arXiv subjects

Dario Fadda

Publications and source records attributed to Dario Fadda.

54 records · Page 3Linked to original sources

arXiv2006

A population of high-redshift type-2 quasars-I. Selection Criteria and Optical Spectra

We discuss the relative merits of mid-infrared and X-ray selection of type-2 quasars. We describe the mid-infrared, near-infrared and radio selection criteria used to find a population of redshift z~2 type-2 quasars which we previously argued suggests that most supermassive black hole growth in the Universe is obscured (Martinez-Sansigre et al., 2005). We present the optical spectra obtained from the William Herschel Telescope, and we compare the narrow emission line luminosity, radio luminosity and maximum size of jets to those of objects from radio-selected samples. This analysis suggests that these are genuine radio-quiet type-2 quasars, albeit the radio-bright end of this population. We also discuss the possibility of two different types of quasar obscuration, which could explain how the ~2-3:1 ratio of type-2 to type-1 quasars preferred by modelling our population can be reconciled with the ~1:1 ratio predicted by unified schemes.

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arXiv2006

The Spitzer Space Telescope Extra-Galactic First Look Survey: 24 micron data reduction, catalog, and source identification

We present the reduction of the 24 micron data obtained during the first cosmological survey performed by the Spitzer Space Telescope (First Look Survey, FLS). The survey consists of a shallow observation of 2.5x2 sq deg centered at 17h18m +59d30m (main survey) and a deeper observation of 1x0.5 sq deg centered at 17h17m +59d45m(verification survey). Issues with the reduction of the 24 micron MIPS data are discussed and solutions to attenuate instrumental effects are proposed and applied to the data. Approximately 17000 sources are extracted with a SNR greater than five. The photometry of the point sources is evaluated through PSF fitting using an empirical PSF derived from the data. Aperture corrections and the absolute calibration have been checked using stars in the field. Astrometric and photometric errors depend on the SNR of the source varying between 0.35-1 arcsec and 5-15%, respectively, for sources detected at 20-5 sigma. The flux of the 123 extended sources have been estimated through aperture photometry. The extended sources cover less than 0.3% of the total area of the survey. Based on simulations, the main and verification surveys are 50% complete at 0.3 and 0.15 mJy, respectively. Counterparts have been searched for in optical and radio catalogs. More than 80% of the 24 micron sources have a reliable optical counterpart down to R=25.5. 16% of the sources have a 20 cm counterpart down to 0.1 mJy and ~ 80% of the radio-infrared associations have a reliable optical counterpart. A residual map is obtained by subtracting point sources detected at the 3-sigma level and interpolating the regions occupied by extended sources. Several galactic clouds with low and intermediate velocities are identified by comparison with neutral Hydrogen data from this field.

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arXiv2006

Star Formation and Extinction in Redshift z~2 Galaxies: Inferences from Spitzer MIPS Observations

Using very deep Spitzer/MIPS 24 micron observations, we present an analysis of the bolometric luminosities (L[bol]) and UV extinction properties of more than 200 spectroscopically identified, optically selected (UGR) z~2 galaxies in the GOODS-N field. The large spectroscopic sample is supplemented with near-IR selected (BzK/DRG) galaxies and submm sources at similar redshifts in the same field, providing a representative collection of relatively massive (M*>1e10 Msun) galaxies at high redshifts. We focus on the redshift range 1.5-2.6, where MIPS is sensitive to the strength of the mid-IR PAH features in the galaxy spectra (rest-frame 5-8.5 micron). We demonstrate, using stacked X-ray data and a subset of galaxies with H-alpha measurements, that L(5-8.5) provides a reliable estimate of L(IR) for most star forming galaxies at z~2. The range of L(IR) in the samples considered extends from ~1e10 to >1e12 Lsun, with a mean of 2e11 Lsun. Using 24 micron observations to infer dust extinction in high redshift galaxies, we find that, as in the local universe, the obscuration (L[IR]/L[1600]) is strongly dependent on L(bol), and ranges in value from <1 to \~1000. However, the obscuration is ~10 times smaller at a given L(bol) at z~2 than at z=0. We show that the values of L(IR) and obscuration inferred from the UV spectral slope beta generally agree well with the values inferred from L(5-8.5) for L(bol)<1e12 Lsun. For galaxies with L(bol)>1e12 Lsun, it is common for UV-based estimates to underpredict L(IR) by a factor of ~10-100. Using the specific SFR as a proxy for cold gas fraction, we find a wide range in the evolutionary state of galaxies at z~2, from those which have just begun to form stars to those which have already accumulated most of their stellar mass and are about to become, or already are, passively-evolving. [Abridged]

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arXiv2005

Most supermassive black hole growth is obscured by dust

We present an alternative method to X-ray surveys for hunting down the high-redshift type-2 quasar population, using Spitzer and VLA data on the Spitzer First Look Survey. By demanding objects to be bright at 24 microns but faint at 3.6 microns, and combining this with a radio criterion, we find 21 type-2 radio-quiet quasar candidates at the epoch at which the quasar activity peaked. Optical spectroscopy with the WHT confirmed 10 of these objects to be type-2s with 1.4 < z < 4.2 while the rest are blank. There is no evidence for contamination in our sample, and we postulate that our 11 blank-spectrum candidates are obscured by kpc-scale dust as opposed to dust from a torus around the accretion disk. By carefully modelling our selection criteria, we conclude that, at high redshift, 50-80 % of the supermassive black hole growth is obscured by dust.

