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 0.04r500(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 <0.02r500, 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 (fgas) within r500 and find a significant correlation with X-ray/BCG projected offset. The mean fgas of the ‘small offset’ clusters (<0.02r500) is 0.106 ± 0.005 (σ= 0.03) compared to 0.145 ± 0.009 (σ= 0.04) for those with an offset >0.02r500, 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.
We study the mid-infrared (MIR) properties of galaxies in 30 massive galaxy clusters at 0.02 <= z <= 0.40, using panoramic Spitzer/MIPS 24 Delta *mm and near-infrared data, including 27 new ...observations from the LoCuSS and ACCESS surveys. This is the largest sample of clusters to date with such high-quality and uniform MIR data covering not only the cluster cores, but extending into the infall regions. We use these data to revisit the so-called Butcher-Oemler (BO) effect, measuring the fraction of massive infrared luminous galaxies (K < K + 1.5, L IR > 5 X 1010 L ) within r 200, finding a steady increase in the fraction with redshift from ~3% at z = 0.02 to ~10% by z = 0.30, and an rms cluster-to-cluster scatter about this trend of 0.03. The best-fit redshift evolution model of the form f SF (1 + z) n has n = 5.7+2.1 -1.8, which is stronger redshift evolution than that of L IR in both clusters and the field. We find that, statistically, this excess is associated with galaxies found at large cluster-centric radii, specifically r 500 < r < r 200, implying that the MIR BO effect can be explained by a combination of both the global decline in star formation in the universe since z ~ 1 and enhanced star formation in the infall regions of clusters at intermediate redshifts. This picture is supported by a simple infall model based on the Millennium Simulation semianalytic galaxy catalogs, whereby star formation in infalling galaxies is instantaneously quenched upon their first passage through the cluster, in that the observed radial trends of f SF trace those inferred from the simulations. The observed f SF values, however, lie systematically above the predictions, suggesting an overall excess of star formation, either due to triggering by environmental processes, or a gradual quenching. We also find that f SF does not depend on simple indicators of the dynamical state of clusters, including the offset between the brightest cluster galaxy and the peak of the X-ray emission. This is consistent with the picture described above in that most new star formation in clusters occurs in the infall regions, and is thus not sensitive to the details of cluster-cluster mergers in the core regions.
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 (tcool∝r∼1.3), reaching tcool < 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.
We investigate the thermodynamic and chemical structure of the intracluster medium (ICM) across a statistical sample of 20 galaxy clusters analysed with the Chandra X-ray satellite. In particular, we ...focus on the scaling properties of the gas density, metallicity and entropy and the comparison between clusters with and without cool cores (CCs). We find marked differences between the two categories except for the gas metallicity, which declines strongly with radius for all clusters (Z∝r−0.31), outside ∼0.02r500. The scaling of gas entropy is non-self-similar and we find clear evidence of bimodality in the distribution of logarithmic slopes of the entropy profiles. With only one exception, the steeper sloped entropy profiles are found in CC clusters whereas the flatter slope population are all non-CC clusters. We explore the role of thermal conduction in stabilizing the ICM and conclude that this mechanism alone is sufficient to balance cooling in non-CC clusters. However, CC clusters appear to form a distinct population in which heating from feedback is required in addition to conduction. Under the assumption that non-CC clusters are thermally stabilized by conduction alone, we find the distribution of Spitzer conduction suppression factors, fc, to be lognormal, with a log (base 10) mean of −1.50 ± 0.03 (i.e. fc= 0.032) and log standard deviation 0.39 ± 0.02.
We present a study of the distribution of X-ray active galactic nuclei (AGNs) in a representative sample of 26 massive clusters at 0.15 < z < 0.30, combining Chandra observations sensitive to X-ray ...point sources of luminosity L sub(X) ~ 10 super(42) erg s super(-1) at the cluster redshift with extensive and highly complete spectroscopy of cluster members down to ~M* sub(K)+2. In the stacked caustic diagram that shows (v sub(los) - )/sigma sub(v) versus r sub(proj)/r sub(500), the X-ray AGN appear to preferentially lie along the caustics, suggestive of an infalling population. Overall, our results provide the strongest observational evidence to date that X-ray AGNs found in massive clusters are an infalling population, and that the cluster environment very effectively suppresses radiatively efficient nuclear activity in its member galaxies. These results are consistent with the view that for galaxies to host an X-ray AGN they should be the central galaxy within their dark matter halo and have a ready supply of cold gas.
