arXiv subjects

G. De Zotti

Publications and source records attributed to G. De Zotti.

At least 181 records · Page 10Linked to original sources

arXiv2001

Theoretical predictions on the clustering of SCUBA galaxies and implications for small-scale fluctuations at sub-mm wavelengths

This paper investigates the clustering properties of SCUBA-selected galaxies in the framework of a unifying scheme relating the formation of QSOs and spheroids (Granato et al. 2000). The theoretical angular correlation function is derived for different bias functions, corresponding to different values of the ratio $M_{\rm halo}/M_{\rm sph}$ between the mass of the dark halo and the final mass in stars. SCUBA sources are predicted to be strongly clustered, with a clustering strength increasing with mass. We show that the model accounts for the clustering of Lyman-break Galaxies, seen as the optical counterpart of low- to intermediate-mass primeval spheroidal galaxies and is also consistent with the observed angular correlation function of Extremely Red Objects. Best agreement is obtained for $M_{\rm halo}/M_{\rm sph}=100$. We also consider the implications for small scale fluctuations observed at sub-mm wavelengths by current or forthcoming experiments aimed at mapping the Cosmic Microwave Background (CMB). The predicted amplitude of the clustering signal in the 350 GHz channel of the Planck mission strongly depends on the halo-to-bulge mass ratio and may be of comparable amplitude to primary CMB anisotropies for multipole numbers $l\simgt 50$.

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arXiv2001

Joint Formation of QSOs and Spheroids: QSOs as clocks of star formation in Spheroids

Direct and indirect observational evidence leads to the conclusion that high redshift QSOs did shine in the core of early type proto-galaxies during their main episode of star formation. Exploting this fact, we derive the rate of formation of this kind of stellar systems at high redshift by using the QSO Luminosity Function. The elemental proportions in elliptical galaxies, the descendents of the QSO hosts, suggest that the star formation was more rapid in more massive objects. We show that this is expected to occur in Dark Matter haloes, when the processes of cooling and heating is considered. This is also confirmed by comparing the observed sub-mm counts to those derived by coupling the formation rate and the star formation rate of the spheroidal galaxies with a detailed model for their SED evolution. In this scenario SCUBA galaxies and Lyman Break Galaxies are early type proto-galaxies forming the bulk of their stars before the onset of QSO activity.

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arXiv2000

Neural networks and separation of Cosmic Microwave Background and astrophysical signals in sky maps

The Independent Component Analysis (ICA) algorithm is implemented as a neural network for separating signals of different origin in astrophysical sky maps. Due to its self-organizing capability, it works without prior assumptions on the signals, neither on their frequency scaling, nor on the signal maps themselves; instead, it learns directly from the input data how to separate the physical components, making use of their statistical independence. To test the capabilities of this approach, we apply the ICA algorithm on sky patches, taken from simulations and observations, at the microwave frequencies, that are going to be deeply explored in a few years on the whole sky, by the Microwave Anisotropy Probe (MAP) and by the {\sc Planck} Surveyor Satellite. The maps are at the frequencies of the Low Frequency Instrument (LFI) aboard the {\sc Planck} satellite (30, 44, 70 and 100 GHz), and contain simulated astrophysical radio sources, Cosmic Microwave Background (CMB) radiation, and Galactic diffuse emissions from thermal dust and synchrotron. We show that the ICA algorithm is able to recover each signal, with precision going from 10% for the Galactic components to percent for CMB; radio sources are almost completely recovered down to a flux limit corresponding to $0.7σ_{CMB}$, where $σ_{CMB}$ is the rms level of CMB fluctuations. The signal recovering possesses equal quality on all the scales larger then the pixel size. In addition, we show that the frequency scalings of the input signals can be partially inferred from the ICA outputs, at the percent precision for the dominant components, radio sources and CMB.