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arXiv2005

The obscuration by dust of most of the growth of supermassive black holes

Supermassive black holes underwent periods of exponential growth during which we seem them as quasars in the distant Universe. The summed emission from these quasars generates the cosmic X-ray background, the spectrum of which has been used to argue that most black-hole growth is obscured. There are clear examples of obscured black-hole growth in the form of `type-2' quasars, but their numbers are fewer than expected from modelling of the X-ray background. Here we report on the direct detection of a population of distant type-2 quasars which is at least comparable in size to the well-known unobscured type-1 population. We selected objects that have mid-infrared and radio emissions characteristic of quasars, but which are faint at near-infrared and optical wavelengths. This population is responsible for most of the black hole growth in the young Universe and, throughout cosmic history, black-hole growth occurs in the dusty, gas-rich centres of active galaxies.

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arXiv2004

ISOCAM observations in the Lockman Hole - I The 14.3 micron shallow survey: data reduction, catalogue, and optical identifications

We present the image and catalogue of the 14.3 micron shallow survey of 0.55 square degrees in the region of the Lockman Hole (10h52m03s +57d21m46s, J2000) with the Infrared Space Observatory (ISO). The data have been analyzed with the recent algorithm by Lari et al. (2001) conceived to exploit ISO data in an optimal way, especially in the case of shallow surveys with low redundancy. Photometry has been accurately evaluated through extensive simulations and also the absolute calibration has been checked using a set of 21 stars detected at 14.3 micron, optical, and near-IR bands. On the basis of simulations, we evaluate that the survey is 80%, 50%, and 20% complete at 0.8, 0.6, and 0.45 mJy, respectively. Below the 20% completeness limit, fluxes are generally overestimated since the sources are preferentially detected if their positions correspond to positive oscillations of the noise. Moreover, from a comparison with the deep survey, we estimate that only sources brighter than 0.45 mJy are highly reliable. Only 5% of these sources do not have optical counterparts down to r'=25. Since none of the Spitzer imaging bands cover the 14.3 micron wavelength range, this data set will remain unique until the advent of the James Webb Space Telescope.

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arXiv2004

The First Measurements of Galaxy Clustering from IRAC Data of the Spitzer First Look Survey

We present the first results of the angular auto-correlation function of the galaxies detected by the Infrared Array Camera (IRAC) instrument in the First Look Survey (FLS) of the Spitzer Space Telescope. We detect significant signals of galaxy clustering within the survey area. The angular auto-correlation function of the galaxies detected in each of the four IRAC instrument channels is consistent with a power-law form $w(θ)=Aθ^{1-γ}$ out to $θ= 0.2\arcdeg$, with the slope ranging from $γ= 1.5$ to 1.8. We estimate the correlation amplitudes $A$ to be $2.95 \times 10^{-3}$, $2.03 \times 10^{-3}$, $4.53 \times 10^{-3}$, and $2.34 \times 10^{-3}$ at $θ=1\arcdeg$ for galaxies detected in the IRAC 3.6$μ$m, 4.5$μ$m, 5.8$μ$m, and 8.0$μ$m instrument channels, respectively. We compare our measurements at 3.6$μ$m with the previous K-band measurements, and discuss the implications of these results.

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arXiv2004

Luminous Infrared Starbursts in a Cluster of Galaxies

Analysing mid--infrared ISOCAM images of the cluster of galaxies J1888.16CL, we identified among its members several particularly active galaxies with total infrared luminosities well above 10^{11} Lsun. If powered by dust enshrouded starbursts, as suggested by their optical spectra, these Luminous Infrared Galaxies would exhibit star formation rates surprisingly high in the cluster environment. The triggering mechanism is unclear but could be tidal collisions within sub-structures or infalling groups.

