological scales. [source]

Probing cosmology and galaxy cluster structure with the Sunyaev,Zel'dovich decrement versus X-ray temperature scaling relation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009
Cien Shang
ABSTRACT Scaling relations among galaxy cluster observables, which will become available in large future samples of galaxy clusters, could be used to constrain not only cluster structure, but also cosmology. We study the utility of this approach, employing a physically motivated parametric model to describe cluster structure and applying it to the expected relation between the Sunyaev,Zel'dovich decrement (S,) and the emission-weighted X-ray temperature (Tew). The slope and normalization of the entropy profile, the concentration of the dark matter potential, the pressure at the virial radius and the level of non-thermal pressure support as well as the mass and redshift dependence of these quantities are described by free parameters. With a suitable choice of fiducial parameter values, the cluster model satisfies several existing observational constraints. We employ a Fisher matrix approach to estimate the joint errors on cosmological and cluster structure parameters from a measurement of S, versus Tew in a future survey. We find that different cosmological parameters affect the scaling relation differently: predominantly through the baryon fraction (,m and ,b), the virial overdensity (w0 and wa for low- z clusters) and the angular diameter distance (w0 and wa for high- z clusters; ,DE and h). We find that the cosmology constraints from the scaling relation are comparable to those expected from the number counts (dN/dz) of the same clusters. The scaling-relation approach is relatively insensitive to selection effects and it offers a valuable consistency check; combining the information from the scaling relation and dN/dz is also useful to break parameter degeneracies and help disentangle cluster physics from cosmology. Our work suggests that scaling relations should be a useful component in extracting cosmological information from large future cluster surveys. [source]

Mass selection bias in galaxy cluster peculiar velocities from the kinetic Sunyaev,Zel'dovich effect

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
Alan C. Peel
ABSTRACT Upcoming surveys for galaxy clusters using the Sunyaev,Zel'dovich effect are potentially sensitive enough to create a peculiar velocity catalogue. The statistics of these peculiar velocities are sensitive to cosmological parameters. We develop a method to explore parameter space using N -body simulations in order to quantify dark matter halo velocity statistics which will be useful for cluster peculiar velocity observations. We show that mass selection bias from a kinetic Sunyaev,Zel'dovich velocity catalogue forecasts rms peculiar velocities with a much more complicated ,m dependency than suggested by linear perturbation theory. In addition, we show that both two-point functions for velocities disagree with linear theory predictions out to ,40 h,1 Mpc separations. A pedagogical appendix is included developing linear theory notation with respect to the two-point peculiar velocities functions. [source]

The kinematical structure of gravitationally lensed arcs

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006
Ole Möller
ABSTRACT In this paper, the expected properties of the velocity fields of strongly lensed arcs behind galaxy clusters are investigated. The velocity profile along typical lensed arcs is determined by ray-tracing light rays from a model source galaxy through parametric cluster toy models consisting of individual galaxies embedded in a dark cluster halo. We find that strongly lensed arcs of high-redshift galaxies show complex velocity structures that are sensitive to the details of the mass distribution within the cluster, in particular at small scales. From fits to the simulated imaging and kinematic data, we demonstrate that reconstruction of the source velocity field is in principle feasible. Two-dimensional kinematic information obtained with integral field units on large ground-based telescopes in combination with adaptive optics will allow the reconstruction of rotation curves of lensed high redshift galaxies. This makes it possible to determine the mass-to-light ratios of galaxies at redshifts z > 1 out to about 2,3 scalelengths with better than ,20 per cent accuracy. We also discuss the possibilities of using two-dimensional kinematic information along the arcs to give additional constraints on the cluster lens mass models. [source]

The effects of thermal conduction on the intracluster medium of the Virgo cluster

