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1.
We present an analysis of 20 galaxy clusters observed with the Chandra X-ray satellite, focusing on the temperature structure of the intracluster medium and the cooling time of the gas. Our sample is drawn from a flux-limited catalogue but excludes the Fornax, Coma and Centaurus clusters, owing to their large angular size compared to the Chandra field of view. We describe a quantitative measure of the impact of central cooling, and find that the sample comprises nine clusters possessing cool cores (CCs) and 11 without. The properties of these two types differ markedly, but there is a high degree of uniformity amongst the CC clusters, which obey a nearly universal radial scaling in temperature of the form   T ∝ r ∼0.4  , within the core. This uniformity persists in the gas cooling time, which varies more strongly with radius in CC clusters  ( t cool∝ r ∼1.3)  , reaching   t cool < 1 Gyr  in all cases, although surprisingly low central cooling times (<5 Gyr) are found in many of the non-CC systems. The scatter between the cooling time profiles of all the clusters is found to be remarkably small, implying a universal form for the cooling time of gas at a given physical radius in virialized systems, in agreement with recent previous work. Our results favour cluster merging as the primary factor in preventing the formation of CCs.  相似文献   

2.
Mayall II = G1 is one of the most luminous globular clusters (GCs) known in M31. New deep, high-resolution observations with the Advanced Camera for Surveys on the Hubble Space Telescope are used to provide accurate photometric data to the smallest radii yet. In particular, we present the precise variation of ellipticity and position angle, and of surface brightness for the core of the object. Based on these accurate photometric data, we redetermine the structural parameters of G1 by fitting a single-mass isotropic King model. We derive a core radius,   r c= 0.21 ± 0.01  arcsec (= 0.78 ± 0.04  pc)  , a tidal radius,   r t= 21.8 ± 1.1  arcsec (= 80.7 ± 3.9  pc)  , and a concentration index   c = log ( r t/ r c) = 2.01 ± 0.02  . The central surface brightness is 13.510 mag arcsec−2. We also calculate the half-light radius, at   r h= 1.73 ± 0.07  arcsec (= 6.5 ± 0.3  pc)  . The results show that, within 10 core radii, a King model fits the surface brightness distribution well. We find that this object falls in the same region of the   MV   versus  log   R h  diagram as ω Centauri, M54 and NGC 2419 in the Milky Way. All three of these objects have been claimed to be the stripped cores of now defunct dwarf galaxies. We discuss in detail whether GCs, stripped cores of dwarf spheroidals and normal dwarf galaxies form a continuous distribution in the   MV   versus  log   R h  plane, or if GCs and dwarf spheroidals constitute distinct classes of objects; we present arguments in favour of this latter view.  相似文献   

3.
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4.
We study the distribution of projected offsets between the cluster X-ray centroid and the brightest cluster galaxy (BCG) for 65 X-ray-selected clusters from the Local Cluster Substructure Survey, with a median redshift of   z = 0.23  . We find a clear correlation between X-ray/BCG projected offset and the logarithmic slope of the cluster gas density profile at  0.04 r 500(α  ), implying that more dynamically disturbed clusters have weaker cool cores. Furthermore, there is a close correspondence between the activity of the BCG, in terms of detected Hα and radio emission, and the X-ray/BCG offset, with the line-emitting galaxies all residing in clusters with X-ray/BCG offsets of ≤15 kpc. Of the BCGs with  α < −0.85  and an offset <0.02 r 500, 96 per cent (23/24) have optical emission and 88 per cent (21/24) are radio active, while none has optical emission outside these criteria. We also study the cluster gas fraction ( f gas) within r 500 and find a significant correlation with X-ray/BCG projected offset. The mean f gas of the 'small offset' clusters (<0.02 r 500) is  0.106 ± 0.005 (σ= 0.03  ) compared to  0.145 ± 0.009 (σ= 0.04  ) for those with an offset >0.02 r 500, indicating that the total mass may be systematically underestimated in clusters with larger X-ray/BCG offsets. Our results imply a link between cool core strength and cluster dynamical state consistent with the view that cluster mergers can significantly perturb cool cores, and set new constraints on models of the evolution of the intracluster medium.  相似文献   

