The X-ray luminosity function of local galaxies

We present an estimate of the local X-ray luminosity function and emissivity for different subsamples of galaxies namely Seyferts, LINERS, star-forming and passive (no-emission-line) galaxies. This is performed by convolving their optical luminosity function, as derived from the Ho et al. spectroscopic sample of nearby galaxies with the corresponding L_x/L_B relation. The local galaxy emissivity is about 1.6 X 10^{39} h erg/sec Mpc^3 in agreement with the results of Lahav et al. derived from cross-correlation techniques of the X-ray background with optical and infrared galaxy catalogues. From our analysis, it becomes evident that the largest fraction of the galaxy emissivity comes from galaxies associated with AGN (Seyferts but also LINERS) while the contribution of star-forming and passive galaxies is small. This independently supports the view that most of the yet unidentified X-ray sources in deep \rosat fields which are associated with faint optical galaxies, do harbour an AGN.Comment: 4 pages, 2 figures, MNRAS Pink pages (in press

Viable f(T) models are practically indistinguishable from LCDM

We investigate the cosmological predictions of several $f(T)$ models, with up to two parameters, at both the background and the perturbation levels. Using current cosmological observations (geometric supernovae type Ia, cosmic microwave background and baryonic acoustic oscillation and dynamical growth data) we impose constraints on the distortion parameter, which quantifies the deviation of these models from the concordance $\Lambda$ cosmology at the background level. In addition we constrain the growth index $\gamma$ predicted in the context of these models using the latest perturbation growth data in the context of three parametrizations for $\gamma$. The evolution of the best fit effective Newton constant, which incorporates the $f(T)$-gravity effects, is also obtained along with the corresponding $1\sigma$ error regions. We show that all the viable parameter sectors of the $f(T)$ gravity models considered practically reduce these models to $\Lambda$CDM. Thus, the degrees of freedom that open up to $\Lambda$CDM in the context of $f(T)$ gravity models are not utilized by the cosmological data leading to an overall disfavor of these models.Comment: 16 pages, 9 figures, changes match published versio

The Clustering of XMM-Newton Hard X-ray Sources

We present the clustering properties of hard (2-8 keV) X-ray selected sources detected in a wide field (~2 deg^{2}) shallow [f(2-8 keV)~ 10^{-14} erg cm^{-2} s^{-1}] and contiguous XMM-Newton survey. We perform an angular correlation function analysis using a total of 171 sources to the above flux limit. We detect a ~ 4\sigma correlation signal out to 300 arcsec with w(theta < 300^{''}) ~ 0.13 +- 0.03. Modeling the two point correlation function as a power law of the usual form we find: theta_o=48.9^{+15.8}_{-24.5} arcsec and gamma=2.2 +- 0.30. Fixing the correlation function slope to gamma=1.8 we obtain theta_o=22.2^{+9.4}_{-8.6} arcsec. Using Limber's integral equation and a variety of possible luminosity functions of the hard X-ray population, we find a relatively large correlation length, ranging from r_o ~ 9 to 19 h^{-1} Mpc (for gamma=1.8 and the concordance cosmological model), with this range reflecting also different evolutionary models for the source luminosities and clustering characteristics.Comment: In "Multiwavelength AGN Surveys" (Cozumel, December 8-12 2003), ed. R. Maiolino and R. Mujica, Singapore: World Scientific, 200

Large scale structure in the HI Parkes All-Sky Survey: Filling the Voids with HI galaxies?

We estimate the two-point correlation function in redshift space of the recently compiled HIPASS neutral hydrogen (HI) sources catalogue, which if modeled as a power law, $\xi(r)=(r_{0}/r)^{\gamma}$, the best-fitting parameters for the HI selected galaxies are found to be $r_{0}=3.3 \pm 0.3 h^{-1}$ Mpc with $\gamma=1.38 \pm 0.24$. Fixing the slope to its universal value $\gamma=1.8$, we obtain $r_{0}= 3.2\pm 0.2 h^{-1}$ Mpc. Comparing the measured two point correlation function with the predictions of the concordance cosmological model, we find that at the present epoch the HI selected galaxies are anti-biased with respect to the underlying matter fluctuation field with their bias value being $b_{0}\simeq 0.68$. Furthermore, dividing the HI galaxies into two richness subsamples we find that the low mass HI galaxies have a very low present bias factor ($b_{0}\simeq 0.48$), while the high mass HI galaxies trace the underlying matter distribution as the optical galaxies ($b_{0}\simeq 1$). Using our derived present-day HI galaxy bias we estimate their redshift space distortion parameter, and correct accordingly the correlation function for peculiar motions. The resulting real-space correlation length is $r^{\rm re}_{0}=1.8 \pm 0.2 h^{-1}$Mpc and $r^{\rm re}_{0}=3.9 \pm 0.6 h^{-1}$Mpc for the low and high mass HI galaxies, respectively. The low-mass HI galaxies appear to have the lowest correlation length among all extragalactic populations studied to-date. Also, we have correlated the IRAS-PSCz reconstructed density field, smoothed over scales of 5$h^{-1}$ Mpc, with the positions of the HI galaxies, to find that indeed the HI galaxies are typically found in negative overdensity regions (\delta\rho/\rho_{\rm PSCz} \mincir 0).Comment: 9 pages, 8 figures, MNRAS in pres

Thermodynamical aspects of running vacuum models

The thermal history of a large class of running vacuum models in which the effective cosmological term is described by a truncated power series of the Hubble rate, whose dominant term is $\Lambda (H) \propto H^{n+2}$, is discussed in detail. Specifically, by assuming that the ultra-relativistic particles produced by the vacuum decay emerge into space-time in such a way that its energy density $\rho_r \propto T^{4}$, the temperature evolution law and the increasing entropy function are analytically calculated. For the whole class of vacuum models explored here we findthat the primeval value of the comoving radiation entropy density (associated to effectively massless particles) starts from zero and evolves extremely fast until reaching a maximum near the end of the vacuum decay phase, where it saturates. The late time conservation of the radiation entropy during the adiabatic FRW phase also guarantees that the whole class of running vacuum models predicts thesame correct value of the present day entropy, $S_{0} \sim 10^{87-88}$ (in natural units), independently of the initial conditions. In addition, by assuming Gibbons-Hawking temperature as an initial condition, we find that the ratio between the late time and primordial vacuum energy densities is in agreement with naive estimates from quantum field theory, namely, $\rho_{\Lambda 0}/\rho_{\Lambda I} \sim10^{-123}$. Such results are independent on the power $n$ and suggests that the observed Universe may evolve smoothly between two extreme, unstable, nonsingular de Sitter phases.Comment: 15 pages in free style, 2 figures, to appear in European Phys. Journal C.,(this work generalizes that of arXiv:1412.5196