We present an analysis of the levels and evolution of star formation activity in a representative sample of 30 massive galaxy clusters at 0.15 < z < 0.30 from the Local Cluster Substructure Survey, ...combining wide-field Spitzer/MIPS 24 mum data with extensive spectroscopy of cluster members. The specific SFRs of massive (M > ~ 10 super(10) M sub(middot in circle)) star-forming cluster galaxies within r sub(200) are found to be systematically ~28% lower than their counterparts in the field at fixed stellar mass and redshift, a difference significant at the 8.7sigma level. This is the unambiguous signature of star formation in most (and possibly all) massive star-forming galaxies being slowly quenched upon accretion into massive clusters, their star formation rates (SFRs) declining exponentially on quenching timescales in the range 0.7-2.0 Gyr. We measure the mid-infrared Butcher-Oemler effect over the redshift range 0.0-0.4, finding rapid evolution in the fraction (functionof sub(SF)) of massive (M sub(K) < - 23.1) cluster galaxies within r sub(200) with SFRs > 3 M sub(middot in circle) yr super(-1), of the form functionof sub(SF) alpha (1 + z) super(7.6 + or - 1.1). We dissect the origins of the Butcher-Oemler effect, revealing it to be due to the combination of a ~3 x decline in the mean specific SFRs of star-forming cluster galaxies since z ~ 0.3 with a ~1.5 x decrease in number density. Two-thirds of this reduction in the specific SFRs of star-forming cluster galaxies is due to the steady cosmic decline in the specific SFRs among those field galaxies accreted into the clusters. The remaining one-third reflects an accelerated decline in the star formation activity of galaxies within clusters. The slow quenching of star formation in cluster galaxies is consistent with a gradual shut down of star formation in infalling spiral galaxies as they interact with the intracluster medium via ram-pressure stripping or starvation mechanisms. The observed sharp decline in star formation activity among cluster galaxies since z ~ 0.4 likely reflects the increased susceptibility of low-redshift spiral galaxies to gas removal mechanisms as their gas surface densities decrease with time. We find no evidence for the build-up of cluster S0 bulges via major nuclear starburst episodes.
We present a statistical analysis of a sample of 20 strong lensing clusters drawn from the Local Cluster Substructure Survey, based on high-resolution Hubble Space Telescope imaging of the cluster ...cores and follow-up spectroscopic observations using the Keck-I telescope. We use detailed parametrized models of the mass distribution in the cluster cores, to measure the total cluster mass and fraction of that mass associated with substructures within R≤ 250 kpc. These measurements are compared with the distribution of baryons in the cores, as traced by the old stellar populations and the X-ray emitting intracluster medium. Our main results include: (i) the distribution of Einstein radii is lognormal, with a peak and 1σ width of 〈log10θE(z= 2)〉= 1.16 ± 0.28; (ii) we detect an X-ray/lensing mass discrepancy of 〈MSL/MX〉= 1.3 at 3σ significance – clusters with larger substructure fractions displaying greater mass discrepancies, and thus greater departures from hydrostatic equilibrium and (iii) cluster substructure fraction is also correlated with the slope of the gas density profile on small scales, implying a connection between cluster–cluster mergers and gas cooling. Overall our results are consistent with the view that cluster–cluster mergers play a prominent role in shaping the properties of cluster cores, in particular causing departures from hydrostatic equilibrium, and possibly disturbing cool cores. Our results do not support recent claims that large Einstein radius clusters present a challenge to the cold dark matter paradigm.
The baryon budget on the galaxy group/cluster boundary Sanderson, Alastair J. R; O'Sullivan, Ewan; Ponman, Trevor J ...
Monthly notices of the Royal Astronomical Society,
03/2013, Letnik:
429, Številka:
4
Journal Article
Recenzirano
Odprti dostop
We present a study of the hot gas and stellar content of five optically selected poor galaxy clusters, including a full accounting of the contribution from intracluster light (ICL) and a combined hot ...gas and hydrostatic X-ray mass analysis with XMM-Newton observations. We find weighted mean stellar (including ICL), gas and total baryon mass fractions within r
500 of 0.026 ± 0.003, 0.070 ± 0.005 and 0.096 ± 0.006, respectively, at a corresponding weighted mean M
500 of (1.08+ 0.21
− 0.18) × 1014 M. Even when accounting for the intracluster stars, four out of five clusters show evidence for a substantial baryon deficit within r
500, with baryon fractions (f
b) between 50 ± 6 and 59 ± 8 per cent of the universal mean level (i.e. Ωb/Ωm), the remaining cluster having f
b = 75±11 per cent. For the three clusters where we can trace the hot halo to r
500 we find no evidence for a steepening of the gas density profile in the outskirts with respect to a power law, as seen in more massive clusters. We find that in all cases, the X-ray mass measurements are larger than those originally published on the basis of the galaxy velocity dispersion (σ) and an assumed σ-M
500 relation, by a factor of 1.7-5.7. Despite these increased masses, the stellar fractions (in the range 0.016-0.034, within r
500) remain consistent with the trend with mass published by Gonzalez et al., from which our sample is drawn.