5.
The Fornax cluster galaxies NGC 1399 and NGC 1404 are ideal for studying the effects of a cluster environment on globular cluster systems. Here we present new optical imaging of these two galaxies from both the Hubble Space Telescope 's Wide Field and Planetary Camera 2 and the Cerro Tololo Inter-American Observatory 1.5-m telescope. The combination of both data sets provides a unique insight on the spatial and colour distribution of globular clusters. From B − I colours, we find that both galaxies have a broad globular cluster metallicity distribution that is inconsistent with a single population. Two Gaussians provide a reasonable representation of the metallicity distribution in each galaxy. The metal-rich subpopulation is more centrally concentrated than the metal-poor one. We show that the radial metallicity gradient can be explained by the changing relative mix of the two globular cluster subpopulations. We derive globular cluster surface density profiles, and find that they are flatter (i.e., more extended) than the underlying starlight. The total number of globular clusters and specific frequency are calculated to be N =5700±500, SN =11.5±1.0 for NGC 1399, and N =725±145, SN =2.0±0.5 for NGC 1404. Our results are compared with the expectations of globular cluster formation scenarios.  相似文献   

6.
The universal baryonic mass fraction  (Ωb/Ωm)  can be sensitively constrained using X-ray observations of galaxy clusters. In this paper, we compare the baryonic mass fraction inferred from measurements of the cosmic microwave background with the gas mass fractions ( f gas) of a large sample of clusters taken from the recent literature. In systems cooler than 4 keV, f gas declines as the system temperature decreases. However, in higher temperature systems, f gas( r 500) converges to  ≈(0.12 ± 0.02)( h /0.72)−1.5  , where the uncertainty reflects the systematic variations between clusters at r 500. This is significantly lower than the maximum-likelihood value of the baryon fraction from the recently released Wilkinson Microwave Anisotropy Probe ( WMAP ) 3-yr results. We investigate possible reasons for this discrepancy, including the effects of radiative cooling and non-gravitational heating, and conclude that the most likely solution is that Ωm is higher than the best-fitting WMAP value (we find  Ωm= 0.36+0.11−0.08  ), but consistent at the 2σ level. Degeneracies within the WMAP data require that σ8 must also be greater than the maximum likelihood value for consistency between the data sets.  相似文献   

7.
The X-ray properties of a sample of 11 high-redshift  (0.6 < z < 1.0)  clusters observed with Chandra and/or XMM–Newton are used to investigate the evolution of the cluster scaling relations. The observed evolution in the normalization of the   L – T , M – T , M g– T   and M – L relations is consistent with simple self-similar predictions, in which the properties of clusters reflect the properties of the Universe at their redshift of observation. Under the assumption that the model of self-similar evolution is correct and that the local systems formed via a single spherical collapse, the high-redshift L – T relation is consistent with the high- z clusters having virialized at a significantly higher redshift than the local systems. The data are also consistent with the more realistic scenario of clusters forming via the continuous accretion of material.
The slope of the L – T relation at high redshift  ( B = 3.32 ± 0.37)  is consistent with the local relation, and significantly steeper than the self-similar prediction of   B = 2  . This suggests that the same non-gravitational processes are responsible for steepening the local and high- z relations, possibly occurring universally at   z ≳ 1  or in the early stages of the cluster formation, prior to their observation.
The properties of the intracluster medium at high redshift are found to be similar to those in the local Universe. The mean surface-brightness profile slope for the sample is  β= 0.66 ± 0.05  , the mean gas mass fractions within   R 2500( z )  and   R 200( z )  are  0.069 ± 0.012  and  0.11 ± 0.02  , respectively, and the mean metallicity of the sample is  0.28 ± 0.11 Z⊙  .  相似文献   

8.
We present gas temperature, density, entropy and cooling time profiles for the cores of a sample of 15 galaxy groups observed with Chandra . We find that the entropy profiles follow a power-law profile down to very small fractions of R 500. Differences between the gas profiles of groups with radio-loud and radio-quiet brightest group galaxies are only marginally significant, and there is only a small difference in the   L X: T X  relations, for the central regions we study with Chandra , between the radio-loud and radio-quiet objects in our sample, in contrast to the much larger difference found on scales of the whole group in earlier work. However, there is evidence, from splitting the sample based on the mass of the central black holes, that repeated outbursts of active galactic nuclei (AGN) activity may have a long-term cumulative effect on the entropy profiles. We argue that, to first order, energy injection from radio sources does not change the global structure of the gas in the cores of groups, although it can displace gas on a local level. In most systems, it appears that AGN energy injection serves primarily to counter the effects of radiative cooling, rather than being responsible for the similarity breaking between groups and clusters.  相似文献   