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arXiv1999

An evolutionary model for GHz Peaked Spectrum Sources. Predictions for high frequency surveys

We have explored, in the general framework of the ``young source'' scenario, evolutionary models for GHz Peaked Spectrum (GPS) galaxies and quasars which reproduce the observed counts, redshift and peak frequency distributions of currently available samples. Substantially different cosmological evolution properties are found for the two populations: the quasar luminosity function must evolve strongly up to $z\sim 1$, while the data on galaxies may be consistent with no evolution. The models show that GPS sources (mostly quasars) may comprise quite a significant fraction of bright ($S> 1 $Jy) radio sources at $ν\geq 30 $GHz if the intrinsic distribution of peak frequencies extends up to $\sim 1000 $GHz. In any case, however, their fraction decreases rapidly with decreasing flux and their contribution to small scale fluctuations in the frequency range covered by the forthcoming space missions MAP and Planck Surveyor is expected to be minor.

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arXiv1999

Polarization fluctuations due to extragalactic sources

We have derived the relationship between polarization and intensity fluctuations due to point sources. In the case of a Poisson distribution of a population with uniform evolution properties and constant polarization degree, polarization fluctuations are simply equal to intensity fluctuations times the average polarization degree. Conservative estimates of the polarization degree of the classes of extragalactic sources contributing to fluctuations in the frequency ranges covered by the forthcoming space missions MAP and Planck Surveyor indicate that extragalactic sources will not be a strong limiting factor to measurements of the polarization of the Cosmic Microwave Background.

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arXiv1999

The Planck Surveyor mission: astrophysical prospects

Although the Planck Surveyor mission is optimized to map the cosmic microwave background anisotropies, it will also provide extremely valuable information on astrophysical phenomena. We review our present understanding of Galactic and extragalactic foregrounds relevant to the mission and discuss on one side, Planck's impact on the study of their properties and, on the other side, to what extent foreground contamination may affect Planck's ability to accurately determine cosmological parameters. Planck's multifrequency surveys will be unique in their coverage of large areas of the sky (actually, of the full sky); this will extend by two or more orders of magnitude the flux density interval over which mm/sub-mm counts of extragalactic sources can be determined by instruments already available (like SCUBA) or planned for the next decade (like the LSA-MMA or the space mission FIRST), which go much deeper but over very limited areas. Planck will thus provide essential complementary information on the epoch-dependent luminosity functions. Bright radio sources will be studied over a poorly explored frequency range where spectral signatures, essential to understand the physical processes that are going on, show up. The Sunyaev-Zeldovich effect, with its extremely rich information content, will be observed in the direction of a large number of rich clusters of Galaxies. Thanks again to its all sky coverage, Planck will provide unique information on the structure and on the emission properties of the interstellar medium in the Galaxy. At the same time, the foregrounds are unlikely to substantially limit Planck's ability to measure the cosmological signals. Even measurements of polarization of the primordial Cosmic Microwave background fluctuations appear to be feasible.

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arXiv1999

Extragalactic Radio Sources and CMB anisotropies

Confusion noise due to extragalactic sources is a fundamental astrophysical limitation for experiments aimed at accurately determining the power spectrum of the Cosmic Microwave Background (CMB) down to arcmin angular scales and with a sensitivity $ΔT/T \simeq 10^{-6}$. At frequencies $\lsim 200-300$ GHz, the most relevant extragalactic foreground hampering the detection of intrinsic CMB anisotropies is constituted by radio loud Active Galactic Nuclei (AGN), including ``flat--spectrum'' radiogalaxies, quasars, BL-LACs and blazars. We review our present understanding of astrophysical properties, spectra, and number counts of the above classes of sources. We also study the angular power spectrum of fluctuations due both to Poisson distributed and clustered radio sources and give preliminary predictions on the power spectrum of their polarized components. Furthermore, we discuss the capabilities of future space missions (NASA's MAP, Bennett et al. 1995; ESA's Planck Surveyor, Bersanelli et al. 1996) in studying bright radio sources over an almost unexplored frequency interval where spectral signatures, essential for the understanding of the physical processes, show up.

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arXiv1998

Radio and Far-IR Extragalactic Sources at Planck Frequencies

We discuss the main uncertainties affecting estimates of small scale fluctuations due to extragalactic sources in the Planck Surveyor frequency bands. Conservative estimates allow us to confidently conclude that, in the frequency range 100--200 GHz, the contaminating effect of extragalactic sources is well below the expected anisotropy level of the cosmic microwave background (CMB), down to angular scales of at least $\simeq 10'$. Hence, an accurate subtraction of foreground fluctuations is not critical for the determination of the CMB power spectrum up to multipoles $\ell \simeq 1000$. In any case, Planck's wide frequency coverage will allow to carefully control foreground contributions. On the other hand, the all sky surveys at 9 frequencies, spanning the range 30--900 GHz, will be unique in providing complete samples comprising from several hundreds to many thousands of extragalactic sources, selected in an essentially unexplored frequency interval. New classes of sources may be revealed in these data.