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2005
Edward C. D. Pope
ABSTRACT Thermal conduction has been suggested as a possible mechanism by which sufficient energy is supplied to the central regions of galaxy clusters to balance the effect of radiative cooling. Recent high-resolution observations of the nearby Virgo cluster make it an ideal subject for an attempt to reproduce the properties of the cluster by numerical simulations, because most of the defining parameters are comparatively well known. Here, we present the results of a simulated high-resolution, 3D Virgo cluster for different values of thermal conductivity ( times the full Spitzer value). Starting from an initially isothermal cluster atmosphere, we allow the cluster to evolve freely over time-scales of roughly 1.3,4.7 × 109 yr. Our results show that thermal conductivity at the Spitzer value can increase the central intracluster medium (ICM) radiative cooling time by a factor of roughly 3.6. In addition, for larger values of thermal conductivity, the simulated temperature and density profiles match the observations significantly better than for the lower values. However, no physically meaningful value of thermal conductivity was able to postpone the cooling catastrophe (characterized by a rapid increase in the central density) for longer than a fraction of the Hubble time nor explain the absence of a strong cooling flow in the Virgo cluster today. We also calculate the effective adiabatic index of the cluster gas for both simulation and observational data and compare the values with theoretical expectations. Using this method, it appears that the Virgo cluster is being heated in the cluster centre by a mechanism other than thermal conductivity. Based on this and our simulations, it is also likely that the thermal conductivity is suppressed by a factor of at least 10 and probably more. Thus, we suggest that thermal conductivity, if present at all, has the effect of slowing down the evolution of the ICM, by radiative cooling, but only by a factor of a few. [source]

Constraining dark energy with X-ray galaxy clusters, supernovae and the cosmic microwave background

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2005
David Rapetti
ABSTRACT We present new constraints on the evolution of dark energy from an analysis of cosmic microwave background, supernova and X-ray galaxy cluster data. Our analysis employs a minimum of priors and exploits the complementary nature of these data sets. We examine a series of dark energy models with up to three free parameters: the current dark energy equation of state w0, the early-time equation of state wet, and the scalefactor at transition at. From a combined analysis of all three data sets, assuming a constant equation of state and that the Universe is flat, we measure w0=,1.05+0.10,0.12. Including wet as a free parameter and allowing the transition scalefactor to vary over the range 0.5 < at < 0.95 where the data sets have discriminating power, we measure w0=,1.27+0.33,0.39 and wet=,0.66+0.44,0.62. We find no significant evidence for evolution in the dark energy equation-of-state parameter with redshift. Marginal hints of evolution in the supernovae data become less significant when the cluster constraints are also included in the analysis. The complementary nature of the data sets leads to a tight constraint on the mean matter density ,m and alleviates a number of other parameter degeneracies, including that between the scalar spectral index ns, the physical baryon density ,bh2 and the optical depth ,. This complementary nature also allows us to examine models in which we drop the prior on the curvature. For non-flat models with a constant equation of state, we measure w0=,1.09+0.12,0.15 and obtain a tight constraint on the current dark energy density ,de= 0.70 ± 0.03. For dark energy models other than a cosmological constant, energy,momentum conservation requires the inclusion of spatial perturbations in the dark energy component. Our analysis includes such perturbations, assuming a sound speed c2s= 1 in the dark energy fluid as expected for quintessence scenarios. For our most general dark energy model, not including such perturbations would lead to spurious constraints on wet, which would be tighter than those mentioned above by approximately a factor of 2 with the current data. [source]

The environmental dependence of radio-loud AGN activity and star formation in the 2dFGRS

