Highly beamed relativistic e super(+ or -)-pair energy distributions result in double photon collisions of the beamed gamma rays from TeV blazars at cosmological distances with the isotropically ...distributed extragalactic background light (EBL) in the intergalactic medium. The typical energies k sub(0) Asymptotically = to 10 super(-7) in units of m sub(e)c super(2) of the EBL are more than 10 orders of magnitude smaller than the observed gamma-ray energies k sub(1) > or =, slanted 10 super(7). Using the limit k sub(0) << k sub(1), we demonstrate that the angular distribution of the generated pairs in the lab frame is highly beamed in the direction of the initial gamma-ray photons. For the astrophysically important case of power-law distributions of the emitted gamma-ray beam up to the maximum energy M interacting with Wien-type N(k sub(0)) is proportional to (ProQuest: Formulae and/or non-USASCII text omitted) exp(-k sub(0)/Theta) soft photon distributions with total number density N sub(0), we calculate analytical approximations for the electron production spectrum. For distant objects with luminosity distances d sub(L) >>y r sub(0) = (sigmaTN sub(0)) super(-1) = (ProQuest: Formulae and/or non-USASCII text omitted) Mpc (with Thomson cross section sigmaT), the implied large values of the optical depth tau sub(0) = d sub(L)/r sub(0) indicate that the electron production spectra differ at energies inside and outside the interval (Thetaln tau sub(0)) super(-1), tau sub(0)/Theta, given the maximum gamma-ray energy M >> Theta super(-1). In the case M >> Theta super(-1) the production spectrum is strongly peaked near E Asymptotically = to Theta super(-1), being exponentially reduced at small energies and decreasing with the steep power law is proportional to E super(-1-)p up to the maximum energy E =M - (1/2).
Context. The XMM-Large Scale Structure survey, covering an area of 11.1 sq. deg., contains more than 6000 X-ray point-like sources detected with the XMM-Newton to a flux of 3 × 10-15 erg s-1 cm-2 in ...the 0.5−2 keV band. The vast majority of these sources have optical (CFHTLS), infrared (SWIRE IRAC and MIPS), near-infrared (UKIDSS), and/or ultraviolet (GALEX) counterparts. Aims. We wish to investigate the environmental properties of the different types of the XMM-LSS X-ray sources by defining their environment using the i′-band CFHTLS W1 catalog of optical galaxies to a magnitude limit of 23.5 mag. Methods. We have classified 4435 X-ray selected sources on the basis of their spectra, SEDs, and X-ray luminosity, and estimated their photometric redshifts, which have a 4−11 band photometry with an accuracy of σ△z/(1+zsp) = 0.076 with 22.6% outliers for i′ < 26 mag. We estimated the local overdensities of 777 X-ray sources that have spectro-z or photo-z calculated by using more than seven bands (accuracy of σ△z/(1+zsp) = 0.061 with 13.8% outliers) within the volume-limited region defined by 0.1 ≤ z ≤ 0.85 and −23.5 < Mi′ < −20. Results. Although X-ray sources may be found in variety of environments, a high fraction (≳55−60%), as verified by comparing with the random expectations, reside in overdense regions. The galaxy overdensities within which X-ray sources reside show a positive recent redshift evolution (at least for the range studied; z ≲ 0.85). We also find that X-ray selected galaxies, when compared to AGN, inhabit significantly higher galaxy overdensities, although their spatial extent appear to be smaller than that of AGN. Hard AGN (HR ≥ −0.2) are located in more overdense regions than soft AGN (HR < −0.2), which is clearly seen in both redshift ranges, although it appears to be stronger in the higher redshift range (0.55 < z < 0.85). Furthermore, the galaxy overdensities (with δ ≳ 1.5) within which soft AGN are embedded appear to evolve more rapidly compared to the corresponding overdensities around hard AGN.
Context. Quickly growing computing facilities and an increasing number of extragalactic observations encourage the application of data-driven approaches to uncover hidden relations from astronomical ...data. In this work we raise the problem of distance reconstruction for a large number of galaxies from available extensive observations. Aims. We propose a new data-driven approach for computing distance moduli for local galaxies based on the machine-learning regression as an alternative to physically oriented methods. We use key observable parameters for a large number of galaxies as input explanatory variables for training: magnitudes in U , B , I , and K bands, corresponding colour indices, surface brightness, angular size, radial velocity, and coordinates. Methods. We performed detailed tests of the five machine-learning regression techniques for inference of m − M : linear, polynomial, k -nearest neighbours, gradient boosting, and artificial neural network regression. As a test set we selected 91 760 galaxies at z < 0.2 from the NASA/IPAC extragalactic database with distance moduli measured by different independent redshift methods. Results. We find that the most effective and precise is the neural network regression model with two hidden layers. The obtained root–mean–square error of 0.35 mag, which corresponds to a relative error of 16%, does not depend on the distance to galaxy and is comparable with methods based on the Tully–Fisher and Fundamental Plane relations. The proposed model shows a 0.44 mag (20%) error in the case of spectroscopic redshift absence and is complementary to existing photometric redshift methodologies. Our approach has great potential for obtaining distance moduli for around 250 000 galaxies at z < 0.2 for which the above-mentioned parameters are already observed.