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arXiv1998

Extragalactic Point Sources and the Planck Surveyor Mission

We review estimates of small scale fluctuations due to extragalactic point sources in the Planck Surveyor frequency bands. While our undestanding of the spectral and evolutionary properties of these sources is far from complete, conservative estimates allow us to confidently conclude that, in the frequency range 100--200 GHz, their contaminating effect is well below the expected anisotropy level of the cosmic microwave background (CMB), down to angular scales of at least $\simeq 10'$. Hence, an accurate subtraction of foreground fluctuations is not critical for the determination of the CMB power spectrum up to multipoles $\ell \simeq 1000$. In any case, Planck's wide frequency coverage will allow to carefully control foreground contributions.On the other hand, the all sky surveys at 9 frequencies, spanning the range 30--900 GHz, will be unique in providing complete samples comprising from several hundreds to many thousands of extragalactic sources, selected in an essentially unexplored frequency region. New classes of sources may be revealed in these data. The familiar ``flat''-spectrum radio sources should show spectral features carrying essential information on their physical properties. Crucial information will be provided to understand the nature of radio sources with strongly inverted spectra. Scenarios for the cosmological evolution of galaxies will be extensively tested.

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arXiv1998

Extragalactic Source Counts and Contributions to the Anisotropies of the Cosmic Microwave Background. Predictions for the Planck Surveyor mission

We present predictions for the counts of extragalactic sources, the contributions to fluctuations and their spatial power spectrum in each channel foreseen for the Planck Surveyor (formerly COBRAS/SAMBA) mission. The contribution to fluctuations due to clustering of both radio and far--IR sources is found to be generally small in comparison with the Poisson term; however the relative importance of the clustering contribution increases and may eventually become dominant if sources are identified and subtracted down to faint flux limits. The central Planck frequency bands are expected to be ``clean'': at high galactic latitude (|b|>20), where the reduced galactic noise does not prevent the detection of the extragalactic signal, only a tiny fraction of pixels is found to be contaminated by discrete extragalactic sources. Moreover, removal of contaminating signals is eased by the substantial difference between their power spectrum and that of primordial fluctuations.

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arXiv1997

Source Counts and Background Radiation

Our present understanding of the extragalactic source counts and background radiation at infrared and sub-mm wavelengths is reviewed. Available count data are used to constrain evolutionary models of galaxies and Active Nuclei. The CIRB, on the other hand, provides crucial information on the integrated past IR emissivity, including sources so faint to be never accessible. The near-IR and sub-mm are suited to search for the CIRB. These cosmological windows are ideal to detect redshifted photons from the two most prominent broad emission features in galaxy spectra: the stellar photospheric 1 micron peak and the one at 100 micron due to dust re-radiation. The recently claimed detection of an isotropic diffuse component at 100-200 microns would support the evidence for strong cosmic evolution from $z=0$ to $z\sim 2$ of faint gas-rich late-type galaxies, as inferred from direct long-$λ$ counts. Furthermore, an equally intense CIRB flux measured in the wavelength range 200 to 500 $μm$ may be in support of models envisaging a dust-enshrouded phase during formation of spheroidal galaxies (e.g. Franceschini et al., 1994): the background spectrum at such long wavelengths would indicate an high redshift ($z>2-3$) for this active phase of star formation, implying a relevant constraint on structure formation scenarios. We argue that, given the difficulties for sky surveys in this spectral domain, only sporadic tests of these ideas will be achieved in the next several years. Only a mission like FIRST, in combination with large mm arrays and optical telescopes, will allow to thoroughly investigate, via photometric and spectroscopic surveys, these IR-bright early phases in the evolution of galaxies and AGNs.