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2004
P. N. Best
ABSTRACT By combining the 2-degree Field Galaxy Redshift Survey with the NRAO VLA Sky Survey at 1.4 GHz, the environments of radio-loud active galactic nuclei (AGN) in the nearby Universe are investigated using both local projected galaxy densities and a friends-of-friends group-finding algorithm. Radio-loud AGN are preferentially located in galaxy groups and poor-to-moderate richness galaxy clusters. The AGN fraction appears to depend more strongly on the large-scale environment (group, cluster, etc.) in which a galaxy is located than on its more local environment, except at the lowest galaxy surface densities where practically no radio-loud AGN are found. The ratio of absorption-line to emission-line AGN changes dramatically with environment, with essentially all radio-loud AGN in rich environments showing no emission lines. This result could be connected with the lack of cool gas in cluster galaxies, and may have important consequences for analyses of optically selected AGN, which are invariably selected on emission-line properties. The local galaxy surface density of the absorption-line AGN is strongly correlated with radio luminosity, implying that the radio luminosities may be significantly boosted in dense environments due to confinement by the hot intracluster gas. The environments of a radio-selected sample of star-forming galaxies are also investigated to provide an independent test of optical studies. In line with those studies, the fraction of star-forming galaxies is found to decrease strongly with increasing local galaxy surface density; this correlation extends across the whole range of galaxy surface densities, with no evidence for the density threshold found in some optical studies. [source]

Cluster correlations in redshift space

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002
N.D. Padilla
We test an analytic model for the two-point correlations of galaxy clusters in redshift space using the Hubble volume N -body simulations. The correlation function of clusters shows no enhancement along the line of sight, owing to the lack of any virialized structures in the cluster distribution. However, the distortion of the clustering pattern arising from coherent bulk motions is clearly visible. The distribution of cluster peculiar motions is well described by a Gaussian, except in the extreme high-velocity tails. The simulations produce a small but significant number of clusters with large peculiar motions. The form of the redshift-space power spectrum is strongly influenced by errors in measured cluster redshifts in extant surveys. When these errors are taken into account, the model reproduces the power spectrum recovered from the simulation to an accuracy of 15 per cent or better over a decade in wavenumber. We compare our analytic predictions with the power spectrum measured from the APM cluster redshift survey. The cluster power spectrum constrains the amplitude of density fluctuations, as measured by the linear rms variance in spheres of radius 8 h,1 Mpc, denoted by ,8. When combined with the constraints on ,8 and the density parameter , derived from the local abundance of clusters, we find a best-fitting cold dark matter model with and , for a power spectrum shape that matches that measured for galaxies. However, for the best-fitting value of , and given the value of Hubble's constant from recent measurements, the assumed shape of the power spectrum is incompatible with the most readily motivated predictions from the cold dark matter paradigm. [source]

Relativistic corrections to the multiple scattering effect on the Sunyaev,Zel'dovich effect in the isotropic approximation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2001
Naoki Itoh
We extend the formalism for the calculation of the relativistic corrections to the Sunyaev,Zel'dovich effect for clusters of galaxies and include the multiple scattering effects in the isotropic approximation. We present the results of the calculations by the Fokker,Planck expansion method as well as by the direct numerical integration of the collision term of the Boltzmann equation. The multiple scattering contribution is found to be very small compared with the single scattering contribution. For high-temperature galaxy clusters of the ratio of both the contributions is ,0.2 per cent in the Wien region. In the Rayleigh,Jeans region the ratio is ,0.03 per cent. Therefore the multiple scattering contribution is safely neglected for the observed galaxy clusters. [source]

Searching for cluster substructure using APM and ROSAT data

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2001
V. Kolokotronis
We present a detailed study of the morphological features of 22 rich galaxy clusters. Our sample is constructed from a cross-correlation of optical data with X-ray (0.1,2.4 keV) ROSAT pointed observations. We systematically compare cluster images and morphological parameters in an attempt to reliably identify possible substructure in both optical and the X-ray images. To this end, we compute various moments of the optical and X-ray surface-brightness distribution such as the ellipticities, centre-of-mass shifts and ellipsoidal orientations. We assess the significance of our results using Monte Carlo simulations. We find significant correlations between the optical and X-ray morphological parameters, indicating that in both parts of the spectrum it is possible to identify correctly the dynamical state of a cluster. Most of our clusters (17/22) have a good one-to-one correspondence between the optical and the X-ray images, and about 10 appear to have strong indications of substructure. This corresponds to a minimum percentage of order ,45 per cent, which is in very good accordance with other similar analyses. Finally, five out of 22 systems (,22 per cent) seem to have distinct subclumps in the optical which are not verified in the X-ray images, and thus are suspect of being due to optical projection effects. These results will serve as a useful guide in interpreting subsequent analyses of large optical cluster catalogues. [source]

