arXiv subjects

A. Franceschini

Publications and source records attributed to A. Franceschini.

214 records · Page 12Linked to original sources

arXiv1998

BeppoSAX detection of the Fe K line in the starburst galaxy NGC253

Preliminary results obtained from BeppoSAX observation of the starburst galaxy NGC253 are presented. X-ray emission from the object is clearly extended but most of the emission is concentrated on the optical nucleus. Preliminary analysis of the LECS and MECS data obtained using the central 4' region indicates that the continuum is well fitted by two thermal components at 0.9 keV and 7 keV. Fe K line at 6.7 keV is detected for the first time in this galaxy; the line has an equivalent width of ~300eV. The line energy and the shape of the 2-10 keV continuum strongly support thermal origin of the hard X-ray emission of NGC253. From the measurement of the Fe K line the abundances can be unambiguously constrained to ~0.25 the solar value. Other lines clearly detected are Si, S and Fe XVIII/Ne, in agreement with ASCA results.

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arXiv1998

Supermassive Black Holes in Early-Type Galaxies: Relationship with Radio Emission and Constraints on the Black Hole Mass Function

Using recently published estimates - based on high spatial resolution spectroscopy - of the mass M_{BH} of nuclear black holes for a sample of nearby galaxies, we explore the dependence of galaxy nucleus emissivity at various wavelengths on M_{BH}. We confirm an almost linear scaling of the black hole mass with the baryonic mass of the host spheroidal galaxy. A remarkably tight relationship is also found with both nuclear and total radio centimetric flux, with a very steep dependence of the radio flux on M_{BH} (P\propto M_{BH}^{2.5}). The high-frequency radio power is thus a very good tracer of a super-massive black hole, and a good estimator of its mass. This, together with the lack of significant correlations with the low-energy X-ray and far-IR flux, supports the view that advection-dominated accretion is ruling the energy output in the low-accretion rate regime. Using the tight dependence of total radio power on M_{BH} and the rich statistics of radio emission of galaxies, we derive an estimate of the mass function of remnants in the nearby universe. This is compared with current models of quasar and AGN activity and of the origin of the hard X-ray background (HXRB). As for the former, continuous long-lived AGN activity is excluded by the present data with high significance, whereas the assumption of a short-lived, possibly recurrent, activity pattern gives remarkable agreement. The presently estimated black hole mass function also implies that the HXRB has been produced by a numerous population (\sim 10^{-2} Mpc^{-3}) of moderately massive (M_{BH}\sim 10^7 M_\odot) black holes.

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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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arXiv1997

Millimetric Properties of IRAS Galaxies (III): Luminosity Functions and Contributions to the Sub-Mm Sky Background

We exploit observations at 1.25 mm with the ESO-SEST telescope of a southern galaxy sample, selected from the IRAS PSC and complete to $S_{60}=2 Jy$, to derive the FIR and mm luminosity functions and the conditional probability distributions of FIR and mm luminosity of galaxies. The reliability of these estimates is ensured by the good observed correlation of the far-infrared and mm emissions. This detailed knowledge of the millimetric properties of galaxies is used to simulate the extragalactic sub-mm sky (background intensity, small-scale anisotropy signals, and discrete source statistics) -- that will be investigated soon by a variety of ground-based and space observatories. We find, in particular, that the recent tentative detection of a sub-mm background would require, if confirmed, strong evolution with cosmic time of the galaxy long-wavelength emissivity. We finally emphasize the difficulty to test such evolution through observations from currently available millimetric sites on ground.

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arXiv1997

Far-IR and MM Properties of Quasars

We use photometric data, from the optical to the mm for a large sample of optically selected radio-quiet quasars, at low and high redshifts, to test emission models from circum-nuclear dusty torii around them. Model parameters, such as dust mass, temperature distributions and torus sizes are inferred, under the assumption that the dust is heated only by the central nuclear source. Dependences of best-fit parameters on luminosity and redshift are studied and the contribution of dust in the host galaxy to the observed fluxes is briefly mentioned. Only tentative conclusions can be drawn from this yet large dataset and detailed modelling, because crucial spectral information is missing at wavelengths where the QSO dust emission is more prominent, i.e. at $λ\simeq 50$ to 600 $μm$. Poor information in this wavelength range affects our understanding not only of the high-redshift objects, but also of the low-redshift sample. We then expect fundamental contributions by the FIRST mission in this field, which is obviously related to the more general one of galaxy formation.

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arXiv1996

MM Observations of IRAS Galaxies: Dust Properties, Luminosity Functions and Contributions to the Sub-MM Background

We have studied the FIR/{\it mm} spectrum of IR galaxies by combining IRAS photometry with new {\it mm} data on a complete southern IRAS galaxy sample. The observed spectra and a dust model emphasize a dicothomy in the galaxy population: half of the objects with a lot of warm dust are characterized by higher values of the bolometric (UV-FIR) luminosity, of the dust-to-gas mass ratio, of the dust optical depths and extinction, while those dominated by cold ({\it cirrus}) dust show opposite trends. From these data we derive the {\it mm} luminosity function of galaxies and estimate their contribution to the sub-{\it mm} background (BKG).