9.
Recent observations show that the cooling flows in the central regions of galaxy clusters are highly suppressed. Observed active galactic nuclei (AGN)-induced cavities/bubbles are a leading candidate for suppressing cooling, usually via some form of mechanical heating. At the same time, observed X-ray cavities and synchrotron emission point towards a significant non-thermal particle population. Previous studies have focused on the dynamical effects of cosmic ray pressure support, but none has built successful models in which cosmic ray heating is significant. Here, we investigate a new model of AGN heating, in which the intracluster medium is efficiently heated by cosmic rays, which are injected into the intra-cluster medium (ICM) through diffusion or the shredding of the bubbles by Rayleigh–Taylor or Kelvin–Helmholtz instabilities. We include thermal conduction as well. Using numerical simulations, we show that the cooling catastrophe is efficiently suppressed. The cluster quickly relaxes to a quasi-equilibrium state with a highly reduced accretion rate and temperature and density profiles which match observations. Unlike the conduction-only case, no fine-tuning of the Spitzer conduction suppression factor f is needed. The cosmic ray pressure, P c/ P g ≲ 0.1 and ∇ P c≲ 0.1ρ g , is well within observational bounds. Cosmic ray heating is a very attractive alternative to mechanical heating, and may become particularly compelling if Gamma-ray Large Array Space Telescope ( GLAST ) detects the γ-ray signature of cosmic rays in clusters.  相似文献   

10.
We analyse the evolutionary history of galaxies formed in a hierarchical scenario consistent with the concordance Lambda cold dark matter (ΛCDM) model focusing on the study of the relation between their chemical and dynamical properties. Our simulations consistently describe the formation of the structure and its chemical enrichment within a cosmological context. Our results indicate that the luminosity–metallicity and the stellar mass–metallicity (LZR and MZR) relations are naturally generated in a hierarchical scenario. Both relations are found to evolve with redshift. In the case of the MZR, the estimated evolution is weaker than that deduced from observational works by approximately 0.10 dex. We also determine a characteristic stellar mass, M c≈ 3 × 1010 M⊙, which segregates the simulated galaxy population into two distinctive groups and which remains unchanged since z ∼ 3, with a very weak evolution of its metallicity content. The value and role played by M c is consistent with the characteristic mass estimated from the SDSS galaxy survey by Kauffmann et al. Our findings suggest that systems with stellar masses smaller than M c are responsible for the evolution of this relation at least from z ≈ 3. Larger systems are stellar dominated and have formed more than 50 per cent of their stars at   z ≥ 2  , showing very weak evolution since this epoch. We also found bimodal metallicity and age distributions from z ∼ 3, which reflects the existence of two different galaxy populations. Although SN feedback may affect the properties of galaxies and help to shape the MZR, it is unlikely that it will significantly modify M c since, from   z = 3  this stellar mass is found in systems with circular velocities larger than 100 km s−1.  相似文献   

11.
We have constructed an analytical model of active galactic nuclei (AGN) feedback and studied its implications for elliptical galaxies and galaxy clusters. The results show that momentum injection above a critical value will eject material from low-mass elliptical galaxies, and leads to an X-ray luminosity, L X, that is  ∝σ8−10  , depending on the AGN fuelling mechanism, where σ is the velocity dispersion of the hot gas. This result agrees well with both observations and semi-analytic models. In more massive ellipticals and clusters, AGN outflows quickly become buoyancy dominated. This necessarily means that heating by a central cluster AGN redistributes the intracluster medium (ICM) such that the mass of hot gas, within the cooling radius, should be  ∝ L X(< r cool)/[ g ( r cool)σ]  , where   g ( r cool)  is the gravitational acceleration at the cooling radius. This prediction is confirmed using observations of seven clusters. The same mechanism also defines a critical ICM cooling time of  ∼0.5 Gyr  , which is in reasonable agreement with recent observations showing that star formation and AGN activity are triggered below a universal cooling time threshold.  相似文献   

