I. M. Gioia

High redshift X-ray galaxy clusters II. The LX-T relationship revisited

Aims. In this paper we re-visit the observational relation between X-ray luminosity and temperature for high-z galaxy clusters and compare it with the local LX–T and with theoretical models. Methods. To these ends we use a sample of 17 clusters extracted from the Chandra archive supplemented with additional clusters from the literature, either observed by Chandra or XMM–Newton, to form a final sample of 39 high redshift (0.25 < z < 1.3) objects. Different statistical approaches are adopted to analyze the LX–T relation. Results. The slope of the LX–T relation of high redshift clusters is steeper than expected from the self-similar model predictions and steeper, even though still compatible within the errors, than the local LX–T slope. The distant cluster LX–T relation shows a significant evolution with respect to the local Universe: high-z clusters are more luminous than the local ones by a factor ≈ 2a t any given temperature. The evolution with redshift of the LX–T relation cannot be described by a single power law nor by the evolution predicted by the self-similar model. Conclusions. We find a strong evolution, similar or stronger than the self-similar model, from z = 0t oz ≤ 0. 3f ollowed by a much weaker, if any, evolution at higher redshifts. The weaker evolution is compatible with non-gravitational models of structure formation. According to us a statistically significant sample of nearby clusters (z < 0.25) should be observed with the current available X-ray telescopes to completely exclude observational effects due to different generation detectors and to understand this novel result.

The radio luminosity function of the NEP distant cluster radio galaxies

A complete sample of 18 X-ray selected clusters of galaxies belonging to the ROSAT North Ecliptic Pole (NEP) survey has been observed with the Very Large Array at 1.4 GHz in B configuration. These are the most distant clusters in the X-ray survey with redshift in the range

Chandra point-source counts in distant galaxy clusters

0.3 -1 , except for three sources, belonging to the same cluster, which have a higher peak brightness limit of 0.26xa0mJyxa0beam -1 . The NEP field source counts are in good agreement with the source counts of a comparison survey, the VLA-VIRMOS deep field survey, indicating that the NEP sample is statistically complete. Thirty-two out of the 79xa0sources are within 0.2xa0Abell radii, twenty-two of them are considered cluster members based on spectroscopic redshifts or their optical magnitude and morphological classification. The cluster radio galaxies are used to construct the Radio Luminosity Function (RLF) of distant X-ray selected clusters. A comparison with two nearby cluster RLFs shows that the NEP RLF lies above the local ones, has a steeper slope at low radio powers ( ≤ 10 24 xa0Wu2009Hz -1 ) and shows no evidence for a break at

High redshift X-ray galaxy clusters I. The impact of point sources on the cluster properties

{approx}6 times 10^{24}

The stellar population of the Rosat North Ecliptic Pole survey

xa0Wu2009Hz -1 which is observed in the nearby cluster RLFs. We discuss briefly the origin and possible explanations of the differences observed in the radio properties of nearby and distant clusters of galaxies. The main result of this study is that the RLF of the distant X-ray clusters is very different from that of the local rich Abell clusters.

The radio luminosity function of radio galaxies in distant clusters

Context. With the superb angular resolution of the Chandra Observatory, it is now possible to detect X‐ray point sources, either e mbedded in galaxy clusters or along the cluster line of sight, which cou ld not be resolved by previous instruments. This now allows studies of source counts in distant cluster fields. Aims. We want to analyze the inner region of clusters of galaxies to check for the presence of any overdensity of X-ray point sources embedded in the gas diffuse emission. These point sources are possible AGN belonging to the clusters and could contaminate the cluster emission. Methods. We used a sample of 18 distant (0.25 < z < 1.01) galaxy clusters from the Chandra archive to construct the log N− log S , in both the soft and hard energy bands, for the X‐ray point sources detected in the central cluster region to be compared with the counts of point sources detected in similarly deep fields without clusters. Results. We find a ∼ 2 � excess of cluster region sources at the bright end of the log N− log S . The radial distribution of the brightest X‐ray point sources confirms this excess and indicates that it is co nfined to the inner 0.5 Mpc of the cluster region. Conclusions. The results suggest the possible existence of X‐ray sources belonging to the cluster (most probably AGN, given their 0.5‐10 keV luminosity ranging from 10 43 to 10 44 erg s −1 ): on average one every three clusters. Unlike previous studies, which have mainly investigated the point-source population in the vicinity of the galaxy clust ers, the present study analyzes the content of point sources within the 1 Mpc region covered by the cluster extent. Our work confirms the findings o f other investigators who analyzed the central 1 Mpc region of more massive clusters and/or groups in a similar redshift range. The X‐ray source excess found here is much smaller than the excess of radio galaxies found recently in high-z X‐ray selected clusters, possibly due to the better sensitivity of the radio observations.

High frequency radio continuum observations of bright spiral galaxies

Context. The current generation of X-ray observatories like Chandra allows studies with very fine spatial details. It is now possible to resolve X-ray point sources projected into the cluster diffuse emission and exclude them from the analysis to estimate the “correct” X-ray observables. Aims. We wish to verify the incidence of point sources on the cluster thermal emission and to evaluate the impact of their non-thermal emission on the determination of cluster properties. Methods. To these ends we use a sample of 18 high-z (0.25 0.7 where temperature and luminosity increase up to 24% and 22%, respectively. Conclusions. The conclusions are that point sources should be removed to correctly estimate the cluster properties. However the inclusion of the point sources does not impact significantly the slope and normalization of the Lbol–T relationship since for each cluster the correction to be applied to T and Lbol produces a moderate shift in the Lbol–T plane almost parallel to the best-fit of the “correct” Lbol–T relation.

A complete sample of radio sources identified with elliptical galaxies - Radio luminosity function and other properties

Context. X-ray surveys are a very efficient mean of detecting young stars and therefore allow us to study the young stellar population in the solar neighborhood and the local star formation history in the last billion of years. Aims. We want to study the young stellar population in the solar neighborhood, to constrain its spatial density and scale height as well as the recent local star formation history. Methods. We analyze the stellar content of the ROSAT North Ecliptic Pole survey, and compare the observations with the predictions derived from stellar galactic model. Since the ROSAT NEPxa0survey is sensitive at intermediate fluxes is able to sample both the youngest stars and the intermediate age stars (younger than 10 9 xa0years), linking the shallow and deep flux surveys already published in the literature. Results. We confirm the existence of anxa0excess of yellow stars in our neighborhood previously seen in shallow survey, which is likely due to axa0young star population not accounted for in the model. However the excellent agreement between observations and predictions of dMxa0stars casts some doubt on the real nature of this active population.