We present a study of the overdensity of X-ray-selected active galactic nuclei (AGN) in 33 galaxy clusters in the XMM-LSS field (The XMM-Newton Large Scale Structure Survey), up to redshift z = 1.05 ...and further divided into a lower (0.14 ≤ z ≤ 0.35) and a higher redshift (0.43 ≤ z ≤ 1.05) subsample. Previous studies have shown that the presence of X-ray-selected AGN in rich galaxy clusters is suppressed, since their number is significantly lower than what is expected from the high galaxy overdensities in the area. In the current study we have investigated the occurrence of X-ray-selected AGN in low (⟨ Lx,bol ⟩ = 2.7 × 1043 erg/s) and moderate (⟨ Lx,bol ⟩ = 2.4 × 1044 erg/s) X-ray luminosity galaxy clusters in an attempt to trace back the relation between high-density environments and nuclear activity. Owing to the wide contiguous XMM-LSS survey area, we were able to extend the study to the cluster outskirts. We therefore determined the projected overdensity of X-ray point-like sources around each cluster out to 6r500 radius, within δr500 = 1 annulus, with respect to the field expectations based on the X-ray source log N − log S of the XMM-LSS field. To provide robust statistical results we also conducted a consistent stacking analysis separately for the two z ranges. We investigated whether the observed X-ray overdensities are to be expected thanks to the obvious enhancement of galaxy numbers in the cluster environment by also estimating the corresponding optical galaxy overdensities, and we assessed the possible enhancement or suppression of AGN activity in clusters. We find a positive X-ray projected overdensity in both redshift ranges at the first radial bins, which however has the same amplitude as that of optical galaxies. Therefore, no suppression (or enhancement) of X-ray AGN activity with respect to the field is found, in sharp contrast to previous results based on rich galaxy clusters, implying that the mechanisms responsible for the suppression are not as effective in lower density environments. After a drop to roughly the background level between 2 and 3r500, the X-ray overdensity exhibits a rise at larger radii, significantly greater than the corresponding optical overdensity. The radial distance of this overdensity “bump”, corresponding to ~1.5−3 Mpc, depends on the richness of the clusters, as well as on the overall X-ray overdensity profile. Finally, using the redshift information, photometric or spectroscopic, of the optical counterparts, we derive the spatial overdensity profile of the clusters. We find that the agreement between X-ray and optical overdensities in the first radial bins is also suggested in the 3-dimensional analysis. However, we argue that the X-ray overdensity “bump” at larger radial distance is at least partially a result of flux boosting by gravitational lensing of background quasi-stellar objects, confirming previous results. For high-redshift clusters, the enhancement of X-ray AGN activity in their outskirts appears to be intrinsic. We argue that a spatial analysis is crucial for disentangling irrelevant phenomena affecting the projected analysis, but we are still not able to report statistically significant results on the spatial overdensity of AGN in clusters or their outskirts because we lack the necessary numbers.
Aims. Our aim is to study the large-scale structure of different types of AGN using the medium-deep XMM-LSS survey. Methods. We measure the two-point angular correlation function of ~5700 and 2500 ...X-ray point-like sources over the ~11 sq. deg. XMM-LSS field in the soft (0.5–2 keV) and hard (2–10 keV) bands. For the conversion from the angular to the spatial correlation function we used the Limber integral equation and the luminosity-dependent density evolution model of the AGN X-ray luminosity function. Results. We have found significant angular correlations with the power-law parameters γ = 1.81 ± 0.02, θ0 = 1.3′′ ± 0.2′′ for the soft, and γ = 2.00 ± 0.04, θ0 = 7.3′′ ± 1.0′′ for the hard bands. The amplitude of the correlation function w(θ) is higher in the hard than in the soft band for fx ≲ 10-14 erg s-1 cm-2 and lower above this flux limit. We confirm that the clustering strength θ0 grows with the flux limit of the sample, a trend which is also present in the amplitude of the spatial correlation function, but only for the soft band. In the hard band, it remains almost constant with r0 ≃ 10h-1 Mpc, irrespective of the flux limit. Our analysis of AGN subsamples with different hardness ratios shows that the sources with a hard-spectrum are more clustered than soft-spectrum ones. This result may be a hint that the two main types of AGN populate different environments. Finally, we find that our clustering results correspond to an X-ray selected AGN bias factor of ~2.5 for the soft band sources (at a median \bar{z}$z̅ ≃ 1.1 ≃ 1.1) and ~3.3 for the hard band sources (at a median $\bar{z}$z̅ ≃ 1 ≃ 1), which translates into a host dark matter halo mass of ~1013h-1M⊙ and ~1013.7h-1M⊙ for the soft and hard bands, respectively.
