arXiv subjects

S. Basilakos

Publications and source records attributed to S. Basilakos.

At least 73 records · Page 4Linked to original sources

arXiv2005

The Shape-Alignment relation in $Λ$CDM Cosmic Structures

In this paper we study the supercluster - cluster morphological properties using one of the largest ($2\times 512^{3}$ SPH+N-body simulations of large scale structure formation in a $Λ$CDM model, based on the publicly available code GADGET. We find that filamentary (prolate-like) shapes are the dominant supercluster and cluster dark matter halo morphological feature, in agreement with previous studies. However, the baryonic gas component of the clusters is predominantly spherical. We investigate the alignment between cluster halos (using either their DM or baryonic components) and their parent supercluster major-axis orientation, finding that clusters show such a preferential alignment. Combining the shape and the alignment statistics, we also find that the amplitude of supercluster - cluster alignment increases although weakly with supercluster filamentariness.

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arXiv2005

The XMM-Newton/2dF survey-VIII: The extended X-ray sources

We present a sample of eight extended X-ray sources detected in the wide-field (~2.3 deg^2), bright (2-10 ksec) XMM-Newton/2dF survey, reaching a flux limit of ~2 X 10^{-14} erg/s/cm^2. Of these, seven are identified as secure X-ray clusters in the soft 0.3-2 keV band using a standard wavelet algorithm on either the PN or the MOS images. Spectroscopic or photometric redshifts are available for five clusters, spanning a range between 0.12 and 0.68. The X-ray spectral fittings show temperatures between 1 and 4.6 keV, characteristic of poor clusters and groups of galaxies. We derive for the first time the XMM-Newton cluster number count log N-log S distribution albeit with poor statistics. Both the log N-\log S and the Luminosity-Temperature relation are in good agreement with previous ROSAT results.

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arXiv2005

The Serendipitous XMM-Newton Cluster Athens Survey (SEXCLAS): Sample selection and the cluster log N - log S

In this paper we serendipitously identify X-ray cluster candidates using XMM-Newton archival observations complemented by 5-band optical photometric follow-up observations (r~23 mag) as part of the X-ray Identification (XID) programme. Our sample covers an area of ~2.1 sq. deg (15 XMM-Newton fields) and comprises a total of 21 (19 serendipitous + 2 target) extended X-ray sources to the limit f(0.5-2keV) ~ 6x10^{-15} cgs with a high probability (> 99.9%) of being extended on the XMM-Newton images. Of the 21 cluster candidates 7 are spectroscopically confirmed in the literature. Exploiting the optical data available for these fields we discover that 68% of the X-ray cluster candidates are associated with optical galaxy overdensities. We also attempt to constrain the redshifts of our cluster candidates using photometric methods. We thus construct the photometric redshift distribution of galaxies in the vicinity of each X-ray selected cluster candidate and search for statistically significant redshift peaks against that of the background distribution of field galaxies. Comparison of the photometric with spectroscopic redshift estimates for the confirmed clusters suggest that our simple method is robust out to z~0.5. For clusters at higher z, deeper optical data are required to estimate reliable photometric redshifts. Finally, using the sample of the 19 serendipitous X-ray selected cluster candidates we estimate their surface density down to f(0.5-2keV) ~ 6x10^{-15} cgs and find it to be in fair agreement with previous and recent studies.

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arXiv2004

XMM-{\em Newton} Observations of Optically Selected SDSS Clusters

We explore the X-ray properties of a subset of the optically selected SDSS cluster sample of Goto et al. (2002), by analysing seven public XMM-{\em Newton} pointings, with exposure times ranging from $\sim$ 4 to 46 ksec. There are in total 17 SDSS clusters out of which only eight are detected at X-ray wavelengths with $\rm f_{0.5-2keV}\magcir 1.2 \times 10^{-14}$ ergs cm$^{-2}$ s$^{-1}$. For the remaining 9 SDSS clusters we estimate their 3$σ$ luminosity upper limits (corresponding to $L_x\mincir 5\times 10^{42}$ ergs/sec in the 0.5-2 keV band). This relatively low luminosity suggests that if real structures, these galaxy aggregations correspond to poor groups of galaxies. Using the SDSS photometric catalogue we also derive the cluster optical $r$-band luminosities. The resulting scaling relations ($L_{\rm opt}-L_x$, $L_{\rm opt}-T_x$) are consistent with those of other recent studies.

