N. Sacchi

The Herschel Reference Survey

The Herschel Reference Survey is a Herschel guaranteed time key project and will be a benchmark study of dust in the nearby universe. The survey will complement a number of other Herschel key projects including large cosmological surveys that trace dust in the distant universe. We will use Herschel to produce images of a statistically-complete sample of 323 galaxies at 250, 350, and 500 μm. The sample is volume-limited, containing sources with distances between 15 and 25 Mpc and flux limits in the K band to minimize the selection effects associated with dust and with young high-mass stars and to introduce a selection in stellar mass. The sample spans the whole range of morphological types (ellipticals to late-type spirals) and environments (from the field to the center of the Virgo Cluster) and as such will be useful for other purposes than our own. We plan to use the survey to investigate (i) the dust content of galaxies as a function of Hubble type, stellar mass, and environment; (ii) the connection between the dust content and composition and the other phases of the interstellar medium; and (iii) the origin and evolution of dust in galaxies. In this article, we describe the goals of the survey, the details of the sample and some of the auxiliary observing programs that we have started to collect complementary data. We also use the available multifrequency data to carry out an analysis of the statistical properties of the sample.

The HELLAS2XMM survey. VI. X-ray absorption in the 1df AGN sample through a spectral analysis

The spectroscopic analysis of 117 serendipitous sources in the HELLAS2XMM 1df (1 degree field) survey is de- scribed. Of these, 106 sources, of which 86% have a spectroscopic redshift, are used to evaluate the fraction of X-ray absorbed (log NH > 22) Active Galactic Nuclei (AGN) in the 2−10 keV flux range 0.8−20 × 10 −14 erg cm −2 s −1 . This fraction turns out lower than what is predicted by two well known Cosmic X-Ray Background synthesis models, and the discrepancy is signifi- cant at the 99.999% level. This result consolidates the findings recently obtained by other authors. In the flux interval explored, the data are consistent with an intrinsic distribution of the absorbing columns (flat per decade above log NH > 21) independent of luminosity and redshift, together with an AGN luminosity function evolving purely in luminosity. It is shown that, on the other hand, extrapolation to lower fluxes fails to reproduce the results inferred from the Chandra Deep Field North survey. It is found that about 40% of the high luminosity sources in our sample have best fit log NH > 22, and the surface density of these X-ray obscured QSOs can then be estimated at about 48 per square degree, at the flux limit of ∼10 −14 erg cm −2 s −1 of the HELLAS2XMM 1df survey. As a side issue, 5 or 6 out of 60 sources, that is about 10%, identified with broad line AGN, turn out to be affected by log NH > 22 absorption.

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.

Spectroscopic Identifications of Spitzer Sources in the SWIRE/XMM-Newton/ELAIS-S1 Field: A Large Fraction of Active Galactic Nucleus with High F(24 μm)/F(R) Ratio

We present a catalog of optical spectroscopic identifications of sources detected by Spitzer at 3.6 or 24 μm down to ~10 and ~280 μJy, respectively, in the SWIRE/XMM-Newton/ELAIS-S1 field and classified via line width analysis and diagnostic diagrams. A total of 1376 sources down to R ~ 24.2 mag have been identified (1362 detected at 3.6 μm, 419 at 24 μm, and 405 at both) by low-resolution optical spectroscopy carried out with FORS2, VIMOS, and EFOSC2 at the Very Large Telescope and 3.6 m ESO telescope. The spectroscopic campaigns have been carried out over the central 0.6 deg^2 area of ELAIS-S1 which, in particular, has also been observed by XMM-Newton and Chandra. We find the first direct optical spectroscopic evidence that the fraction of active galactic nuclei (AGNs; mostly AGN2) increases with increasing F(24 μm)/F(R) ratio, reaching values of 70(±20)% in the range 316 < F(24 μm)/F(R) < 1000. We present an Infrared Array Camera-Multiband Imaging Photometer color-color diagram able to separate AGN1 from obscured AGN2 candidates. After having corrected for the spectroscopic incompleteness of our sample, the result is that the AGN fraction at F(24 μm) ~0.8 mJy is ~22(±7)% and decreases slowly to ~19(±5)% down to F(24 μm) ~ 0.3 mJy.

The Contribution of AGNs and Star-forming Galaxies to the Mid-Infrared as Revealed by Their Spectral Energy Distributions

We present the broadband SEDs of the largest available highly complete (72%) spectroscopic sample of MIR-selected galaxies and AGNs at intermediate redshift. The sample contains 203 extragalactic sources from the 15 μm ELAIS-SWIRE survey, all with measured spectroscopic redshift. Most of these sources have full multiwavelength coverage from the FUV (GALEX) to the FIR (Spitzer) and lie in the redshift range 0.1 10 mJy, while that obtained from optical spectroscopy is never >30%, even at higher flux densities. Our results will be very useful for updating all models aimed at interpreting the deep IR survey data and in particular for constraining the nature and role of dust-obscured systems in the intermediate/high-redshift universe.

