arXiv subjects

M. Plionis

Publications and source records attributed to M. Plionis.

At least 55 records · Page 3Linked to original sources

arXiv2011

A Strategy to Measure the Dark Energy Equation of State using the HII galaxy Hubble Relation & X-ray AGN Clustering: Preliminary Results

We explore the possibility of setting stringent constraints to the Dark Energy equation of state using alternative cosmic tracers like: (a) the Hubble relation using HII galaxies, which can be observed at much higher redshifts (z~3.5) than those currently traced by SNIa samples, and (b) the large-scale structure using the clustering of X-ray selected AGN,which have a redshift distribution peaking at z~1. We use extensive Monte-Carlo simulations to define the optimal strategy for the recovery of the dark-energy equation of state using the high redshift (z~2) Hubble relation, but accounting also for the effects of gravitational lensing, which for such high redshifts can significantly affect the derived cosmological constraints. Based on a "Figure of Merit" analysis, we provide estimates for the number of 2<z<3.5 tracers needed to reduce the cosmological solution space, presently provided by the Constitution SNIa set, by a desired factor. We find that it is much more efficient to increase the number of tracers than to reduce their individual uncertainties. Finally, we propose a framework to put constraints on the dark energy equation of state by using the joint likelihood of the X-ray AGN clustering and of the Hubble relation cosmological analyses. A preliminary joint analysis using the X-ray AGN clustering of the 2XMM survey and the Hubble relation of the Constitution SNIa set provide: Omega_m= 0.31+-0.01 and w=-1.06+-0.05. We also find that the joint SNIa-2XMM analysis provides significantly more stringent cosmological constraints, increasing the Figure of Merit by a factor ~2, with respect to that of the joint SNIa-BAO analysis.

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arXiv2011

The generalized evolution of linear bias: a tool to test gravity

We derive an exact analytical solution for the redshift evolution of linear and scale-independent bias, by solving a second order differential equation based on linear perturbation theory. This bias evolution model is applicable to all different types of dark energy and modified gravity models. We propose that the combination of the current bias evolution model with data on the bias of extragalactic mass tracers could provide an efficient way to discriminate between "geometrical" dark energy models and dark energy models that adhere to general relativity.

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arXiv2011

Dynamics and Constraints of the Massive Gravitons Dark Matter Flat Cosmologies

We discuss the dynamics of the universe within the framework of Massive Graviton Dark Matter scenario (MGCDM) in which gravitons are geometrically treated as massive particles. In this modified gravity theory, the main effect of the gravitons is to alter the density evolution of the cold dark matter component in such a way that the Universe evolves to an accelerating expanding regime, as presently observed. Tight constraints on the main cosmological parameters of the MGCDM model are derived by performing a joint likelihood analysis involving the recent supernovae type Ia data, the Cosmic Microwave Background (CMB) shift parameter and the Baryonic Acoustic Oscillations (BAOs) as traced by the Sloan Digital Sky Survey (SDSS) red luminous galaxies. The linear evolution of small density fluctuations is also analysed in detail. It is found that the growth factor of the MGCDM model is slightly different ($\sim1-4%$) from the one provided by the conventional flat $Λ$CDM cosmology. The growth rate of clustering predicted by MGCDM and $Λ$CDM models are confronted to the observations and the corresponding best fit values of the growth index ($γ$) are also determined. By using the expectations of realistic future X-ray and Sunyaev-Zeldovich cluster surveys we derive the dark-matter halo mass function and the corresponding redshift distribution of cluster-size halos for the MGCDM model. Finally, we also show that the Hubble flow differences between the MGCDM and the $Λ$CDM models provide a halo redshift distribution departing significantly from the ones predicted by other DE models. These results suggest that the MGCDM model can observationally be distinguished from $Λ$CDM and also from a large number of dark energy models recently proposed in the literature.

