Local Galaxy Clusters Research Articles

Evolution atz≥ 0.5 of the X-ray properties of simulated galaxy clusters: comparison with observational constraints

We analyse the X-ray properties of a sample of local and high-redshift galaxy clusters extracted from a large cosmological hydrodynamical simulation. This simulation has been realized using the tree + SPH (smoothed particle hydrodynamic) code gadget-2 for a Λ-cold dark matter (ΛCDM) model. It includes radiative cooling, star formation and supernova feedback and allows the thermodynamic structure of clusters to be resolved radially up to redshift z= 1 in a way that is not yet completely accessible to observations. We consider only objects with Tew > 2 keV to avoid the large scatter in the physical properties present at the scale of groups and compare their properties to recent observational constraints. In our analysis, we adopt an approach that mimics observations, associating with each measurement an error comparable with recent observations and providing best-fitting results via robust techniques. Within the clusters, baryons are distributed among (i) a cold neutral phase, with a relative contribution that increases from less than 1 per cent to 3 per cent at higher redshift, (ii) stars, which contribute about 20 per cent, and (iii) the X-ray-emitting plasma, which contributes 80 (76) per cent at z= 0 (1) to the total baryonic budget. A depletion of the cosmic baryon fraction of ∼7 per cent (at z= 0) and 5 per cent (at z= 1) is measured at the virial radius, Rvir, in good agreement with adiabatic hydrodynamical simulations. We confirm that, also at redshift z > 0.5, power-law relations hold between the gas temperature T, the bolometric luminosity L, the central entropy S, the gas mass Mgas and the total gravitating mass Mtot, and that these relations are steeper than predicted by simple gravitational collapse. A significant negative evolution in the L–T and L–Mtot relations and positive evolution in the S–T relation are detected at 0.5 < z < 1 in this set of simulated galaxy clusters. This is partially consistent with recent analyses of the observed properties of z≳ 0.5 X-ray galaxy clusters. By fixing the slope to the values predicted by simple gravitational collapse, at high redshift we measure normalizations lower than the observed estimates by 10–40 per cent in the L–T, Mtot–T, Mgas–T, fgas–T and L–Mtot relations. This suggests either that the amount of hot X-ray-emitting plasma measured in the central regions of simulated systems is smaller than the observed one or that systematically higher values of gas temperatures and total masses than actually measured are recovered in the present simulated data set.

A sample of radio galaxies spanning three decades in radio luminosity - I. The host galaxy properties and black hole masses

The results of analysis of HST I-band imaging of a sample of 41 radio galaxies spanning three orders of magnitude in radio luminosity at redshift z~0.5 are presented. The full sample is drawn from four complete, low-frequency selected radio samples with progressively fainter flux-density limits (3CRR, 6CE, 7CRS and the new TexOx-1000 sample). Modelling of the HST imaging data shows that the host galaxies have surface-brightness distributions consistent with those expected for classic ellipticals (Sersic parameter, beta~0.25), with beta in the range 0.17 =0.23\pm0.01. The host-galaxy luminosities are comparable with those of galaxies drawn from the bright end of the local cluster galaxy luminosity function, spanning the range 0.7L^{\star} =10^{8.87\pm 0.04}\Msun. Finally, a significant (~3 sigma) correlation is found between black-hole mass and 151-MHz radio luminosity for those objects in the sample with either high-excitation nuclear spectra or classical double radio structures. (abridged)

Studying dark energy with galaxy cluster surveys

Galaxy cluster surveys provide a powerful means of studying the density and nature of the dark energy. The redshift distribution of detected clusters in a deep, large solid angle SZE or X-ray survey is highly sensitive to the dark energy equation of state. Accurate constraints at the 5% level on the dark energy equation of state require that systematic biases in the mass estimators must be controlled at better than the ∼10% level. Observed regularity in the cluster population and the availability of multiple, independent mass estimators suggests these precise measurements are possible. Using hydrodynamical simulations that include preheating, we show that the level of preheating required to explain local galaxy cluster structure has a dramatic effect on X-ray cluster surveys, but only a mild effect on SZE surveys. This suggests that SZE surveys may be optimal for cosmology while X-ray surveys are well suited for studies of the thermal history of the intracluster medium.