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arXiv1996

1.25 mm Observations of a Complete Sample of IRAS Galaxies: (II) Dust Properties

We present 1.25 mm continuum data for a southern galaxy sample selected from the IRAS PSC and complete to S_60=2 Jy. Two thirds of the galaxies have been detected and significant limits on the remaining objects have been set. We find, on a statistical basis, indications that the dust emission in these galaxies is somewhat more centrally concentrated than that of the optical light, possibly tracing a higher metal content in the inner galactic regions. This result also allows to estimate the aperture corrections to the millimetric data. The latter, together with IRAS photometric data, have been used to compare the broad-band FIR/mm spectra with a simple dust model. According to their far-IR/mm spectrum, the sample galaxies show a dichothomy: almost half of the objects, those displaying bright 25-60um fluxes ascribed to warm dust residing in 'starburst' regions, are characterized by higher values of the bolometric (optical + FIR) luminosity, of the dust-to-gas mass ratio, of the dust optical depths and of the overall extinction. A complementary class of objects dominated by cold dust ('cirrus') shows opposite trends. Because of the favourable observational setup, selection wavelength and completeness, we believe these data provide an exhaustive and unbiased view of dust properties in spiral galaxies.

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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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arXiv1995

COSMOLOGICAL EVOLUTION OF EXTRAGALACTIC SOURCES IN THE INFRARED AND CONTRIBUTIONS TO THE BACKGROUND RADIATION

Infrared surveys provide essential insights on galaxy evolution. If near-IR studies suggest mild evolution of stellar populations with cosmic time, indications of a substantial evolution have been seen in the far-IR, although the available information is largely insufficient to delineate precise evolutionary properties. A consistent picture encompassing all the currently available data may be obtained assuming that dust extinction hides the early evolutionary phases of spheroidal galaxies in the optical band, while the corresponding dust re-radiation in the far-IR may have been, during the early evolutionary phases, orders of magnitude larger than today. Hyperluminous IRAS galaxies might be extreme examples of this situation. Additional indications that at least some spheroidal galaxies may have been very dusty during their early evolution are provided by recent data on high redshift radio galaxies and quasars. Galaxies are the likely dominant contributors to the IR background. However, in the framework of unified models for Active Galactic Nuclei, a large number of nuclei hidden by a dusty torus may be expected. Implications for the IR background are discussed.

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arXiv1995

SUB--DEGREE ANISOTROPY OBSERVATIONS: GROUND--BASED, BALLOON--BORNE AND SPACE EXPERIMENTS

Extensive, accurate imaging of the Cosmic Background Radiation temperature anisotropy at sub--degree angular resolution is widely recognized as one of the most crucial goals for cosmology and astroparticle physics in the next decade. We review the scientific case for such measurements in relation with sky coverage, attainable sensitivity, confusing foreground radiation components, and experimental strategies. Although ground--based and balloon--borne experiments will provide valuable results, only a well--designed, far--Earth orbit space mission covering a wide spectral range and a significant part of all the sky will provide decisive answers on the mechanism of structure formation.

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arXiv1994

On the Relationship between Optical and X-Ray Luminosity of Quasars

The issue of the X-ray to optical luminosity relationship ($L_o-L_x$) is addressed for both optically and X-ray selected quasar samples. We have applied a generalized regression algorithm for the case of samples involving censored data, with errors on both the dependent and the independent variable. Contrary to some previous results, we find that such relationship is consistent with being a linear one ($L_x\propto L_o$). We argue that previous reports of non-linear relationships (i.e. $L_x\propto L_o^e$ with $e < 1$) are due to the neglect of the influence of the photometric errors, the precise knowledge of which strongly influences the reliability of the results. Further progresses in the determination of the $L_o-L_x$ relationship can be achieved with ROSAT observations of the new generation of large bright quasar surveys.

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arXiv1994

Millimeter Observations of a Complete Sample of IRAS Galaxies: Dust Emission and Absorption in Spirals

We report on observations performed at 1.25 $mm$ of a southern galaxy sample, selected from the IRAS PSC and complete to $S_{60}=2 Jy$. We detected 18 sources and set significant limits on 10 further objects. We use these data to discuss the spatial distribution of cold dust, the broad-band far-IR/mm spectra, the overall amount of dust, the gas-to-dust mass ratio, the dust optical depth, and the overall extinction, for such a representative galaxy sample. These results are also supported by a successful comparison with values inferred for the Galaxy. Because of the favourable observational setup, selection wavelength and completeness, we believe these data provide an unbiased view of dust properties in spiral galaxies.

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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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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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arXiv1993

Background Radiation

We present a short overview on the extragalactic background radiation from radio to X-rays, with an eye to the relation to galaxy formation and emphasizing on astrophysical backgrounds (as opposed to cosmological). As the radio background, the wealth of data on source counts make possible reliable estimations of the contribution from extragalactic sources. At 178 MHz we obtain an antenna temperature of about 22 K, with the 60% of the flux contributed by radiogalaxies and quasars and the remaining part from sub-mJy radio sources. At high frequencies the compact sources, generally QSOs, dominate. An intense far-IR/sub-mm background is predicted if a large fraction of the hard X-ray Background (XRB) comes from starburst galaxies but this might face problems with the upper limits on the far-IR isotropic flux. To clarify the problem spectral measurements should be complemented with the analysis of small scale anisotropies. The flattening of the QSO counts at B=19.5 suggests that their contribution to the optical/near-IR background is small in comparison to that of galaxies. As for the XRB, a consisting picture obtains considering two AGN populations: a "soft X-ray spectrum" population and a "hard X-ray spectrum" one, mainly contributing to the soft and to the hard XRB respectively. A third population of extremely absorbed or of reflection dominated AGN could either be the dominant contributor of the XRB above 3 keV.

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