We present new photometric observations of H1413+117 acquired during seasons between 2001 and 2008 in order to estimate the time delays between the lensed quasar images and to characterize at best ...the on-going micro-lensing events. We propose a highly performing photometric method called the adaptive point spread function fitting and have successfully tested this method on a large number of simulated frames. This has enabled us to estimate the photometric error bars affecting our observational results. We analysed the V- and R-band light curves and V-R colour variations of the A-D components which show short- and long-term brightness variations correlated with colour variations. Using the ... and dispersion methods, we estimated the time delays on the basis of the R-band light curves over the seasons between 2003 and 2006. We have derived the new values: ...tAB = -17.4 plus or minus 2.1, ...tAC = -18.9 plus or minus 2.8 and ...tAD = 28.8 plus or minus 0.7 d using the ... method (B and C are leading, D is trailing) with 1s confidence intervals. We also used available observational constraints (resp. the lensed image positions, the flux ratios in mid-IR and two sets of time delays derived in the present work) to update the lens redshift estimation. We obtained zl=1.95... which is in good agreement with previous estimations. We propose to characterize two kinds of micro-lensing events: micro-lensing for the A, B, C components corresponds to typical variations of ~10-4 mag d-1 during all the seasons, while the D component shows an unusually strong micro-lensing effect with variations of up to ~10-3 mag d-1 during 2004 and 2005. (ProQuest: ... denotes formulae/symbols omitted.)
The XXL Survey Koulouridis, E; Poggianti, B; Altieri, B ...
Astronomy and astrophysics (Berlin),
08/2016, Letnik:
592
Journal Article, Web Resource
Recenzirano
Odprti dostop
Context. This article belongs to the first series of XXL publications. It presents multifibre spectroscopic observations of three 0.55 deg super(2) fields in the XXL Survey, which were selected on ...the basis of their high density of X-ray-detected clusters. The observations were obtained with the AutoFib2+WYFFOS (AF2) wide-field fibre spectrograph mounted on the 4.2 m William Herschel Telescope. Aims. The paper first describes the scientific rationale, the preparation, the data reduction, and the results of the observations, and then presents a study of active galactic nuclei (AGN) within three superclusters. Methods. To determine the redshift of galaxy clusters and AGN, we assign high priority to a) the brightest cluster galaxies (BCGs), b) the most probable cluster galaxy candidates, and c) the optical counterparts of X-ray point-like sources. We use the outcome of the observations to study the projected (2D) and the spatial (3D) overdensity of AGN in three superclusters. Results. We obtained redshifts for 455 galaxies in total, 56 of which are counterparts of X-ray point-like sources. We were able to determine the redshift of the merging supercluster XLSSC-e, which consists of six individual clusters at z~ 0.43, and we confirmed the redshift of supercluster XLSSC-d at z~ 0.3. More importantly, we discovered a new supercluster, XLSSC-f, that comprises three galaxy clusters also at z~ 0.3. We find a significant 2D overdensity of X-ray point-like sources only around the supercluster XLSSC-f. This result is also supported by the spatial (3D) analysis of XLSSC-f, where we find four AGN with compatible spectroscopic redshifts and possibly one more with compatible photometric redshift. In addition, we find two AGN (3D analysis) at the redshift of XLSSC-e, but no AGN in XLSSC-d. Comparing these findings with the optical galaxy overdensity we conclude that the total number of AGN in the area of the three superclusters significantly exceeds the field expectations. All of the AGN found have luminosities below 7 x 10 super(42) ergs super(-1). Conclusions. The difference in the AGN frequency between the three superclusters cannot be explained by the present study because of small number statistics. Further analysis of a larger number of superclusters within the 50 deg super(2) of the XXL is needed before any conclusions on the effect of the supercluster environment on AGN can be reached.
The growth of brightest cluster galaxies (BCGs) is closely related to the properties of their host cluster. We present evidence for dry mergers as the dominant source of BCG mass growth at z ≲ 1 in ...the XXL 100 brightest cluster sample. We use the global red sequence, Hα emission and mean star formation history to show that BCGs in the sample possess star formation levels comparable to field ellipticals of similar stellar mass and redshift. XXL 100 brightest clusters are less massive on average than those in other X-ray selected samples such as LoCuSS or HIFLUGCS. Few clusters in the sample display high central gas concentration, rendering inefficient the growth of BCGs via star formation resulting from the accretion of cool gas. Using measures of the relaxation state of their host clusters, we show that BCGs grow as relaxation proceeds. We find that the BCG stellar mass corresponds to a relatively constant fraction 1 per cent of the total cluster mass in relaxed systems. We also show that, following a cluster scale merger event, the BCG stellar mass lags behind the expected value from the Mcluster–MBCG relation but subsequently accretes stellar mass via dry mergers as the BCG and cluster evolve towards a relaxed state.
