J. E. Dias

The XMM-Newton survey of the ELAIS-S1 field - I. Number counts, angular correlation function and X-ray spectral properties

Aims. The formation and evolution of cosmic structures can be probed by studying the evolution of the luminosity function of the Active Galactic Nuclei (AGNs), galaxies and clusters of galaxies and of the clustering of the X-ray active Universe, compared to the IR-UV active Universe. Methods. To this purpose, we have surveyed with XMM-Newton the central ∼0. 6d eg 2 region of the ELAIS-S1 field down to flux limits of ∼5.5 × 10 −16 erg cm −2 s −1 (0.5–2 keV, soft band, S), ∼2 × 10 −15 erg cm −2 s −1 (2–10 keV, hard band, H), and ∼4 × 10 −15 erg cm −2 s −1 (5–10 keV, ultra hard band, HH). We present here the analysis of the XMM-Newton observations, the number counts in different energy bands and the clustering properties of the X-ray sources. Results. We detect a total of 478 sources, 395 and 205 of which detected in the S and H bands respectively. We identified 7 clearly extended sources and estimated their redshift through X-ray spectral fits with thermal models. In four cases the redshift is consistent with z = 0.4, so we may have detected a large scale structure formed by groups and clusters of galaxies through their hot intra-cluster gas emission. We have computed the angular correlation function of the sources in the S and H bands finding best fit correlation angles θ0 = 5.2 ± 3. 8a rcsec andθ0 = 12.8 ± 7.8 arcsec in the two bands respectively. The correlation angle of H band sources is therefore formally ∼2.5 times that of the S band sources, although the difference is at only ∼1σ confidence level. A rough estimate of the present-day correlation length r0 can be obtained inverting the Limber equation and assuming an appropriate redshift distribution dN/dz. The results range between 12.8 and 9.8 h −1 Mpc in the S band and between 17.9 and 13.4 h −1 Mpc in the H band, with 30–40% statistical errors, assuming either smooth redshift distributions or redshift distributions with spikes accounting for the presence of significant structure at z = 0.4. The relative density of the S band sources is higher near the clusters and groups at z ∼ 0.4 and extends toward East and toward South/West. This suggests that the structure is complex, with a size comparable to the full XMM-Newton field. Conversely, the highest relative source densities of the H band sources are located in the central-west region of the field.

The contribution of very massive high-redshift SWIRE galaxies to the stellar mass function

Context. In the last couple of years a population of very massive (M_* > 10^(11) M_⊙), high-redshift (z ≥ 2) galaxies has been identified, but its role in galaxy evolution has not yet been fully understood. Aims. It is necessary to perform a systematic study of high-redshift massive galaxies, in order to determine the shape of the very massive tail of the stellar mass function and determine the epoch of their assembly. Methods. We selected high-z massive galaxies at 5.8 μm, in the SWIRE ELAIS-S1 field (1 deg^2). Galaxies with the 1.6 μm stellar peak redshifted into the IRAC bands (z ≃ 1−3, called “IR-peakers”) were identified. Stellar masses were derived by means of spectrophotometric fitting and used to compute the stellar mass function (MF) at z = 1−2 and 2−3. A parametric fit to the MF was performed, based on a Bayesian formalism, and the stellar mass density of massive galaxies above z = 2 determined. Results. We present the first systematic study of the very-massive tail of the galaxy stellar mass function at high redshift. A total of 326 sources were selected. The majority of these galaxies have stellar masses in excess of 10^(11) M_⊙ and lie at z > 1.5. The availability of mid-IR data turned out to be a valuable tool to constrain the contribution of young stars to galaxy SEDs, and thus their M_*/L ratio. The influence of near-IR data and of the chosen stellar library on the SED fitting are also discussed. The z = 2−3 stellar mass function between 10^(11) and ~10^(12) M_⊙ is probed with unprecedented detail. A significant evolution is found not only for galaxies with M ~ 10^(11) M_⊙, but also in the highest mass bins considered. The comoving number density of these galaxies was lower by more than a factor of 10 at z = 2−3, with respect to the local estimate. SWIRE 5.8 μm peakers more massive than 1.6 × 10^(11) M_⊙ provide 30−50% of the total stellar mass density in galaxies at z = 2−3.