12.
We present an extensive study of the double β model for the X-ray surface brightness profiles of clusters, and derive analytically the gas density and total masses of clusters under the hydrostatic equilibrium hypothesis. It is shown that the employment of the double β model instead of the conventional single β model can significantly improve the goodness-of-fit to the observed X-ray surface brightness profiles of clusters, which will in turn lead to a better determination of the gas and total mass distributions in clusters. In particular, the observationally fitted β parameter for the extended component in a double β model may become larger. This opens a new possibility of resolving the long-standing β discrepancy for clusters. Using an ensemble of 33 ROSAT PSPC observed clusters drawn from the Mohr, Mathiesen & Evrard sample, we find that the asymptotic value of β fit is 0.83±0.33 at large radii, consistent with both the average spectroscopic parameter β spec=0.78±0.37 and the result given by numerical simulations.  相似文献   

13.
We present the result of a photometric and Keck low-resolution imaging spectrometer (LRIS) spectroscopic study of dwarf galaxies in the core of the Perseus Cluster, down to a magnitude of   M B =−12.5  . Spectra were obtained for 23 dwarf-galaxy candidates, from which we measure radial velocities and stellar population characteristics from absorption line indices. From radial velocities obtained using these spectra, we confirm 12 systems as cluster members, with the remaining 11 as non-members. Using these newly confirmed cluster members, we are able to extend the confirmed colour–magnitude relation for the Perseus Cluster down to   M B =−12.5  . We confirm an increase in the scatter about the colour–magnitude relationship below   M B =−15.5  , but reject the hypothesis that very red dwarfs are cluster members. We measure the faint-end slope of the luminosity function between   M B =−18  and −12.5, finding  α=−1.26 ± 0.06  , which is similar to that of the field. This implies that an overabundance of dwarf galaxies does not exist in the core of the Perseus Cluster. By comparing metal and Balmer absorption line indices with α-enhanced single stellar population models, we derive ages and metallicities for these newly confirmed cluster members. We find two distinct dwarf elliptical populations: an old, metal-poor population with ages ∼8 Gyr and metallicities  [Fe/H] < −0.33  , and a young, metal-rich population with ages <5 Gyr and metallicities  [Fe/H] > −0.33  . Dwarf galaxies in the Perseus Cluster are therefore not a simple homogeneous population, but rather exhibit a range in age and metallicity.  相似文献   

14.
We discuss the optical properties, X-ray detections and active galactic nucleus (AGN) populations of four clusters at   z ∼ 1  in the Subaru–XMM Deep Field (SXDF). The velocity distribution and plausible extended X-ray detections are examined, as well as the number of X-ray point sources and radio sources associated with the clusters. We find that the two clusters that appear virialized and have an extended X-ray detection contain few, if any, AGN, whereas the two pre-virialized clusters have a large AGN population. This constitutes evidence that the AGN fraction in clusters is linked to the clusters' evolutionary stage. The number of X-ray AGN in the pre-virialized clusters is consistent with an overdensity of a factor of ∼200; the radio AGN appear to be clustered with a factor of 3 to 6 higher. The median K -band luminosities of   LK = 1.7 ± 0.7 L *  for the X-ray sources and   LK = 2.3 ± 0.1 L *  for the radio sources support the theory that these AGN are triggered by galaxy interaction and merging events in sub-groups with low internal velocity distributions, which make up the cluster environment in a pre-virialization evolutionary stage.  相似文献   

15.
We carry out a comprehensive joint analysis of high-quality HST /ACS and Chandra measurements of A1689, from which we derive mass, temperature, X-ray emission and abundance profiles. The X-ray emission is smooth and symmetric, and the lensing mass is centrally concentrated indicating a relaxed cluster. Assuming hydrostatic equilibrium we deduce a 3D mass profile that agrees simultaneously with both the lensing and X-ray measurements. However, the projected temperature profile predicted with this 3D mass profile exceeds the observed temperature by ∼30 per cent at all radii, a level of discrepancy comparable to the level found for other relaxed clusters. This result may support recent suggestions from hydrodynamical simulations that denser, more X-ray luminous small-scale structure can bias observed temperature measurements downward at about the same (∼30 per cent) level. We determine the gas entropy at  0.1 r vir  (where r vir is the virial radius) to be ∼800 keV cm2, as expected for a high-temperature cluster, but its profile at  >0.1 r vir  has a power-law form with index ∼0.8, considerably shallower than the ∼1.1 index advocated by theoretical studies and simulations. Moreover, if a constant entropy 'floor' exists at all, then it is within a small region in the inner core,   r < 0.02 r vir  , in accord with previous theoretical studies of massive clusters.  相似文献   