The XXL Survey Koulouridis, E.; Ricci, M.; Giles, P. ...
Astronomy and astrophysics (Berlin),
12/2018, Letnik:
620
Journal Article
Recenzirano
Odprti dostop
Context. We present the results of a study of the active galactic nucleus (AGN) density in a homogeneous and well-studied sample of 167 bona fide X-ray galaxy clusters (0.1 < z < 0.5) from the XXL ...Survey, from the cluster core to the outskirts (up to 6r500). The results can provide evidence of the physical mechanisms that drive AGN and galaxy evolution within clusters, testing the efficiency of ram pressure gas stripping and galaxy merging in dense environments. Aims. The XXL cluster sample mostly comprises poor and moderately rich structures (M = 1013–4 × 1014 M⊙), a poorly studied population that bridges the gap between optically selected groups and massive X-ray selected clusters. Our aim is to statistically study the demographics of cluster AGNs as a function of cluster mass and host galaxy position. Methods. To investigate the effect of the environment on AGN activity, we computed the fraction of spectroscopically confirmed X-ray AGNs (LX 0.5-10 keV > 1042 erg cm−1) in bright cluster galaxies with Mi* − 2 < M < Mi* + 1 $M_i^*-2<M<M_i^*+1$ Mi*−2<M<Mi*+1, up to 6r500 radius. The corresponding field fraction was computed from 200 mock cluster catalogues with reshuffled positions within the XXL fields. To study the mass dependence and the evolution of the AGN population, we further divided the sample into low- and high-mass clusters (below and above 1014M⊙, respectively) and two redshift bins (0.1–0.28 and 0.28–0.5). Results. We detect a significant excess of X-ray AGNs, at the 95% confidence level, in low-mass clusters between 0.5r500 and 2r500, which drops to the field value within the cluster cores (r < 0.5r500). In contrast, high-mass clusters present a decreasing AGN fraction towards the cluster centres, in agreement with previous studies. The high AGN fraction in the outskirts is caused by low-luminosity AGNs, up to LX 0.5-10 keV = 1043 erg cm−1. It can be explained by a higher galaxy merging rate in low-mass clusters, where velocity dispersions are not high enough to prevent galaxy interactions and merging. Ram pressure stripping is possible in the cores of all our clusters, but probably stronger in deeper gravitational potentials. Compared with previous studies of massive or high-redshift clusters, we conclude that the AGN fraction in cluster galaxies anti-correlates strongly with cluster mass. The AGN fraction also increases with redshift, but at the same rate with the respective fraction in field galaxies.
The XXL Survey Melnyk, O.; Elyiv, A.; Smolčić, V. ...
Astronomy and astrophysics (Berlin),
12/2018, Letnik:
620
Journal Article
Recenzirano
Odprti dostop
Context. This work is part of a series of studies focusing on the environment and the properties of the X-ray selected active galactic nuclei (AGN) population from the XXL survey. The present survey, ...given its large area, continuity, extensive multiwavelength coverage, and large-scale structure information, is ideal for this kind of study. Here, we focus on the XXL-South (XXL-S) field. Aims. Our main aim is to study the environment of the various types of X-ray selected AGN and investigate its possible role in AGN triggering and evolution. Methods. We studied the large-scale (>1 Mpc) environment up to redshift z = 1 using the nearest neighbour distance method to compare various pairs of AGN types. We also investigated the small-scale environment (<0.4 Mpc) by calculating the local overdensities of optical galaxies. In addition, we built a catalogue of AGN concentrations with two or more members using the hierarchical clustering method and we correlated them with the X-ray galaxy clusters detected in the XXL survey. Results. It is found that radio detected X-ray sources are more obscured than non-radio ones, though not all radio sources are obscured AGN. We did not find any significant differences in the large-scale clustering between luminous and faint X-ray AGN, or between obscured and unobscured ones, or between radio and non-radio sources. At local scales (<0.4 Mpc), AGN typically reside in overdense regions, compared to non-AGN; however, no differences were found between the various types of AGN. A majority of AGN concentrations with two or more members are found in the neighbourhood of X-ray galaxy clusters within <25–45 Mpc. Our results suggest that X-ray AGN are typically located in supercluster filaments, but they are also found in over- and underdense regions.
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