The content of active galactic nuclei in the z = 0.83 cluster MS 1054--0321

We present a survey of X-ray point sources in a 91-ks Chandra ACIS-S observation of the z = 0.83 cluster MS 1054-0321. We detect 47 X-ray sources within the 8.3 x 8.3 arcmin 2 field, of which two are immediately confirmedfrom pre-existing spectroscopy to be luminous active galactic nuclei (AGN) [L ( 0 . 5 - 8 k e V ) ∼ 10 4 3 erg s - 1 ] at the redshift of the cluster. At fluxes brighter than S ( 0 . 5 - 8 k e V ) = 5 x 10 - 1 5 erg s - 1 cm - 2 , we find a ∼2σ excess compared with predictions from field surveys, consistent with an excess of approximately six AGN. If these sources are associated with the cluster, they too are AGN with luminosities of order L ( 0 . 5 - 8 k e V ) ∼ 10 4 3 erg s - 1 . Combined with the recent identification of seven cluster AGN from deep radio observations by Best et al., these observations suggest significantly enhanced AGN activity in MS 1054-03 compared with local galaxy clusters. Interestingly, the excess of X-ray detected AGN is found at radial distances of between 1 and 2 Mpc, suggesting that they may be associated with infalling galaxies. The radio AGN are seen within the inner megaparsec of the cluster and are largely undetected in the X-ray, suggesting that they are either intrinsically less luminous and/or heavily obscured.

Evolutionary Synthesis Models for Galaxy Transformation in Clusters

The galaxy population in rich local galaxy clusters shows a ratio of one quarter elliptical galaxies, two quarters S0 galaxies, and one quarter spiral galaxies. Observations of clusters at redshift 0.5 show a perspicuously different ratio, the dominant galaxy type are spiral galaxies with a fraction of two quarters while the number of S0 galaxies decreases to a fraction of one quarter (Dressler et al.1997). This shows an evolution of the galaxy population in clusters since redshift0.5 and it has been suspected that galaxy transformation processes during the infall into a cluster are responsible for this change. These could be merging, starburst or ram-pressure stripping. We use our evolutionary synthesis models to describe various possible effects of those interactions on the star formation of spiral galaxies infalling into clusters. We study the effects of starbursts of various strengths as well as of the truncation of star formation at various epochs on the color and luminosity evolution of model galaxies of various spectral types. As a first application we present the comparison of our models with observed properties of the local S0 galaxy population to constrain possible S0 formation mechanisms in clusters. Application to other types of galaxies is planned for the future.

The Chandra Deep Field North Survey. IX. Extended X-Ray Sources

The ≈1 Ms Chandra Deep Field North observation is used to study the extended X-ray sources in the region surrounding the Hubble Deep Field North (HDF-N), yielding the most sensitive probe of extended X-ray emission at cosmological distances to date. A total of six such sources are detected, the majority of which align with small numbers of optically bright galaxies. Their angular sizes, band ratios, and X-ray luminosities—assuming they lie at the same distances as the galaxies coincident with the X-ray emission—are generally consistent with the properties found for nearby groups of galaxies. One source is notably different and is likely to be a poor-to-moderate X-ray cluster at high redshift (i.e., z 0.7). This source has a large angular extent, a double-peaked X-ray morphology, and an overdensity of unusual objects [very red objects, optically faint (I ≥ 24) radio and X-ray sources]. Another of the six sources is coincident with several z ≈ 1.01 galaxies located within the HDF-N itself, including the FR I radio galaxy VLA J123644+621133, and is likely to be a group or poor cluster of galaxies at that redshift. We are also able to place strong constraints on the optically detected cluster of galaxies ClG 1236+6215 at z = 0.85 and the wide-angle–tailed radio galaxy VLA J123725+621128 at z ~ 1–2; both sources are expected to have considerable associated diffuse X-ray emission, and yet they have rest-frame 0.5–2.0 keV X-ray luminosities of 3 × 1042 and (3–15) × 1042 ergs s-1, respectively. The environments of both sources are either likely to have a significant deficit of hot intracluster gas compared with local clusters of galaxies, or they are X-ray groups. We find the surface density of extended X-ray sources in this observation to be 167 deg-2 at a limiting soft-band flux of ≈3 × 10-16 ergs cm-2 s-1. No evolution in the X-ray luminosity function of clusters is needed to explain this value.