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arXiv2010

The spherical collapse model in time varying vacuum cosmologies

We investigate the virialization of cosmic structures in the framework of flat FLRW cosmological models, in which the vacuum energy density evolves with time. In particular, our analysis focuses on the study of spherical matter perturbations, as they decouple from the background expansion, "turn around" and finally collapse. We generalize the spherical collapse model in the case when the vacuum energy is a running function of the Hubble rate, $Λ=Λ(H)$. A particularly well motivated model of this type is the so-called quantum field vacuum, in which $Λ(H)$ is a quadratic function, $Λ(H)=n_0+n_2\,H^2$, with $n_0\neq 0$. This model was previously studied by our team using the latest high quality cosmological data to constrain its free parameters, as well as the predicted cluster formation rate. It turns out that the corresponding Hubble expansion history resembles that of the traditional $Λ$CDM cosmology. We use this $Λ(t)$CDM framework to illustrate the fact that the properties of the spherical collapse model (virial density, collapse factor, etc.) depend on the choice of the considered vacuum energy (homogeneous or clustered). In particular, if the distribution of the vacuum energy is clustered, then, under specific conditions, we can produce more concentrated structures with respect to the homogeneous vacuum energy case.

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arXiv2010

Confronting Dark Energy Models using Galaxy Cluster Number Counts

The mass function of cluster-size halos and their redshift distribution are computed for 12 distinct accelerating cosmological scenarios and confronted to the predictions of the conventional flat $Λ$CDM model. The comparison with $Λ$CDM is performed by a two-step process. Firstly, we determine the free parameters of all models through a joint analysis involving the latest cosmological data, using SNe type Ia, the CMB shift parameter and BAO. Apart from a brane world inspired cosmology, it is found that the derived Hubble relation of theremaining models reproduce the $Λ$CDM results approximately with the same degree of statistical confidence. Secondly, in order to attempt distinguish the different dark energy models from the expectations of $Λ$CDM, we analyze the predicted cluster-size halo redshift distribution on the basis of two future cluster surveys: (i) an X-ray survey based on the {\tt eROSITA} satellite, and (ii) a Sunayev-Zeldovich survey based on the South Pole Telescope. As a result, we find that the predictions of 8 out of 12 dark energy models can be clearly distinguished from the $Λ$CDM cosmology, while the predictions of 4 models are statistically equivalent to those of the $Λ$CDM model, as far as the expected cluster mass function and redshift distribution are concerned. The present analysis suggest that such a technique appears to be very competitive to independent tests probing the late time evolution of the Universe and the associated dark energy effects.

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arXiv2010

The Relation Between Halo Shape, Velocity Dispersion and Formation Time

We use dark matter haloes identified in the MareNostrum Universe and galaxy groups identified in the Sloan Data Release 7 galaxy catalogue, to study the relation between halo shape and halo dynamics, parametrizing out the mass of the systems. A strong shape-dynamics, independent of mass, correlation is present in the simulation data, which we find it to be due to different halo formation times. Early formation time haloes are, at the present epoch, more spherical and have higher velocity dispersions than late forming-time haloes. The halo shape-dynamics correlation, albeit weaker, survives the projection in 2D (ie., among projected shape and 1-D velocity dispersion). A similar shape-dynamics correlation, independent of mass, is also found in the SDSS DR7 groups of galaxies and in order to investigate its cause we have tested and used, as a proxy of the group formation time, a concentration parameter. We have found, as in the case of the simulated haloes, that less concentrated groups, corresponding to late formation times, have lower velocity dispersions and higher elongations than groups with higher values of concentration, corresponding to early formation times.

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arXiv2010

Breaking the sigma_8-Omega_m degeneracy using the clustering of high-z X-ray AGN

The clustering of X-ray selected AGN appears to be a valuable tool for extracting cosmological information. Using the recent high-precision angular clustering results of ~30000 XMM-Newton soft (0.5-2 keV) X-ray sources (Ebrero et al. 2009), which have a median redshift of $z\sim 1$, and assuming a flat geometry, a constant in comoving coordinates AGN clustering evolution and the AGN bias evolution model of Basilakos et al. (2008), we manage to break the Omega_m-sigma_8 degeneracy. The resulting cosmological constraints are: Omega_m=0.27 (+0.03 -0.05), w=-0.90 (+0.10 -0.16) and sigma_8=0.74 (+0.14 -0.12), while the dark matter host halo mass, in which the X-ray selected AGN are presumed to reside, is M=2.50 (+0.50 -1.50) X 10^13 h^{-1} M(solar). For the constant Lambda model (w=-1) we find Omega_m=0.24 (+- 0.06) and sigma_8=0.83 (+0.11 -0.16), in good agreement with recent studies based on cluster abundances, weak lensing and the CMB, but in disagreement with the recent bulk flow analysis.