X-ray colour maps of the cores of galaxy clusters

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2000
J. S. Sanders
We present an analysis of X-ray colour maps of the cores of clusters of galaxies, formed from the ratios of counts in different X-ray bands. Our technique groups pixels lying between contours in an adaptively smoothed image of a cluster. We select the contour levels to minimize the uncertainties in the colour ratios, whilst preserving the structure of the object. We extend the work of Allen & Fabian by investigating the spatial distributions of cooling gas and absorbing material in cluster cores. Their sample is almost doubled: we analyse archive ROSAT Position Sensitive Proportional Counter (PSPC) data for 33 clusters from the sample of the 55 brightest X-ray clusters in the sky. Many of our clusters contain strong cooling flows. We present colour maps of a sample of the clusters, in addition to adaptively smoothed images in different bands. Most of the cooling flow clusters display little substructure, unlike several of the non-cooling-flow clusters. We fitted an isothermal plasma model with galactic absorption and constant metallicity to the mid-over-high energy colours in our clusters. Those clusters with known strong cooling flows have inner contours which fit a significantly lower temperature than the outer contours. Clusters in the sample without strong cooling flows show no significant temperature variation. The inclusion of a metallicity gradient alone was not sufficient to explain the observations. A cooling flow component plus a constant temperature phase did account for the colour profiles in clusters with known strong cooling flow components. We also had to increase the levels of absorbing material to fit the low-over-high colours at the cluster centres. Our results provide more evidence that cooling flows accumulate absorbing material. No evidence for increased absorption was found for the non-cooling-flow clusters. [source]

Ram pressure stripping of disk galaxies in galaxy clusters

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009
E. Roediger
Abstract While galaxies move through the intracluster medium of their host cluster, they experience a ram pressure which removes at least a significant part of their interstellar medium. This ram pressure stripping appears to be especially important for spiral galaxies: this scenario is a good candidate to explain the differences observed between cluster spirals in the nearby universe and their field counterparts. Thus, ram pressure stripping of disk galaxies in clusters has been studied intensively during the last decade. I review advances made in this area, concentrating on theoretical work, but continuously comparing to observations (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Dust-enshrouded star formation in XMM-LSS galaxy clusters

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009
S. Temporin
Abstract In recent years, infrared studies of individual galaxy clusters at intermediate redshifts led to the identification of several luminous infrared galaxies (LIR > 1011 L,). These findings were interpreted as an evidence ofvigorous, dust-enshrouded star formation activity, hinting at the presence of an infrared Butcher-Oemler effect. In order to explore this effect, we have undertaken a statistical investigation based on a cluster sample drawn from the XMM-Newton Large Scale Structure survey (XMM-LSS) and exploiting the contiguous coverage of the XMM-LSS field by the Spitzer SWIRE legacy survey. We adopted a stacking technique to investigate the distribution of 24- , m sources out to the cluster periphery and compare it with the distribution of sources in a sample of control fields. Among putative cluster members, selected by means of photometric redshifts, we identified surprisingly high numbers of bright 24- , m sources toward intermediate redshift clusters (z > 0.3). Our analysis indicates the presence of an excess of bright mid-IR sources at redshift z > 0.3 at clustercentric radii ,200,500 kpc (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Probing evolutionary mechanisms in galaxy clusters: H I in Abell 1367