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.

The XMM-Newton survey of the ELAIS-S1 field - II. Optical identifications and multiwavelength catalogue of X-ray sources

We present the optical identifications and a multi-band catalogue of a sample of 478 X-ray sources detected in the XMM-Newton and Chandra surveys of the central 0.6 deg 2 of the ELAIS-S1 field. The most likely optical/infrared counterpart of each X-ray source was identified using the chance coincidence probability in the R and IRAC 3.6 μm bands. This method was complemented by the precise positions obtained through Chandra observations. We were able to associate a counterpart to each X-ray source in the catalogue. Approximately 94% of them are detected in the R band, while the remaining are blank fields in the optical down to R ∼ 24.5, but have a near-infrared counterpart detected by IRAC within 6 �� from the XMM-Newton centroid. The multi-band catalogue, produced using the positions of the identified optical counterparts, contains photometry in ten photometric bands, from B to the MIPS 24 μm band. The spectroscopic follow-up allowed us to determine the redshift and classification for 237 sources (∼50% of the sample) brighter than R = 24. The spectroscopic redshifts were complemented by reliable photometric redshifts for 68 sources. We classified 47% of the sources with spectroscopic redshift as broad-line active galactic nuclei (BL AGNs) with z = 0.1−3.5, while sources without broadlines (NOT BL AGNs) are about 46% of the spectroscopic sample and are found up to z = 2.6. The remaining fraction is represented by extended X-ray sources and stars. We spectroscopically identified 11 type 2 QSOs among the sources with F(2−10 keV)/F(R) > 8, with redshift between 0.9 and 2.6, high 2−10 keV luminosity (log L2−10 keV ≥ 43.8 erg s −1 ) and hard X-ray colors suggesting large absorbing columns at the rest frame (log NH up to 23.6 cm −2 ). BL AGNs show on average blue optical-to-near-infrared colors, softer X-ray colors and X-ray-to-optical colors typical of optically selected AGNs. Conversely, narrow-line sources show redder optical colors, harder X-ray flux ratio and span a wider range of X-ray-to-optical colors. On average the Spectral Energy Distributions (SEDs) of high-luminosity BL AGNs resemble the power-law typical of unobscured AGNs. The SEDs of NOT BL AGNs are dominated by the galaxy emission in the optical/near-infrared, and show a rise in the mid-infrared which suggests the presence of an obscured active nucleus. We study the infrared-to-optical colors and near-infrared SEDs to infer the properties of the AGN host galaxies.

The ESO-Spitzer Imaging extragalactic Survey (ESIS)II. VIMOS I, z wide field imaging of ELAIS-S1 and selection of distant massive galaxies

Context. The ESO-Spitzer Imaging extragalactic Survey (ESIS) is the optical follow up of the Spitzer Wide-area Infra-Red Extragalactic survey (SWIRE) in the ELAIS-S1 region of the sky. Aims. In the era of observational cosmology, the main efforts are focused on the study of galaxy evolution and its environmental dependence. Wide area, multiwavelength, extragalactic surveys are needed in order to probe sufficiently large volumes, minimize cosmic variance and find significant numbers of rare objects. Methods. We present VIMOS I and z band imaging belonging to the ESIS survey. A total of ~4 deg^2 was targeted in I and ~1 deg^2 in z. Accurate data processing includes removal of fringing, and mosaicking of the complex observing pattern. Completeness levels and photometric uncertainties are estimated through simulations. The multi-wavelength data available in the area are exploited to identify high-redshift galaxies, using the IR-peak technique. Results. More than 300 000 galaxies have been detected in the I band and ~50 000 in the z band. Object coordinates are defined within an uncertainty of ~0.2 arcsec rms, with respect to GSC 2.2. We reach a 90% average completeness at 23.1 and 22.5 mag (Vega) in the I and z bands, respectively. On the basis of IRAC colors, we identify galaxies having the 1.6

AGN counts at 15µm XMM observations of the ELAIS-S1-5 sample

\mu

The central region of spiral galaxies as seen by Herschel

m stellar peak shifted to z = 1-3. The new I, z band data provide reliable constraints to help avoid low-redshift interlopers and reinforce this selection. Roughly 1000 galaxies between z = 2-3 are identified over the ESIS ~4 deg^2, at the SWIRE 5.8