We present a study of the structural and scaling properties of the gas distributions in the intracluster medium (ICM) of 31 nearby ($z < 0.2$) clusters observed with XMM-Newton, which together ...comprise the Representative XMM-Newton Cluster Structure Survey (REXCESS). In contrast to previous studies, this sample is unbiased with respect to X-ray surface brightness and cluster dynamical state, and it fully samples the cluster X-ray luminosity function. The clusters cover a temperature range of 2.0-8.5 keV and possess a variety of morphologies. The sampling strategy allows us to compare clusters with a wide range of central cooling times on an equal footing. We applied a recently developed technique for the deprojection and PSF-deconvolution of X-ray surface brightness profiles to obtain non-parametric gas-density profiles out to distances ranging between $0.8~R_{500}$ and $1.5~R_{500}$. We scaled the gas density distributions to allow for the systems' differing masses and redshifts. The central gas densities differ greatly from system to system, with no clear correlation with system temperature. At intermediate radii (~$0.3~R_{500}$), the scaled density profiles show much less scatter, with a clear dependence on system temperature. We find that the density at this radius scales proportionally to the square root of temperature, consistent with the presence of an entropy excess as suggested in previous literature. However, at larger scaled radii this dependence becomes weaker: clusters with $kT > 3$ keV scale self-similarly, with no temperature dependence of gas-density normalisation. The REXCESS sample allows us to investigate the correlations between cluster properties and dynamical state. We find no evidence of correlations between cluster dynamical state and either the gas density slope in the inner regions or temperature, but do find some evidence of a correlation between dynamical state and outer gas density slope. We also find a weak correlation between dynamical state and both central gas normalisation and inner cooling times, but this is only significant at the 10% level. We conclude that, for the X-ray cluster population as a whole, both the central gas properties and the angle-averaged, large-scale gas properties are linked to the cluster dynamical state. We also investigate the central cooling times of the clusters. While the cooling times span a wide range, we find no evidence of a significant bimodality in the distributions of central density, density gradient, or cooling time. Finally, we present the gas mass-temperature relation for the REXCESS sample, finding that $h(z)M_{\rm gas} \propto T^{1.99\pm0.11}$, which is consistent with the expectation of self-similar scaling modified by the presence of an entropy excess in the inner regions of the cluster and consistent with earlier work on relaxed cluster samples. We measure a logarithmic intrinsic scatter in this relation of ~$10\%$, which should be a good measure of the intrinsic scatter in the Mgas-T relation for the cluster population as a whole.
Carbon films were prepared by DC magnetron sputtering at various argon pressures ranging from 1 to 30mTorr. The film sputtered at the lowest pressure (1mTorr) was fully amorphous, showing no evidence ...of the (002) Bragg diffraction peak, and a density of 1.9±0.3g/cc, indicating little, if any, porosity. The film sputtered at the highest pressure (30mTorr) showed a weak and broad (002) Bragg peak and had a density of 1.35±0.15g/cc, indicating significant porosity. The low pressure sputtered films were almost pure carbon (<1% O by weight) while the high pressure sputtered carbon films contained between 13% and 21% O by weight (depending on the measurement method) which was presumably incorporated when the porous films were exposed to air after sputtering. Electrochemical testing in Li/carbon film cells showed that the low pressure sputtered carbon had a reversible specific capacity of about 800mAh/g between 5mV and 2V, an irreversible capacity of 200mAh/g and an average delithiation potential of about 1V vs. Li/Li+. Heating the low pressure sputtered carbon to 900°C in argon decreased the reversible capacity, the irreversible capacity and the average voltage to 600mAh/g, 100mAh/g and 0.75V, respectively. Since the density of these films is almost as large as graphite, these unheated materials have volumetric capacities near 1600mAh/cm3 while the heated materials are near 1200mAh/cm3.
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