16.
We have derived ages and metallicities from co-added spectra of 131 globular clusters associated with the giant elliptical galaxy NGC 4472. Based upon a calibration with Galactic globular clusters, we find that our sample of globular clusters in NGC 4472 span a metallicity range of approximately −1.6≤[Fe/H]≤0 dex. There is evidence of a radial metallicity gradient in the globular cluster system which is steeper than that seen in the underlying starlight. Determination of the absolute ages of the globular clusters is uncertain, but formally, the metal-poor population of globular clusters has an age of 14.5±4 Gyr and the metal-rich population is 13.8±6 Gyr old. Monte Carlo simulations indicate that the globular cluster populations present in these data are older than 6 Gyr at the 95 per cent confidence level. We find that within the uncertainties, the globular clusters are old and coeval, implying that the bimodality seen in the broadband colours primarily reflects metallicity and not age differences.  相似文献   

17.
We revisit the issue of the recent dynamical evolution of clusters of galaxies using a sample of Abell, Corwin & Olowin (ACO) clusters with   z < 0.14  , which has been selected such that it does not contain clusters with multiple velocity components nor strongly merging or interacting clusters, as revealed in X-rays. We use as proxies of the cluster dynamical state the projected cluster ellipticity, velocity dispersion and X-ray luminosity. We find indications for a recent dynamical evolution of this cluster population, which however strongly depends on the cluster richness. Poor clusters appear to be undergoing their primary phase of virialization, with their ellipticity increasing with redshift with a rate  dε/d z ≃ 2.5 ± 0.4  , while the richest clusters show an ellipticity evolution in the opposite direction (with  dε/d z ≃−1.2 ± 0.1  ), which could be due to secondary infall. When taking into account sampling effects due to the magnitude-limited nature of the ACO cluster catalogue we find no significant evolution of the cluster X-ray luminosity, while the velocity dispersion increases with decreasing redshift, independent of the cluster richness, at a rate  dσ v /d z ≃−1700 ± 400 km s−1  .  相似文献   

18.
We explore the relationship between the metallicity of the intracluster gas in clusters of galaxies, determined by X-ray spectroscopy, and the presence of cooling flows. Using ASCA spectra and ROSAT images, we demonstrate a clear segregation between the metallicities of clusters with and without cooling flows. On average, cooling-flow clusters have an emission-weighted metallicity a factor ∼ 1.8 times higher than that of non-cooling-flow systems. We suggest that this is caused by the presence of metallicity gradients in the cooling-flow clusters, coupled with the sharply peaked X-ray surface brightness profiles of these systems. Non-cooling-flow clusters have much flatter X-ray surface brightness distributions and are thought to have undergone recent merger events, which may have mixed the central high-metallicity gas with the surrounding less metal-rich material. We find no evidence for evolution in the emission-weighted metallicities of clusters within z  ∼ 0.3.  相似文献   

19.
Comparing the gravitational acceleration induced on the Local Group of galaxies by different tracers of the underlying density field we estimate, within the linear gravitational instability theory and the linear biasing ansatz, their relative bias factors. Using optical SSRS2 galaxies, IRAS (PSC z ) galaxies and Abell/ACO clusters, we find b O,I≈1.21±0.06 and b C,I≈4.3±0.8, in agreement with other recent studies. Finally, there is an excellent one-to-one correspondence of the PSC z and Abell/ACO cluster dipole profiles, once the latter is rescaled by b C,I, out to at least ∼150  h −1 Mpc.  相似文献   

20.
Cold collapse of a cluster composed of small identical clumps, each of which is in virial equilibrium, is considered. Since the clumps have no relative motion with respect to each other initially, the cluster collapses under its own gravity. At the first collapse of the cluster, most of the clumps are destroyed, but some survive. In order to find the condition for the clumps to survive, we made a systematic study in two-parameter space: the number of the clumps N c and the size of the clump r v . We obtained the condition N c ≫ 1 and n k  ≥ 1, where n k is related to r v and the initial radius of the cluster R ini through the relation R ini/ r v  = 2 N ( n k +5)/6c. A simple analytical argument supports the numerical result. This n k corresponds to the index of the power spectrum of the density fluctuation in the cosmological hierarchical clustering, and thus our result may suggest that in the systems smaller than 2/Ω h 2)Mpc, the first violent collapse is strong enough to sweep away all the substructures that exist before the collapse.  相似文献   

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