The UV-upturn in brightest cluster galaxies

This paper is part of a series devoted to the investigation of a large sample of brightest cluster galaxies (BCGs), their properties and the relationships between these and the properties of the host clusters. In this paper, we compare the stellar population properties derived from high signal-to-noise, optical long-slit spectra with the GALEX ultraviolet (UV) colour measurements for 36 nearby BCGs to understand the diversity in the most rapidly evolving feature in old stellar systems, the UV-upturn. We investigate: (1) the possible differences between the UV-upturn of BCGs and those of a control sample of ordinary ellipticals in the same mass range, as well as possible correlations between the UV-upturn and other general properties of the galaxies; (2) possible correlations between the UV-upturn and the properties of the host clusters; (3) recently proposed scenarios where helium-sedimentation in the cluster centre can produce an enhanced UV-upturn. We find systematic differences between the UV-colours of BCGs and ordinary ellipticals, but we do not find correlations between these colours and the properties of the host clusters. Furthermore, the observations do not support the predictions made by the helium-sedimentation model as an enhancer of the UV-upturn.Comment: 12 pages, accepted for publication in MNRA

Cluster of galaxies around seven radio-loud QSOs at 1<z<1.6: K-band images

We have conducted a NIR study of the environments of seven radio-loud quasars at redshifts 1<z<1.6. In present paper we describe deep $K$ band images obtained for the fields of ~6X6 arcmin around the quasars with 3$\sigma$ limiting magnitudes of K~20.5. These fields were previously studied using deep B and R band images (Sanchez & Gonzalez-Serrano 1999). Using together optical and NIR data, it has been found a significant excess of galaxies which optical-NIR colours, luminosity, spatial scale, and number of galaxies are compatible with clusters at the redshift of the quasar. We have selected a sample of cluster candidates analyzing the R-K vs. K diagram. A ~25% of the candidates present red optical-NIR colours and an ultraviolet excess. This population has been also found in clusters around quasars at the same redshifts (Tanaka et al. 2000; Haines et al. 2001). These galaxies seem to follow a mixed evolution: a main passive evolution plus late starformation processes. The quasars do not inhabit the core of the clusters, being found in the outer regions. This result agrees with the hypothesis that the origin/feeding mechanism of the nuclear activity were merging processes. The quasars inhabit the region were a collision is most probably to produce a merger.Comment: 15 pages. A&A, accepted for publishin

Constraints on a possible variation of the fine structure constant from galaxy cluster data

We propose a new method to probe a possible time evolution of the fine structure constant $\alpha$ from X-ray and Sunyaev-Zeldovich measurements of the gas mass fraction ($f_{gas}$) in galaxy clusters. Taking into account a direct relation between variations of $\alpha$ and violations of the distance-duality relation, we discuss constraints on $\alpha$ for a class of dilaton runaway models. Although not yet competitive with bounds from high-$z$ quasar absorption systems, our constraints, considering a sample of 29 measurements of $f_{gas}$, in the redshift interval $0.14 < z < 0.89$, provide an independent estimate of $\alpha$ variation at low and intermediate redshifts. Furthermore, current and planned surveys will provide a larger amount of data and thus allow to improve the limits on $\alpha$ variation obtained in the present analysis.Comment: 7 pages, 2 figures, accepted for publication in JCA

Social Support, Social and Temporal Comparisons Protect Well-Being and Health between 45 and 70 Years Old in Four Urban Communities

This study examined the impact of social support and of temporal and social comparisons on well-being and selfreported health in four capital cities: Paris, Berlin, Moscow and Beijing. Based on the lifespan control theory, an integrative model investigating the influence of these coping strategies, especially on the psychological regulation of health losses, was tested on 1141 respondents aged 45 to 70 years by using structural equation modelling with multigroup comparisons. Results indicated a good fit of the model to participants' responses. In all contexts, physical weaknesses favoured the use of social and temporal comparison strategies rather than social support. Moreover, across the cities, coping strategies were oriented more toward protecting self-evaluation of health than toward enhancement of well-being. Social comparison decreased the impact of physical weaknesses on health evaluation and on well-being in the four cities, but to a lesser extent in China. Results are discussed regarding the normative cross-cultural aspects that intervene during ageing in the four urban contexts

Cosmological evolution of warm dark matter fluctuations II: Solution from small to large scales and keV sterile neutrinos

We solve the cosmological evolution of warm dark matter (WDM) density fluctuations with the Volterra integral equations of paper I. In the absence of neutrinos, the anisotropic stress vanishes and the Volterra equations reduce to a single integral equation. We solve numerically this equation both for DM fermions decoupling at equilibrium and DM sterile neutrinos decoupling out of equilibrium. We give the exact analytic solution for the density fluctuations and gravitational potential at zero wavenumber. We compute the density contrast as a function of the scale factor a for a wide range of wavenumbers k. At fixed a, the density contrast grows with k for k k_c, (k_c ~ 1.6/Mpc). The density contrast depends on k and a mainly through the product k a exhibiting a self-similar behavior. Our numerical density contrast for small k gently approaches our analytic solution for k = 0. For fixed k < 1/(60 kpc), the density contrast generically grows with a while for k > 1/(60 kpc) it exhibits oscillations since the RD era which become stronger as k grows. We compute the transfer function of the density contrast for thermal fermions and for sterile neutrinos in: a) the Dodelson-Widrow (DW) model and b) in a model with sterile neutrinos produced by a scalar particle decay. The transfer function grows with k for small k and then decreases after reaching a maximum at k = k_c reflecting the time evolution of the density contrast. The integral kernels in the Volterra equations are nonlocal in time and their falloff determine the memory of the past evolution since decoupling. This falloff is faster when DM decouples at equilibrium than when it decouples out of equilibrium. Although neutrinos and photons can be neglected in the MD era, they contribute in the MD era through their memory from the RD era.Comment: 27 pages, 6 figures. To appear in Phys Rev

Continuous Time Random Walks in periodic systems: fluid limit and fractional differential equations on the circle

In this article, the continuous time random walk on the circle is studied. We derive the corresponding generalized master equation and discuss the effects of topology, especially important when Levy flights are allowed. Then, we work out the fluid limit equation, formulated in terms of the periodic version of the fractional Riemann-Liouville operators, for which we provide explicit expressions. Finally, we compute the propagator in some simple cases. The analysis presented herein should be relevant when investigating anomalous transport phenomena in systems with periodic dimensions.Comment: 14 pages, 1 figure. References added. Published versio