TheROSAT‐ESO Flux‐limited X‐Ray (REFLEX) Galaxy Cluster Survey. IV. The X‐Ray Luminosity Function

The X-ray galaxy cluster sample from the REFLEX Cluster Survey, which covers the X-ray brightest galaxy clusters detected in the ROSAT All-Sky Survey in the southern sky, is used to construct the X-ray luminosity function of clusters in the local universe. With 452 clusters detected above an X-ray flux limit of 3 × 10-12 ergs s-1 cm-2 in 4.24 sr of the sky, this sample is the most comprehensive X-ray cluster sample with a well-documented selection function, providing the best current census of the local X-ray galaxy cluster population. In this paper we discuss the construction of the luminosity function and the effects of flux measurement errors and of variations with sample region, and we compare the results with those from previous surveys.

Deuterium and the baryonic density of the universe

Big bang nucleosynthesis (BBN) is the creation of the light nuclei, deuterium, 3 He, 4 He and 7Li during the first few minutes of the universe. Here we discuss recent measurements of the D to H abundance ratio, D/H, in our galaxy and towards quasars. We have achieved an order of magnitude improvement in the precision of the measurement of primordial D/H, using the HIRES spectrograph on the W. M. Keck telescope to measure D in gas with very nearly primordial abundances towards quasars. From 1994 to 1996, it appeared that there could be a factor of 10 range in primordial D/H, but today four examples of low D are secure. High D/H should be much easier to detect, and since there are no convincing examples, it must be extremely rare or non-existent. All data are consistent with a single low value for D/H, and the examples which are consistent with high D/H are readily interpreted as H contamination near the position of D. The new D/H measurements give the most accurate value for the baryon-to-photon ratio, η, and hence the cosmological baryon density. A similar density is required to explain the amount of Ly α absorption from neutral hydrogen in the intergalactic medium (IGM) at redshift z≃3, and to explain the fraction of baryons in local clusters of galaxies. The D/H measurements lead to predictions for the abundances of the other light nuclei, which generally agree with measurements. The remaining differences with some measurements can be explained by a combination of measurement and analysis errors or changes in the abundances after BBN. The measurements do not require physics beyond the standard BBN model. Instead, the agreement between the abundances is used to limit the non-standard physics.

Measurements of The Primordial D/H Abundance Towards Quasars

Big Bang Nucleosynthesis (BBN) is the synthesis of the light nuclei, Deuterium (D or 2H), 3He, 4He and 7Li during the first few minutes of the universe. In this review we concentrate on recent data which give the primordial deuterium (D) abundance.We have measured the primordial D/H in gas with very nearly primordial abundances. We use the Lyman series absorption lines seen in the spectra of quasars. We have measured D/H towards three QSOs, while a fourth gives a consistent upper limit. All QSO spectra are consistent with a single value for D/H: 3.325+0.22−0.25X10−5. From about 1994 − 1996, there was much discussion of the possibility that some QSOs show much higher D/H, but the best such example was shown to be contaminated by H, and no other no convincing examples have been found. Since high D/H should be much easier to detect, and hence it must be extremely rare or non-existent.The new D/H measurements give the most accurate value for the baryon to photon ratio, η, and hence the cosmological baryon density: ωb = 0.0190 ± 0.0009 (1σ) A similar density is required to explain the amount of Lyα absorption from neutral Hydrogen in the intergalactic medium (IGM) at redshift z ≃ 3, and to explain the fraction of baryons in local clusters of galaxies. The D/H measurements lead to predictions for the abundances of the other light nuclei, which generally agree with measurements. The remaining differences with some measurements can be explained by a combination of measurement and analysis errors or changes in the abundances after BBN. The measurements do not require physics beyond the standard BBN model. Instead, the agreement between the abundances is used to limit the non-standard physics.