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arXiv2010

Luminous X-ray AGN in Clusters of Galaxies

We present a study of X-ray AGN overdensities in 16 Abell clusters, within the redshift range 0.073 = 10^{42} erg s^{-1} (at the redshift of the clusters) and within an area of 1 h^{-1}_{72} Mpc radius (excluding the core). To investigate the presence or not of a true enhancement of luminous X-ray AGN in the cluster area, we also derived the corresponding optical galaxy overdensities, using a suitable range of $r$-band magnitudes. We always find the latter to be significantly higher (and only in two cases roughly equal) with respect to the corresponding X-ray overdensities. Over the whole cluster sample, the mean X-ray point-source overdensity is a factor of ~4 less than that corresponding to bright optical galaxies, a difference which is significant at a >0.995 level, as indicated by an appropriate t-student test. We conclude that the triggering of luminous X-ray AGN in rich clusters is strongly suppressed. Furthermore, searching for optical SDSS counterparts of all the X-ray sources, associated with our clusters, we found that about half appear to be background QSOs, while others are background and foreground AGN or stars. The true overdensity of X-ray point sources, associated to the clusters, is therefore even smaller than what our statistical approach revealed.

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arXiv2010

The Dark Energy Equation of State using Alternative High-z Cosmic Tracers

We propose to use alternative cosmic tracers to measure the dark energy equation of state and the matter content of the Universe [w(z) & Omega_m]. Our proposed method consists of two components: (a) tracing the Hubble relation using HII galaxies which can be detected up to very large redshifts, z~4, as an alternative to supernovae type Ia, and (b) measuring the clustering pattern of X-ray selected AGN at a median redshift of z~1. Each component of the method can in itself provide interesting constraints on the cosmological parameters, especially under our anticipation that we will reduce the corresponding random and systematic errors significantly. However, by joining their likelihood functions we will be able to put stringent cosmological constraints and break the known degeneracies between the dark energy equation of state (whether it is constant or variable) and the matter content of the universe and provide a powerful and alternative route to measure the contribution to the global dynamics and the equation of state of dark energy. A preliminary joint analysis of X-ray selected AGN (based on the largest to-date XMM survey; the 2XMM) and the currently largest SNIa sample (Hicken et al.), using as priors a flat universe and the WMAP5 normalization of the power-spectrum, provides: Omega_m=0.27+-0.02 and w=-0.96+-0.07. Equivalent and consistent results are provided by the joint analysis of X-ray selected AGN clustering and the latest Baryonic Acoustic Oscillation measures, providing: Omega_m=0.27+-0.02 and w=-0.97+-0.04.

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arXiv2009

Hubble expansion & Structure Formation in Time Varying Vacuum Models

We investigate the properties of the FLRW flat cosmological models in which the vacuum energy density evolves with time, $Λ(t)$. Using different versions of the $Λ(t)$ model, namely quantum field vacuum, power series vacuum and power law vacuum, we find that the main cosmological functions such as the scale factor of the universe, the Hubble expansion rate $H$ and the energy densities are defined analytically. Performing a joint likelihood analysis of the recent supernovae type Ia data, the Cosmic Microwave Background (CMB) shift parameter and the Baryonic Acoustic Oscillations (BAOs) traced by the Sloan Digital Sky Survey (SDSS) galaxies, we put tight constraints on the main cosmological parameters of the $Λ(t)$ scenarios. Furthermore, we study the linear matter fluctuation field and the growth rate of clustering of the above vacuum models. Finally, we derived the theoretically predicted dark-matter halo mass function and the corresponding distribution of cluster-size halos for all the models studied. Their expected redshift distribution indicates that it will be difficult to distinguish the closely resembling models (constant vacuum, quantum field and power-law vacuum), using realistic future X-ray surveys of cluster abundances. However, cluster surveys based on the Sunayev-Zeldovich detection method give some hope to distinguish the closely resembling models at high redshifts.