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arXiv2004

The XMM-{\em Newton}/2dF Survey - VI: Clustering and Bias of the Soft X-ray Point Sources

We study the clustering properties of X-ray sources detected in the wide area ($\sim 2 $deg$^{2}$) bright, contiguous XMM-{\em Newton}/2dF survey. We detect 432 objects to a flux limit of $5\times 10^{-15}$ ergcm$^{-2}$s$^{-1}$ in the soft 0.5-2 keV band. Performing the standard angular correlation function analysis, a $\sim 3σ$ correlation signal between 0 and 150 arcsec is detected: $w(θ<150^{''})\simeq 0.114\pm 0.037$. If the angular correlation function is modeled as a power law, $w(θ)=(θ_{\circ}/θ)^{γ-1}$, then for its nominal slope of $γ=1.8$ we estimate, after correcting for the integral constraint and the amplification bias, that $θ_{\circ}\simeq 10.4 \pm 1.9$ arcsec. Using a clustering evolution model which is constant in comoving coordinates ($ε=-1.2$), a luminosity dependent density evolution model for the X-ray luminosity function and the concordance cosmological model ($Ω_{\rm m}=1-Ω_Λ=0.3$) we obtain, by inverting Limber's integral equation, a spatial correlation length of $r_{\circ}\sim 16$ $h^{-1}$ Mpc. This value is larger than that of previous {\sc ROSAT} surveys as well as of the optical two-degree quasar redshift survey. Only in models where the clustering remains constant in physical coordinates ($ε=-3$), do we obtain an $r_{\circ}$ value ($\sim 7.5$ $h^{-1}$ Mpc) which is consistent with the above surveys.

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arXiv2004

The Needles in the Haystack survey: searching for X-ray selected normal galaxies

In this paper we present the first results from an on-going serendipitous survey aiming to identify X-ray selected `normal' galaxies (i.e. not AGN dominated) by combining archival XMM-Newton data with the Sloan Digital Sky Survey. In the first 4.5deg2 of this program we have identified a total of 11 `normal' galaxy candidates (8 of them with optical spectroscopy) with fluxes f_X(0.5-8keV)~1e-15-1e-13cgs. These sources are selected to have low X-ray--to--optical flux ratio (logf_X/f_{opt}<-2), soft X-ray spectral properties and optical spectra, when available, consistent with the presence of a stellar ionising continuum. These sources comprise both early and late type systems at redshifts z<0.2 with luminosities L_X(0.5-8keV)~1e39-1e42cgs. This dataset provides the first tight constraint on the surface density of X-ray selected `normal' galaxies at relatively bright fluxes spanning two orders of magnitude (1e-15 - 1e-13cgs). The slope of the `normal' galaxy logN-logS in the above flux range is estimated -1.4\pm0.3 consistent with the euclidean prediction. We also discuss the prospects of `normal' galaxy studies at X-ray wavelengths using both our continuously expanding survey and future X-ray missions.

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arXiv2004

The Cluster M/L and the Value of Omega_m

From an up-to-date compilation of ACO cluster redshifts and velocity dispersions we extract a homogeneous sample of 459 clusters with robust velocity dispersion. Using the virial theorem to estimate cluster masses, and a correlation between Abell galaxy counts and R-band luminosity, we find a median M/L_R of 174 h_50 M_sun/L_sun. However, if we correct the virial masses according to the X-ray vs. virial mass correlation, as derived from our analysis, we obtain M/L_R ~100 h_50 M_sun/L_sun. These two values of M/L imply a mass content of the Universe of Omega_m ~0.26 and 0.15, respectively.

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arXiv2004

The Shape of Poor Groups of Galaxies

We estimate the distribution of intrinsic shapes of UZC-SSRS2 groups of galaxies from the distribution of their apparent shapes. We measure the projected group axial ratio using the moments of their discrete galaxy distribution. Then using the non-parametric kernel method to estimate the smooth apparent axial ratio distribution we numerically invert a set of integral equations to recover the corresponding intrinsic distribution under the assumption that groups are either oblate or prolate spheroids. We find that the prolate spheroidal model fits very well the UZC-SSRS2 group distribution with a true mean axial ratio <β>~0.3 and σ~0.15. This shows that groups of galaxies are significantly more elongated, both on the plane of the sky and in 3 dimensions, than clusters of galaxies. The poorest groups that we consider, those with 4 members, are even more elongated than the overall population with 85% of the groups having β< 0.4.