Investigation of galactic alignment in Local Supercluster galaxy clusters

We investigate the orientations of the galactic axes within 18 selected clusters, substructures of the Local Supercluster. For every cluster we map the parameter Δ11 describing the alignment of the galactic axes with respect to a chosen cluster pole, divided by its formal error σ (Δ11) [s ≡Δ11 / σ (Δ11)]. The cluster pole coordinates change along the entire celestial sphere. The resulting maps are analysed for correlations of their maxima with directions from the cluster centre to (i) the derived `physical' cluster poles, (ii) the Local Supercluster centre, (iii) the Virgo A centre and (iv) the Earth, i.e. along the line of sight (≡ LOS). The strong maxima -- with one exception -- exist only for non-spiral (NS) subsamples, with the maximum well correlated with the LOS direction. Another of the studied directions may occur close to the maximum only if these directions are close to the LOS and they do not correlate with other features visible on the maps. For clusters with a clearly defined maximum of s below 3.0 the conclusion generally does not change. For the spiral (S) subsamples the maps are usually at the random noise level. In these cases a weaker, but still existing correlation with the LOS is observed and no other evident correlations are noted. We conclude that a strong systematic effect, generated by the process of deprojection of a galactic axis from its optical image, is present in the catalogue data. It can mask any weak alignment existing in the analysed clusters. With the use of a simple model for the systematic effect we are able to reproduce the main characteristic features of the maps for non-spiral galaxies. We note, however, that a few clusters show significant differences with respect to this model.

Testing Cold Dark Matter Models at Moderate to High Redshift

The COBE microwave background temperature fluctuations and the abundance of local rich clusters of galaxies provide the two most powerful constraints on cosmological models. When all variants of the standard cold dark matter (CDM) model are subject to the combined constraint, the power spectrum of any model is fixed to $\sim 10%$ accuracy in both the shape and overall amplitude. These constrained models are not expected to differ dramatically in their local large-scale structure properties. However, their evolutionary histories differ, resulting in dramatic differences towards high redshift. We examine in detail six standardized, COBE and cluster normalized CDM models with respect to a large set of independent observations. The observations include correlation function of rich clusters of galaxies, galaxy power spectrum, evolution of rich cluster abundance, gravitational lensing by moderate -to-high redshift clusters, $\lya$ forest, damped $\lya$ systems, high redshift galaxies, reionization of the universe and future CMB experiments. It seems that each of the independent observations examined is or potentially is capable of distinguishing between at least some of the models. The combined power of several or all of these observations is tremendous. Thus, we appear to be on the verge of being able to make dramatic tests of all models in the near future using a rapidly growing set of observations, mostly at moderate to high redshift. Consistency or inconsistency between different observed phenomena on different scales and/or at different epochs with respect to the models will have profound implications for theory of growth of cosmic structure.