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arXiv2009

Could Dark Matter Interactions be an Alternative to Dark Energy ?

We study the global dynamics of the universe within the framework of the Interacting Dark Matter (IDM) scenario. Assuming that the dark matter obeys the collisional Boltzmann equation, we can derive analytical solutions of the global density evolution, which can accommodate an accelerated expansion, equivalent to either the {\em quintessence} or the standard $Λ$ models, with the present time located after the inflection point. This is possible if there is a disequilibrium between the DM particle creation and annihilation processes with the former process dominating, which creates an effective source term with negative pressure. Comparing the predicted Hubble expansion of one of the IDM models (the simplest) with observational data we find that the effective annihilation term is quite small, as suggested by a variety of other recent experiments.

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arXiv2009

Alternative High-z Cosmic Tracers and the Dark Energy Equation of State

We propose to use alternative cosmic tracers to measure the dark energy equation of state and the matter content of the Universe [w(z) & Ω_m]. Our proposed method consists of two components: (a) tracing the Hubble relation using HII-like starburst galaxies, as an alternative to SNIa, which can be detected up to very large redshifts, z~4, and (b) measuring the clustering pattern of X-ray selected AGN at a median redshift of ~1. Each component of the method can in itself provide interesting constraints on the cosmological parameters, especially under our anticipation that we will reduce the corresponding random and systematic errors significantly. However, by joining their likelihood functions we will be able to put stringent cosmological constraints and break the known degeneracies between the dark energy equation of state (whether it is constant or variable) and the matter content of the universe and provide a powerful and alternative rute to measure the contribution to the global dynamics, and the equation of state, of dark energy. A preliminary joint analysis of X-ray selected AGN (based on a small XMM survey) and the currently largest SNIa sample (Kowalski et al 2008), provides: Omega_m=0.28^{+0.02}_{-0.04} and w=-1.0 +-0.1.

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arXiv2009

The Relation between Morphology and Dynamics of Poor Groups of Galaxies

We investigate the relation between the projected morphology (b/a) and the velocity dispersion (sigma_v) of groups of galaxies using two recently compiled group catalogs, one based on the 2MASS redshift survey and the other on the SDSS Data Release 5 galaxy catalog. We find that the sigma_v of groups is strongly correlated with the group projected b/a and size, with elongated and larger groups having a lower sigma_v. Such a correlation could be attributed to the dynamical evolution of groups, with groups in the initial stages of formation, having small sigma_v, a large size and an elongated shape that reflects the anisotropic accretion of galaxies along filamentary structures. The same sort of correlations, however, could also be reproduced in prolate-like groups, if the net galaxy motion is preferentially along the group elongation, since then the groups oriented close to the line of sight will appear more spherical, will have a small projected size and large sigma_v, while groups oriented close to the sky-plane will appear larger in projection, more elongated, and will have smaller sigma_v. We perform tests that relate only to the dynamical evolution of groups (eg., calculating the fraction of early type galaxies in groups) and indeed we find a strong positive (negative) correlation between the group sigma_v (projected major axis) with the fraction of early type galaxies. We conclude that (a) the observed dependencies of the group sigma_v on the group projected size and b/a, should be attributed mostly to the dynamical state of groups and (b) groups in the local universe do not constitute a family of objects in dynamical equilibrium, but rather a family of cosmic structures that are presently at various stages of their virialization process.

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arXiv2009

Richness Dependence of the Recent Evolution of Clusters of Galaxies

We revisit the issue of the recent dynamical evolution of clusters of galaxies using a sample of 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 de/dz ~ 2.5, while the richest clusters show an ellipticity evolution in the opposite direction (with de/dz ~ -1.2), 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 dsigma/dz ~ -1700 km/sec.