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arXiv2004

The Spitzer Space Telescope First-Look Survey: KPNO MOSAIC-1 R-band Images and Source Catalogs

(Abridged) We present R-band images covering more than 11 square degrees of sky obtained with the KPNO 4-m telescope in preparation for the Spitzer Space Telescope First Look Survey. The FLS was designed to characterize the mid-infrared sky at depths 2 orders of magnitude deeper than previous surveys. The extragalactic component is the first cosmological survey done with Spitzer. Two relatively large regions of the sky were observed: the main FLS extra galactic field (17h18m+59d30m) and ELAIS-N1 field (16h10m+54d30m). The overall quality of the images is high. The relative astrometric accuracy is better than 0.1'' and the typical seeing is 1.1''. Images are relatively deep since they reach a median 5-sigma depth limiting magnitude of R=25.5 (Vega). Catalogs have been extracted using SExtractor using thresholds in area and flux for which the number of false detections is below 1% at R=25. Only sources with S/N greater than 3 have been retained in the final catalogs. Comparing the galaxy number counts from our images with those of deeper R-band surveys, we estimate that our observations are 50% complete at R=24.5. These limits in depth are sufficient to identify a substantial fraction of the infrared sources which will be detected by Spitzer.

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arXiv2002

The LARI Method for ISO-CAM/PHOT Data Reduction and Analysis

The techniques and software tools developed for the reduction and analysis of ISO-CAM/PHOT data with the LARI method are presented. The method, designed for the detection of faint sources in ISO raster observations, is based on the assumption of the existence of two different time scales in the detectors' transient behaviour, accounting either for fast or slow detectors' response. The specifically developed IDL software includes: a reduction pipeline performing basic operations such as deglitching and background determination; the fitting procedures proper, modelling the time history of individual pixels and detecting any flux excess with respect to the local background ascribable to potential sources; mapping, source extraction and flux estimation procedures; simulation procedures allowing one to estimate the errors arising from different instrumental and reduction effects. Moreover, an easy-to-use graphical user interface allows one to quickly browse the data and carry out the substantial amount of interactive analysis required when the automatic fit fails and to check the reliability of detected sources. This method provides source lists of great reliability and completeness and an outstanding photometric accuracy, particularly at low redundancy levels, where the reliability of ISO-CAM/PHOT source lists, even at moderately bright flux levels, has been a long-standing issue. In this work a description of the techniques and software tools that have been developed is given, alongside with some highlights from the results obtained thanks to their application to different fields.

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arXiv2001

The AGN contribution to mid-infrared surveys

We provide constraints on the AGN contribution to the mid-IR extragalactic background light from a correlation analysis of deep X-ray and mid-IR observations in two regions centred on the Lockman Hole (LH) and Hubble Deep Field North (HDFN). Among the 76 galaxies detected by XMM in the LH area (200 square arcminutes), 24 show mid-IR emission, but the relative percentage of X-ray sources with mid-IR counterparts increases with the band energy: from 30% of the 0.5-2 keV sources up to 63% of the 5-10 keV sources. In contrast, only a small fraction of the mid-IR sources (around 10%) show X-ray emission. In the region centred on the HDFN (24 square arcminutes), 25% of the mid-IR sources are detected in the X-ray, while 30-40% of the X-ray sources show mid-IR emission. Under the conservative assumption that all XMM sources are AGN-dominated, AGNs contribute 15% of the total mid-IR flux in the LH. For the HDFN we have assumed that AGN-dominated sources are luminous X-ray sources with radio to X-ray SEDs typical of local AGNs, in which case we find that 18% of the mid-IR flux are due to AGN emission. If we put together all the existing information from the deepest HDFN data to the shallow large-area BeppoSAX observations in the ELAIS S1 region using the median mid-IR to X-ray spectral indices as a function of the X-ray flux, we find an AGN contribution to the 15um background of 17%. We conclude that the population of IR luminous galaxies detected in the ISOCAM deep surveys are mainly constituted by dust-obscured starbursts.

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arXiv2001

ISO investigates the nature of extremely-red hard X-ray sources responsible for the X-ray background

We analyse very deep X-ray and mid-IR surveys in common areas of the Lockman Hole and the HDF North to study the sources of the X-ray background (XRB) and to test the standard obscured accretion paradigm. We detect with ISO a rich population of X-ray luminous sources with red optical colours, including a fraction identified with Extremely Red Objects (R-K > 5) and galaxies with SEDs typical of normal massive ellipticals or spirals at z ~ 1. The high 0.5-10 keV X-ray luminosities of these objects (1E43-1E45 erg/s) indicate that the ultimate energy source is gravitational accretion, while the X-ray to IR flux ratios and the X-ray spectral hardness show evidence of photoelectric absorption at low X-ray energies. An important hint on the physics comes from the mid-IR data at 6.7 and 15 um, well reproduced by model spectra of completely obscured quasars under standard assumptions and l.o.s. optical depths tau ~ 30-40. Other predictions of the standard XRB picture, like the distributions of intrinsic bolometric luminosities and the relative fractions of type-I and -II objects (1:3), are also consistent with our results. Obscured gravitational accretion is then confirmed as being responsible for the bulk of the X-ray background, since we detect in the IR the down-graded energy photoelectrically absorbed in X-rays: 63% of the faint 5-10 keV XMM sources are detected in the mid-IR by Fadda et al. (2001). However, although as much as 90% of the X-ray energy production could be converted to IR photons, no more than 20% of the Cosmic IR Background can be attributed to X-ray loud AGNs.