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009
T.C. Scott
Abstract We are carrying out a programme of CO and interferometric H I observations in the nearby galaxy cluster Abell 1367. The aim is to better understand the processes driving the evolution of the ISM in late-type galaxies in the cluster environment. Abell 1367 is a dynamically young cluster. We have determined the H I content and g , i colour from AGES (Cortese 2008) and SDSS, respectively, of its bright late-type galaxies (spirals) in a volume centred on the NW subcluster. We use a combination of these characteristics, both of which are indicative of evolutionary history, to classify each spiral into one of four evolutionary states. This analysis revealed the cluster contains spirals in a wide range of evolutionary states. VLA D-array imaging centred on the NW subcluster indicates most galaxies in that field have their H I intensity maximum offset relative to its optical counterpart, implying a recent and strong disturbance. The direction of this offset is not always consistent with a simple scenario of ram pressure stripping by the cluster's high density intra-cluster medium (ICM) (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

The evolution of cluster early-type galaxies over the past 8 Gyr

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009
A. Fritz
Abstract We present the Fundamental Plane (FP) of early-type galaxies in the clusters of galaxies RXJ1415.1+3612 at z = 1.013. This is the first detailed FP investigation of cluster early-type galaxies at redshift z = 1. The distant cluster galaxies follow a steeper FP relation compared to the local FP. The change in the slope of the FP can be interpreted as a massdependent evolution. To analyse in more detail the galaxy population in high redshift galaxy clusters at 0.8 < z < 1, we combine our sample with a previous detailed spectroscopic study of 38 early-type galaxies in two distant galaxy clusters, RXJ0152.7,1357 at z = 0.83 and RXJ1226.9+3332 at z = 0.89. For all clusters Gemini/GMOS spectroscopy with high signal-to-noise and intermediate-resolution has been acquired to measure the internal kinematics and stellar populations of the galaxies. From HST/ACS imaging, surface brightness profiles, morphologies and structural parameters were derived for the galaxy sample. The least massive galaxies (M = 2 x 1010 M,) in our sample have experienced their most recent major star formation burst at zform , 1.1. For massive galaxies (M > 2 x 1011 M,) the bulk of their stellar populations have been formed earlier zform , 1.6. Our results confirm previous findings by Jørgensen et al. This suggests that the less massive galaxies in the distant clusters have much younger stellar populations than their more massive counterparts. One explanation is that low-mass cluster galaxies have experienced more extended star formation histories with more frequent bursts of star formation with shorter duration compared to the formation history of high-mass cluster galaxies (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Luminosity function of galaxies in rich and poor galaxy clusters

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009
J. Krywult
Abstract Using photometric data from SDSS survey we study the galaxy luminosity function in the 75 rich and poor Abell clusters. The integrated Schechter functions were fitted to unbinned data in the four g, r, i, and z photometric passbands. We find that the luminosity function differs with Abell richness class (R). In all bands the faint-end slope , is steeper in poor clusters than in rich clusters. Moreover, the value of characteristic magnitude M * decreasing with richness class R (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

The properties of fossil groups of galaxies

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009
P. Eigenthaler
Abstract Numerical simulations as well as optical and X-ray observations over the last few years have shown that poor groups of galaxies can evolve to what is called a fossil group. Dynamical friction as the driving process leads to the coalescence of individual galaxies in ordinary poor groups leaving behind nothing more than a central, massive elliptical galaxy supposed to contain the merger history of the whole group. Due to merging timescales for less-massive galaxies and gas cooling timescales of the X-ray intragroup medium exceeding a Hubble time, a surrounding faint-galaxy population having survived this galactic cannibalism as well as an extended X-ray halo similar to that found in ordinary groups, is expected. Recent studies suggest that fossil groups are very abundant and could be the progenitors of brightest cluster galaxies (BCGs) in the centers of rich galaxy clusters. However, only a few objects are known to the literature. This article aims to summarize the results of observational fossil group research over the last few years and presents ongoing work by the authors. Complementary to previous research, the SDSS and RASS surveys have been cross-correlated to identify new fossil structures yielding 34 newly detected fossil group candidates. Observations with ISIS at the 4.2 m William Herschel Telescope on La Palma have been carried out to study the stellar populations of the central ellipticals of 6 fossil groups. In addition multi-object spectroscopy with VLTs VIMOS has been performed to study the shape of the OLF of one fossil system (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