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arXiv2004

The Clustering of XMM-Newton Hard X-ray Sources

This paper presents the clustering properties of hard (2-8 keV) X-ray selected sources detected in a wide field ($\approx \rm 2 deg^{2}$) shallow [$f_X(\rm 2-8 keV)\approx 10^{-14}\rm erg cm^{-2} s^{-1}$] and contiguous XMM-{\it Newton} survey. We perform an angular correlation function analysis using a total of 171 sources to the above flux limit. We detect a $\sim 4σ$ correlation signal out to 300 arcsec with $w(θ< 300^{''})\simeq 0.13 \pm 0.03$. Modeling the two point correlation function as a power law of the form $w(θ)=(θ_{\circ}/θ)^{γ-1}$ we find: $θ_{\circ}=48.9^{+15.8}_{-24.5}$ arcsec and $γ=2.2\pm {0.30}$. Fixing the correlation function slope to $γ=1.8$ we obtain $θ_{\circ}=22.2^{+9.4}_{-8.6}$ arcsec. Using Limber's intergral equation and a variety of possible luminosity functions of the hard X-ray population, we find a relatively large correlation length, ranging from $r_{\circ}\sim 9$ to 19 $h^{-1}$ Mpc (for $γ=1.8$ and the {\em concordance} cosmological model), with this range reflecting also different evolutionary models for the source luminosities and clustering characteristics. The relatively large correlation length is comparable to that of extremely red objects and luminous radio sources.

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arXiv2004

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 ~ 4σcorrelation 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.

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arXiv2004

The XMM/2dF survey III: Comparison between optical and X-ray cluster detection methods

We directly compare X-ray and optical techniques of cluster detection by combining SDSS photometric data with a wide-field ($\sim 1.8$ deg$^{2}$) XMM-{\em Newton} survey in the North Galactic Pole region. The optical cluster detection procedure is based on merging two independent selection methods - a smoothing+percolation technique, and a Matched Filter Algorithm. The X-ray cluster detection is based on a wavelet-based algorithm, incorporated in the SAS v.5.2 package. The final optical sample counts 9 candidate clusters with richness of more than 20 galaxies, corresponding roughly to APM richness class. Three, of our optically detected clusters are also detected in our X-ray survey. The most probable cause of the small number of optical cluster candidates detected in our X-ray survey is that they are relatively poor clusters, fainter than the X-ray flux limit (for extended sources) of our survey $f_{x}(0.3-2 {\rm keV}) \simeq 2 \times 10^{-14} erg cm^{-2} s^{-1}$.

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arXiv2003

Modelling the two point correlation function of galaxy clusters in the Sloan Digital Sky Survey

We study the clustering properties of the recently compiled SDSS cluster catalog using the two point correlation function in redshift space. We divide the total SDSS sample into two richness subsamples, roughly corresponding to Abell $R\ge 0$ and APM clusters, respectively. If the two point correlations are modeled as a power law, $ξ(r)=(r_{\circ}/r)^γ$, then the best-fitting parameters for the two subsamples are $r_{\circ}=20.7^{+4.0}_{-3.8} h^{-1}$ Mpc with $γ=1.6^{+0.4}_{-0.4}$ and $r_{\circ}=9.7^{+1.2}_{-1.2}$ with $γ=2.0^{+0.7}_{-0.5} h^{-1}$ Mpc, respectively. Our results are consistent with the dependence of cluster richness to the cluster correlation length. Finally, comparing the SDSS cluster correlation function with that predictions from three flat cosmological models ($Ω_{\rm m}=0.3$) with dark energy (quintessence), we estimate the cluster redshift space distortion parameter $β\simeq Ω_{\rm m}^{0.6}/b_{\circ}$ and the cluster bias at the present time. For the $Λ$CDM case we find $β=0.2^{+0.029}_{-0.016}$, which is in agreement with the results based on the large scale cluster motions.

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arXiv2003

The XMM-Newton/2dF survey - II. The nature of X-ray faint optically bright X-ray sources

In this paper we investigate the properties of low X-ray-to-optical flux ratio sources detected in a wide area (2.5deg^2) shallow (f(0.5-8keV)~10e-14cgs) XMM-Newton survey. We find a total of 26 sources (5% of the total X-ray selected population) with log f_X/f_{opt}<-0.9 to the above flux limit. Optical spectroscopy is available for 20 of these low X-ray-to-optical flux ratio objects. Most of them are found to be associated with Galactic stars (total of 8) and broad line AGNs (total of 8).We also find two sources with optical spectra showing absorption and/or narrow emission lines and X-ray/optical properties suggesting AGN activity. Another two sources are found to be associated with low redshift galaxies with narrow emission line optical spectra, X-ray luminosities L_X(0.5-8keV)~10e41cgs and logf_X/f_opt ~ -2 suggesting `normal' star-forming galaxies. Despite the small number statistics the sky density of `normal' X-ray selected star-forming galaxies at the flux limit of the present sample is low consistent with previous ROSAT HRI deep surveys. Also, the number density estimated here is in good agreement with both the logN-logS of `normal' galaxies in the Chandra Deep Field North (extrapolated to bright fluxes) and model predictions based on the X-ray luminosity function of local star-forming galaxies.