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arXiv1996

Observability of Early Evolutionary Phases of Galaxies at mm Wavelengths

Several lines of evidence and theoretical arguments suggest that a large fraction of starlight is absorbed by interstellar dust and re-radiated at far-IR wavelengths, particularly during early evolutionary phases of early type galaxies, which may even, under some circumstances, experience an optically thick phase. Therefore far-IR to mm observations are crucial to understand the galaxy evolution. The strong K-correction makes surveys at mm wavelengths ideally suited for studying high-$z$ galaxies. The broad redshift range covered by mm surveys at sub-mJy flux limits offers a good chance for gaining important information also on the geometry of the Universe.

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arXiv1995

Hard X-ray emission from extragalactic IRAS 12um sources: constraints on the unified AGN model and the synthesis of the X-ray background

We analyze the $2-10$~keV X-ray emission of complete samples of AGN and galaxies selected at 12$μ$m and recently compiled by Rush, Malkan \& Spinoglio (1993). As a necessary complement we also study the IR emissivity of the largest sample of hard X--ray selected AGN (Grossan 1992). Our purposes are to probe the unified scheme of active nuclei and to evaluate the local X--ray volume emissivity of low luminosity and `hidden' AGN. We confirm that X-ray emission is basically restricted to AGN, Seyfert 1's being much more powerful emitters than Seyfert 2's. We find no signal of X-ray emission for the remaining galaxies selected at $12μ$m, not even for those having a flat far-infrared spectrum, which would be prime candidates to host hidden AGN. Our result on the local volume emissivity of the Seyfert 1's is fully consistent with those derived from studies of the hard X--ray luminosity function. However the 95$\%$ upper limits to the local emissivity we derive for the Seyfert 2 class and for the most likely `hidden' AGN candidates, severely constrain (and in some cases exclude) models of the X--ray background based on the unified Seyfert scheme. We also find that less than $\sim20$\% of Seyfert 1-like AGN and less than $\sim 50$\% of Seyfert 2-like AGN can be `hidden' in the non-Seyfert galaxy samples assuming that the X-ray to 12$μ$m flux ratios measured in the 12$μ$m selected samples can be considered typical for these classes of objects. In this case, the number ratio of Seyfert 2 to Seyfert 1 galaxies ranges from 1 to 2.

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arXiv1994

The Extragalactic IR Background

Current limits on the intensity of the extragalactic infrared background are consistent with the expected contribution from evolving galaxies. Depending on the behaviour of the star formation rate and of the initial mass function, we can expect that dust extinction during early evolutionary phases ranges from moderate to strong. An example of the latter case may be the ultraluminous galaxy IRAS F$10214 + 4724$. The remarkable lack of high redshift galaxies in faint optically selected samples may be indirect evidence that strong extinction is common during early phases. Testable implications of different scenarios are discussed; ISO can play a key role in this context. Estimates of possible contributions of galaxies to the background under different assumptions are presented. The COBE/FIRAS limits on deviations from a blackbody spectrum at sub-mm wavelengths already set important constraints on the evolution of the far-IR emission of galaxies and on the density of obscured (``Type 2'') AGNs. A major progress in the field is expected at the completion of the analysis of COBE/DIRBE data.

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arXiv1994

Far-IR Properties of Early Type Galaxies

We have investigated the IRAS colours and the far-IR to optical luminosity ratios of a complete sample of elliptical's and S0's brighter than $B_T = 12$. On the average, elliptical galaxies emit in the far-IR less than 1\% of their bolometric luminosity, while S0's are about a factor of three brighter in the far-IR. On the average, the photospheric emission of red giant stars can account for 50--60\% of the $12\mic$ flux from early type galaxies; the contribution from diffuse dust at this wavelength is $< 10\%$ in the case of ellipticals,and may amount to 20--40\% for S0's. An additional, $\sim 30--40\%$, contribution from circumstellar emission from evolved giants with mass loss (particularly OH/IR stars) seems to be required in the case of ellipticals. This suggests a small but significant star formation activity in these galaxies at a look-back time of 1--$2\,$Gyr, corresponding to about 10\% of that typical of a disk galaxy having the same V-band luminosity. As for S0's, the larger diffuse dust emission may swamp to some extent that of circumstellar dust, which is indicated to be, on the average, $\lsim 20\%$. The weak emission from diffuse interstellar dust, detected mostly at $60\mic$ and $100\mic$, has color temperatures similar to those of disk galaxies; as in that case of the latter, a warm dust component is suggested, associated to star-forming regions. The implied star formation would be a few percent of that of disk galaxies of similar V-band luminosity and could account for a fraction of the observed UV branch of early type galaxies.

