413 research outputs found
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
Scalar-Tensor Gravity Cosmology: Noether symmetries and analytical solutions
In this paper, we present a complete Noether Symmetry analysis in the
framework of scalar-tensor cosmology. Specifically, we consider a non-minimally
coupled scalar field action embedded in the FLRW spacetime and provide a full
set of Noether symmetries for related minisuperspaces. The presence of
symmetries implies that the dynamical system becomes integrable and then we can
compute cosmological analytical solutions for specific functional forms of
coupling and potential functions selected by the Noether Approach.Comment: 9 pages, accepted for publication by Phys. Rev.
Viable f(T) models are practically indistinguishable from LCDM
We investigate the cosmological predictions of several 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 cosmology at the
background level. In addition we constrain the growth index predicted
in the context of these models using the latest perturbation growth data in the
context of three parametrizations for . The evolution of the best fit
effective Newton constant, which incorporates the -gravity effects, is
also obtained along with the corresponding error regions. We show
that all the viable parameter sectors of the gravity models considered
practically reduce these models to CDM. Thus, the degrees of freedom
that open up to CDM in the context of 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, , the best-fitting
parameters for the HI selected galaxies are found to be Mpc with . Fixing the slope to its universal
value , we obtain 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 . Furthermore, dividing the HI
galaxies into two richness subsamples we find that the low mass HI galaxies
have a very low present bias factor (), while the high mass
HI galaxies trace the underlying matter distribution as the optical galaxies
(). 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 Mpc and 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 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
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