IC 1370: A merger candidate at the periphery of a z , 0.05 cluster,

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009
S. Temporin
Abstract Recent studies show evidence of galaxy pre-processing within groups and/or filaments before infall in galaxy clusters. We present VIMOS/IFU observations of IC1370, a galaxy that shows morphological signatures of a recent merger and is located at a projected distance of ,1.2 Mpc to the center of the cluster II Zw108 at z , 0.05. This galaxy shows two opposite tidal tails, a boxy bulge with a de Vaucouleurs light profile, and a disk component that contributes ,50 % of the total luminosity. We investigate the history of the galaxy by applying the spectral synthesis technique to the integral field observations and combining the resulting information with morphological and photometric parameters. The same observations allow us to investigate a background (z , 0.09) compact group of 6 galaxies, ofwhich IC 1370 was initially thought to be the central, brightest member (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Early-type dwarf galaxies in clusters: A mixed bag with various origins?

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009
T. Lisker
Abstract The formation ofearly-type dwarf(dE) galaxies, the most numerous objects in clusters, is believed tobe closely connected to the physical processes that drive galaxy cluster evolution, like galaxy harassment and ram-pressure stripping. However, the actual significance ofeach mechanism for building the observed cluster dE population is yet unknown. Several distinct dE subclasses were identified, which show significant differences in their shape, stellar content, and distribution within the cluster. Does this diversity imply that dEs originate from various formation channels? Does "cosmological" formation play a role as well? I try to touch on these questions in this brief overview of dEs in galaxy clusters (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Probing magnetohydrodynamical scenarios for jet production

ASTRONOMISCHE NACHRICHTEN, Issue 5-6 2006
E. M. de Gouveia Dal Pino
Abstract Large-scale magnetic fields created by dynamo action along with magneto-centrifugal and reconnection processes in accretion disks around the sources is currently the most accepted mechanism for jet production. Understanding the production of magnetic fields in jet-accretion phenomena may be the key for understanding the origin of the large-scale magnetic fields in galaxy clusters and in the Universe. Here, we discuss recent observational, theoretical and numerical results on jet phenomena that support the MHD scenarios for jet production. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

The origin and evolution of cluster magnetism

ASTRONOMISCHE NACHRICHTEN, Issue 5-6 2006
A. Shukurov
Abstract Random motions can occur in the intergalactic gas of galaxy clusters at all stages of their evolution. Depending on the poorly known value of the Reynolds number, these motions can or cannot become turbulent, but in any case they can generate random magnetic fields via dynamo action. We argue that magnetic fields inferred observationally for the intracluster medium require dynamo action, and then estimate parameters of random flows and magnetic fields at various stages of the cluster evolution. Polarization in cluster radio halos predicted by the model would be detectable with the SKA. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Simulations of magnetic fields in the cosmos

ASTRONOMISCHE NACHRICHTEN, Issue 5-6 2006
M. Brüggen
Abstract The origin of large-scale magnetic fields in clusters of galaxies remains controversial. The intergalactic magnetic field within filaments should be less polluted by magnetised outflows from active galaxies than magnetic fields in clusters. Therefore, filaments may be a better laboratory to study magnetic field amplification by structure formation than galaxy clusters, which typically host many more active galaxies. We present highly resolved cosmological adaptive mesh refinement simulations of magnetic fields in the cosmos and make predictions about the evolution and structure of magnetic fields in filaments. Comparing our results to observational evidence of magnetic fields in filaments suggests that amplification of seed fields by gravitational collapse is not sufficient to produce intergalactic medium fields. Finally, implications for cosmic ray transport and the impact of magnetic fields on delayed photons from gamma-ray bursts are discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Dark matter behaves itself in galaxy clusters

ASTRONOMY & GEOPHYSICS, Issue 3 2010
Article first published online: 24 MAY 2010
While the nature of dark matter is proving elusive, its location is becoming ever more precisely known thanks to more sophisticated means of measuring its gravitational effects. [source]