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arXiv1994

A Model for the Spectral Energy Distribution of the Ultraluminous Galaxy IRAS F10214+4724

If indeed early type galaxies used up most of their gas to form stars in a time short compared to their collapse time and if a roughly constant fraction of metals is locked up in dust grains, these galaxies may easily become opaque to starlight and emit most of their luminosity in the far-IR. The corresponding spectral energy distribution matches remarkably well the observed continuum spectrum of the ultraluminous galaxy IRAS F$10214+4724$ from UV to sub-mm wavelengths, i.e. over almost four decades in frequency, for a galactic age $\lsim 1\,$Gyr. The bolometric luminosity in this model is $\simeq 2.7\times 10^{14}\Lsol$ ($H_0 =50\,\hbox{km}\,\hbox{s}^{-1} \,\hbox{Mpc}^{-1}$, $Ω=1$), i.e. somewhat lower than implied by previous models. In the present framework, the bolometric luminosity of the galaxy is expected to decrease by a factor $\gsim 30$ during the subsequent evolution.

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arXiv1993

Models for the Evolution of the Spectral Energy Distribution of Elliptical Galaxies from UV to Far--IR Wavelengths

We have worked out evolutionary synthesis models of the broad-band spectral energy distribution of elliptical galaxies over the whole frequency range from UV to far--IR. Internal extinction and far--IR re--emission by interstellar dust have been taken into account in a self--consistent way. Diffuse dust emission has been modelled in terms of two components: warm dust, located in regions of high radiation intensity, and cold dust, heated by the general radiation field. Emission from circumstellar dust clouds at different galactic ages was also taken into account. The models reproduce well the present average broad--band spectrum of nearby ellipticals over about four decades in frequency. Under the assumption of a dust--to--gas ratio proportional to the metallicity, the fraction of bolometric luminosity coming out at far-IR wavelengths strongly evolves during the galaxy lifetime, ranging from very low local values ($\lsim 0.5\%$) to $\sim 30\%$ or more in the first billion years. Some models even imply that early phases are optically thick; these models provide an excellent fit of the observed spectral energy distribution of the high redshift galaxy IRAS F10214$+$4724. Far--IR observations may thus play a key role in investigating the early evolution of elliptical galaxies. A strong far-IR evolution of early-type galaxies might be a crucial ingredient to explain the deep $60\mic$ IRAS counts.

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arXiv1993

Angular Correlations of the X-Ray Background and Clustering of Extragalactic X-Ray Sources

The information content of the autocorrelation function (ACF) of intensity fluctuations of the X-ray background (XRB) is analyzed. The tight upper limits set by ROSAT deep survey data on the ACF at arcmin scales imply strong constraints on clustering properties of X-ray sources at cosmological distances and on their contribution to the soft XRB. If quasars have a clustering radius r_0=12-20 Mpc (H_0=50), and their two point correlation function, is constant in comoving coordinates as indicated by optical data, they cannot make up more 40-50% of the soft XRB (the maximum contribution may reach 80% in the case of stable clustering, epsilon=0). Active Star-forming (ASF) galaxies clustered like normal galaxies, with r_0=10-12 Mpc can yield up to 20% or up to 40% of the soft XRB for epsilon=-1.2 or epsilon=0, respectively. The ACF on degree scales essentially reflects the clustering properties of local sources and is proportional to their volume emissivity. The upper limits on scales of a few degrees imply that hard X-ray selected AGNs have r_0<25 Mpc if epsilon=0 or r_0<20 Mpc if epsilon=-1.2. No significant constraints are set on clustering of ASF galaxies, due to their low local volume emissivity. The possible signal on scales >6 deg, if real, may be due to AGNs with r_0=20 Mpc; the contribution from clusters of galaxies with r_0~50 Mpc is a factor 2 lower.

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