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arXiv2001

The dark side of star formation in galaxy clusters: spectroscopic follow-up of clusters observed with ISOCAM

The evolution of galaxies in cluster environments can be studied using mid-IR observations which trace star forming regions hidden by dust. The optical follow-up of A1689 (z=0.2), observed at 6.7 and 15um by ISOCAM, have revealed a systematic excess of the B-[15um] galaxy color distribution with respect that of Coma and Virgo clusters. This result suggests the existence of a dark side of the Butcher-Oemler effect measured in the optical. We present an analysis of the optical/mid-IR properties of the mid-IR emitters in A1689, comparing in particular the star formation rates based on mid-IR and optical data. Morover, we present preliminary result for J1888, a cluster at z=0.56 deeply observed with ISOCAM, based on recent VLT/FORS and NTT/SOFI observations.

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arXiv1998

Deep ISOCAM Observations of Galaxy Clusters

Ten galaxy clusters, with redshift ranging from 0.2 to 1, have been observed with the ISO camera in the two bands LW2 and LW3 (centered respectively at 6.75 and 15 microns). We present a first analysis for three of these clusters at redshift 0.2, 0.5 and 1.

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arXiv1997

The Near- and Mid-Infrared Continuum Emission of Seyfert Nuclei: Constraints on the Models of Obscuring Tori

For an extended sample of Seyfert galaxies we compile from the literature the infrared fluxes in the four IRAS bands, the ground-based small-beam (~ 5-10") fluxes in the standard Q, N, M, L (or L') bands, and the nuclear (non-stellar) estimated fluxes in the JHK bands. We estimate nuclear fluxes in the L band by applying a correction for stellar light. From the statistical study of the infrared colors and luminosities, we derive the typical SEDs of Seyfert 1 and 2 nuclei and the typical differences in luminosities between the two types of objects in the mid- and near-infrared spectral ranges. The observational data appear to severely challenge many models of dusty tori, which hardly account for the shapes of the SEDs and the degree of anisotropy observed in Seyfert galaxies. In particular, at variance with some earlier claims, very thick and compact tori are basically inconsistent with these observational constraints. The most successful models, though having problems in accounting for several details, can fit the major infrared observational data of both Seyfert 1 and Seyfert 2 nuclei with tori which extend up to several hundreds pc and have fairly low optical thickness.

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arXiv1997

Density estimation with non-parametric methods

One key issue in several astrophysical problems is the evaluation of the density probability function underlying an observational discrete data set. We here review two non-parametric density estimators which recently appeared in the astrophysical literature, namely the adaptive kernel density estimator and the Maximum Penalized Likelihood technique, and describe another method based on the wavelet transform. The efficiency of these estimators is tested by using extensive numerical simulations in the one-dimensional case. The results are in good agreement with theoretical functions and the three methods appear to yield consistent estimates. In order to check these estimators with respect to previous studies, two galaxy redshift samples (the galaxy cluster A3526 and the Corona Borealis region) have been analyzed.

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arXiv1996

The Observational Distribution of Internal Velocity Dispersions in Nearby Galaxy Clusters

We analyze the internal velocity dispersions of a sample of 172 nearby galaxy clusters (z < 0.15), each of which has at least 30 available galaxy redshifts, and spans a large richness range. Cluster membership selection is based on nonparametric methods. In the estimate of galaxy velocity dispersion we consider the effects of possible velocity anisotropies in galaxy orbits, the infall of late-type galaxies, and velocity gradients. The dynamical uncertainties due to the presence of substructures are also taken into account. Our cumulative distribution agrees with previous distributions within their completness in velocity dispersion ( > 800 km/s). We estimate that our distributions is complete for at least a dispersion greater than 650 km/s. In this completeness range a power law fit is in fair agreement with results coming from the X-ray temperature distributions.

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arXiv1996

Compact Radio Sources and Nuclear Activity in Seyfert Galaxies

Using recent high-resolution radio observations of a large sample of Seyfert galaxies (Roy et al. 1994), we analyze the relations between the compact radio core emission and several nuclear and host galaxy properties. Seyfert nuclei hosted in early-type galaxies or in object with nearby companions show stronger radio cores than the norm. Radio core emission show some correlation with total radio emission and with tracers of nuclear activity (mid-IR, hard X-ray and narrow-line emissions). This favours the view that Seyfert radio cores are powered by AGN rather than by radio supernovae.

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