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arXiv2003

The XMM-2df Cluster Survey

We present the results from a shallow (2-10 ksec) XMM/2dF survey. Our survey covers 18 XMM fields ($\sim 5 {\rm deg}^2$) previously spectroscopically followed up with the Anglo-Australian telescope 2-degree field facility. About half of the fields are also covered by the Sloan Digital Sky Survey (SDSS). We are searching for extended sources using the XMM SAS maximum likelihood algorithm in the 0.3-2 keV band and we have detected 14 candidate clusters down to a flux of $\sim2\times10^{-14} cgs$. Our preliminary results show that: i) the redshift distribution peaks at relatively high redshifts ($\sim0.4$) as expected from the Rosati et al. $Φ(L)$, ii) some of our X-ray clusters appear to have optical counterparts.

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arXiv2003

Comparison between optical and X-ray cluster detection methods

In this work we present combined optical and X-ray cluster detection methods in an area near the North Galactic Pole area, previously covered by the SDSS and 2dF optical surveys. The same area has been covered by shallow ($\sim 1.8$ deg$^{2}$) XMM-{\em Newton} observations. The optical cluster detection procedure is based on merging two independent selection methods - a smoothing+percolation technique, and a Matched Filter Algorithm. The X-ray cluster detection is based on a wavelet-based algorithm, incorporated in the SAS v.5.2 package. The final optical sample counts 9 candidate clusters with richness of more than 20 galaxies, corresponding roughly to APM richness class. Three, of our optically detected clusters are also detected in our X-ray survey.

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arXiv2003

Galaxy Bias in Quintessence Cosmological Models

We derive the evolution of the linear bias factor, $b(z)$, in cosmological models driven by an exotic fluid with an equation of state: $p_{x}=wρ_{x}$, where $-1\le w<0$ (quintessence). Our aim is to put constrains on different cosmological and biasing models by combining the recent observational clustering results of optical ({\em 2dF}) galaxies (Hawkings et al.) with those predicted by the models. We find that our bias model when fitted to the {\em 2dF} clustering results predicts different bias evolution for different values of $w$. The models that provide the weak biasing ($b_{\circ} \sim 1.1$) of optical galaxies found in many recent observational studies are flat, $Ω_{\rm m}=0.3$ with $w\le -0.9$. These models however, predict a weak redshift evolution of $b(z)$, not corroborated by N-body simulations.

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arXiv2003

Galaxy Alignments as a Probe of the Dynamical State of Clusters

We present indications, based on a sample of 303 Abell clusters, for a relation between the dynamical state of clusters and the alignments of galaxy members with their parent cluster major axis orientation as well as with the large scale environment within which the clusters are embedded. The statistical results are complemented with a deep, wide-field case study of galaxy alignments in the cluster A521, which is characterised by multiple merging events (Maurogrdato et al 2000, Ferrari et al. 2003) and whose galaxy members show a strong alignment signal out to $\sim 5 h^{-1}$ Mpc. Our results show that galaxy alignments appear to be stronger the more dynamically young is the cluster, especially when found in high-density environments. This relation complements the recently found {``Cluster Substructure - Alignment Connection''} (Plionis & Basilakos 2002) by which dynamically young clusters, found in high-density environments, show stronger cluster-cluster alignments.

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arXiv2003

Cluster Formation Rate in Models with Dark Energy

Based on flat Friedmann-Robertson Walker cold dark matter (CDM) type models driven by non-relativistic matter and an exotic fluid (quintessence) with an equation of state: $p_{Q}=wρ_{Q}$ ($-1\le w<0$), we investigate whether or not the large scale dynamical effects regarding the cluster formation and virialization are related to the cosmic equation of state. Using the non-linear spherical collapse we find that the cluster formation rate, in quintessence models, is intermediate between the open and $Λ$CDM respectively. For the QCDM case, using the virial theorem and energy conservation and assuming a spherical mass overdensity shell, we obtain analytically the ratio between the final (virial) and the turn-around radius. We find that the above ratio is almost independent from the equation of state.

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