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arXiv2008

The Environment of Sy1, Sy2 & Bright IRAS Galaxies

We present a 3-dimensional study of the local (< 100 kpc) environment of Sy1, Sy2 and Bright IRAS Galaxies. For this purpose we use three galaxy samples (Sy1, Sy2, BIRG) located at high galactic latitudes as well as three control sample of non-active galaxies having the same morphological, redshift and diameter size distributions as the corresponding Seyfert or BIRG sample. Using the CfA2 and SSRS galaxy catalogues as well as our own spectroscopic observations, we find that the fraction of BIRGs with a close neighbor is significantly higher than that of their control sample. We also find that Sy2 galaxies demonstrate the same behaviour with BIRG galaxies but not with Sy1s which do not show any excess of companions with respect to their control sample galaxies. An additional analysis of the relation between FIR colors and activity type of the BIRG's shows a significant difference between the colors of strongly-interacting and non-interacting starbursts and a resemblance between the colors of non-interacting starbursts and Sy2s.Our results support an evolutionary scenario leading from Starbursting to a Sy2 and finally to an unobscured Sy1 galaxy, where close interactions play the role of the triggering mechanism.

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arXiv2008

The Halo Mass-Bias Redshift Evolution in the $Λ$CDM Cosmology

We derive an analytic model for the redshift evolution of linear-bias, allowing for interactions and merging of the mass-tracers, by solving a second order differential equation based on linear perturbation theory and the Friedmann-Lemaitre solutions of the cosmological field equations. We then study the halo-mass dependence of the bias evolution, using the dark matter halo distribution in a $Λ$CDM simulation in order to calibrate the free parameters of the model. Finally, we compare our theoretical predictions with available observational data and find a good agreement. In particular, we find that the bias of optical QSO's evolve differently than those selected in X-rays and that their corresponding typical dark matter halo mass is $\sim 10^{13} h^{-1} M_{\odot}$ and $\magcir 5 \times 10^{13} h^{-1} M_{\odot}$, respectively.

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arXiv2007

Luminosity Dependent X-ray AGN Clustering ?

We have analysed the angular clustering of X-ray selected active galactic nuclei (AGN) in different flux-limited sub-samples of the Chandra Deep Field North (CDF-N) and South (CDF-S) surveys. We find a strong dependence of the clustering strength on the sub-sample flux-limit, a fact which explains most of the disparate clustering results of different XMM and Chandra surveys. Using Limber's equation, we find that the inverted CDF-N and CDF-S spatial clustering lengths are consistent with direct spatial clustering measures found in the literature, while at higher flux-limits the clustering length increases considerably; for example, at $fx,limit ~ 10^{-15} erg/s/cm^{-2} we obtain r_0~ 17 +- 5 and 18+- 3 h^{-1} Mpc, for the CDF-N and CDF-S, respectively. We show that the observed flux-limit clustering trend hints towards an X-ray luminosity dependent clustering of X-ray selected, $z\sim 1$, AGNs.

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arXiv2007

Environmental Effects of Dark Matter Haloes: The Clustering-Substructure relation of Group-size Haloes

We estimate the two-point correlation function of dark matter haloes, with masses >10^{13} h^{-1} Mo, that have or not significant substructure. The haloes are identified with a friends of friends algorithm in a large LCDM simulation at two redshift snapshots (z=0 and 1), while halo substructure is determined using an observationally driven method. We find in both epochs a clear and significant signal by which haloes with substructure are more clustered than those with no-substructure. This is true for all the considered halo mass ranges, although for the highest halo masses the signal is noisy and present only out to ~20 h^{-1} Mpc. There is also a smooth increase of the halo correlation length with increasing amplitude of the halo substructure. We also find that substructured haloes are typically located in high-density large-scale environments, while the opposite is true for non-substructured haloes. If the haloes found in high-density regions have a relatively earlier formation time, as suggested by recent works, then they do indeed have more time to cluster than haloes, of a similar mass, which form later in the low-density regions. In such a case one would have naively expected that the former (earlier formed) haloes would typically be dynamically more relaxed than the latter (later formed). However, the higher merging and interaction rate,expected in high-density regions, could disrupt their relatively relaxed dynamical state and thus be the cause for the higher fraction of haloes with substructure found in such regions.

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