Formation of cosmic structures in a light gravitino-dominated universe

We analyse the formation of cosmic structures in models where the dark matter is dominated by light gravitinos with mass of $ 100$ eV -- 1 keV, as predicted by gauge-mediated supersymmetry (SUSY) breaking models. After evaluating the number of degrees of freedom at the gravitinos decoupling ($g_*$), we compute the transfer function for matter fluctuations and show that gravitinos behave like warm dark matter (WDM) with free-streaming scale comparable to the galaxy mass scale. We consider different low-density variants of the WDM model, both with and without cosmological constant, and compare the predictions on the abundances of neutral hydrogen within high-redshift damped Ly--$\alpha$ systems and on the number density of local galaxy clusters with the corresponding observational constraints. We find that none of the models satisfies both constraints at the same time, unless a rather small $\Omega_0$ value ($\mincir 0.4$) and a rather large Hubble parameter ($\magcir 0.9$) is assumed. Furthermore, in a model with warm + hot dark matter, with hot component provided by massive neutrinos, the strong suppression of fluctuation on scales of $\sim 1\hm$ precludes the formation of high-redshift objects, when the low--$z$ cluster abundance is required. We conclude that all different variants of a light gravitino DM dominated model show strong difficulties for what concerns cosmic structure formation. This gives a severe cosmological constraint on the gauge-mediated SUSY breaking scheme.

Testing Cosmological Models with a L[CLC]y[/CLC]α Forest Statistic: The High End of the Optical Depth Distribution

We pay particular attention to the high end of the $\lya$ optical depth distribution of a quasar spectrum. Based on the flux distribution (Miralda-Escud\'e et al 1996), a simple yet seemingly cosmological model -differentiating statistic, $\Delta_{\tau_0}$ -- the cumulative probability of a quasar spectrum with $\lya$ optical depth greater than a high value $\tau_0$ -- is emphasized. It is shown that two different models -- the cold dark matter model with a cosmological constant and the mixed hot and cold dark matter model, both normalized to COBE and local galaxy cluster abundance -- yield quite different values of $\Delta_{\tau_0}$: 0.13 of the former versus 0.058 of the latter for $\tau_0=3.0$ at $z=3$. Moreover, it is argued that $\Delta_{\tau_0}$ may be fairly robust to compute theoretically because it does not seem to depend sensitively on small variations of simulations parameters such as radiation field, cooling, feedback process, radiative transfer, resolution and simulation volume within the plausible ranges of the concerned quantities. Furthermore, it is illustrated that $\Delta_{\tau_0}$ can be obtained sufficiently accurately from currently available observed quasar spectra for $\tau_0\sim 3.0-4.0$, when observational noise is properly taken into account. We anticipate that analyses of observations of quasar $\lya$ absorption spectra over a range of redshift may be able to constrain the redshift evolution of the amplitude of the density fluctuations on small-to-intermediate scales, therefore providing an independent constraint on $\Omega_0$, $\Omega_{0,HDM}$ and $\Lambda_0$.

Evolution of Cluster Ellipticals at 0.2 &lt; [CLC][ITAL]z[/ITAL][/CLC] &lt; 1.2 from [ITAL]Hubble Space Telescope[/ITAL] Imaging,

Two-dimensional surface photometry derived from Hubble Space Telescope imaging is presented for a sample of 225 early-type galaxies (assumed to be cluster members) in the fields of 9 clusters at redshifts $0.17 < z < 1.21$. The 94 luminous ellipticals ($M_{AB}(B)<-20$; selected by morphology alone with no reference to color) form tight sequences in the size-luminosity plane. The position of these sequences shifts, on average, with redshift so that an object of a given size at z=0.55 is brighter by $\Delta M(B)=-0.57 \pm 0.13$ mag than its counterpart (measured with the same techniques) in nearby clusters. At z=0.9 the shift is $\Delta M(B)=-0.96 \pm 0.22$ mag. If the relation between size and luminosity is universal so that the local cluster galaxies represent the evolutionary endpoints of those at high redshift, and if the size-luminosity relation is not modified by dynamical processes then this population of galaxies has undergone significant luminosity evolution since z=1 consistent with expectations based on models of passively evolving, old stellar populations.