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M. Magliocchetti

Publications and source records attributed to M. Magliocchetti.

88 records · Page 5Linked to original sources
arXiv2003

The Radio Loud / Radio Quiet dichotomy: news from the 2dF QSO Redshift Survey

We present a detailed analysis of a sample of radio-detected quasars, obtained by matching together objects from the FIRST and 2dF Quasar Redshift Surveys. The dataset consists of 113 sources, spanning a redshift range 0.3 < z < 2.2, with optical magnitudes 18.25 < b_J < 20.85 and radio fluxes S_{1.4 GHz} < 1 mJy. These objects exhibit properties such as redshift and colour distribution in full agreement with those derived for the whole quasar population, suggestive of an independence of the mechanism(s) controlling the birth and life-time of quasars of their level of radio emission. The long debated question of radio-loud (RL)/radio-quiet (RQ) dichotomy is then investigated for the combined FIRST-2dF and FIRST-LBQS sample, since they present similar selection criteria. We find the fraction of radio detections to increase with magnitude from < 3% at the faintest levels up to 20% for the brightest sources. The classical RL/RQ dichotomy, in which the distribution of radio-to-optical ratios and/or radio luminosities shows a lack of sources, is ruled out by our analysis. We also find no tight relationship between optical and radio luminosities for sources in the considered sample, result that tends to exclude the mass of the quasar black hole as the physical quantity associated to the level of radio emission.

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arXiv2002

The 2dF Galaxy Redshift Survey: The population of nearby radio galaxies at the 1 mJy level

We use redshift determinations and spectral analysis of galaxies in the 2dF Galaxy Redshift Survey to study the properties of local radio sources with $S\ge 1$ mJy. 557 objects drawn from the FIRST survey, corresponding to 2.3 per cent of the total radio sample, are found in the 2dFGRS cataloge within the area $9^h 48^m \simlt {\rm RA}({\rm 2000}) \simlt 14^h 32^m$ and $-2.77^\circ \simlt {\rm dec}({\rm 2000}) \simlt 2.25^\circ$, down to a magnitude limit $b_J= 19.45$. The excellent quality of 2dF spectra allows us to divide these sources into classes, according to their optical spectra. Radio galaxies (i.e. absorption systems, mainly belonging to the class of FR I sources, whose spectra may or may not show emission lines due to AGN activity) make up 63 per cent of the spectroscopic sample; they are characterized by relatively high radio-to-optical ratios, red colours and luminosities $10^{21}\simlt P_{1.4{\rm GHz}} W Hz^${-1}$ sr$^{-1}$ \simlt 10^{24}$. Actively star-forming galaxies instead contribute with 30 per cent to the total sample. These objects -- principally found at low redshifts ($z \simlt 0.1$) -- in general show low radio-to-optical ratios, blue colours and low radio luminosities. We also found 18 Seyfert 2 galaxies and 4 Seyfert 1's. Analysis of the local radio luminosity function shows that radio galaxies are well described by models that assume pure luminosity evolution, at least down to radio powers $P_{1.4{\rm GHz}}\simlt 10^{20.5} W Hz$^{-1}$ sr$^{-1}$ Late-type galaxies, whose relative contribution to the RLF is found to be lower than was predicted by previous work, present a luminosity function which is compatible with the IRAS galaxy LF, and therefore plausibly constitute the radio counterpart of the dusty spirals and starbursts that dominate the counts at 60 $μ$m.

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arXiv2001

QSOs and Spheroidal Galaxies

In view of the extensive evidence of tight inter-relationships between spheroidal galaxies (and galactic bulges) with massive black holes hosted at their centers, a consistent model must deal jointly with the evolution of the two components. We describe one such model, which successfully accounts for the local luminosity function of spheroidal galaxies, for their photometric and chemical properties, for deep galaxy counts in different wavebands, including those in the (sub)-mm region which proved to be critical for current semi-analytic models stemming from the standard hierarchical clustering picture, for clustering properties of SCUBA galaxies, of EROs, and of LBGs, as well as for the local mass function of massive black holes and for quasar evolution. Predictions that can be tested by surveys carried out by SIRTF are presented.

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arXiv2001

Clustering properties and gravitational lensing of forming spheroidal galaxies

We show that the predictions of the recent physical model by Granato et al. (2001) are fully consistent with the available estimates of the clustering properties of galaxies at (sub)-mm wavelengths. The model is then exploited to compute the expected power spectrum of fluctuations due to clustering at the frequencies of the High Frequency Instrument (HFI) of ESA's {\sc Planck} satellite. The clustering signal is found to be detectable in regions of low interstellar dust emission. A further distinctive prediction of the adopted model is a remarkably high fraction of gravitationally lensed sources at bright mm/sub-mm fluxes. In fact, according to the adopted model, only thanks to gravitational lensing, a significant number of high-$z$ forming spheroidal galaxies will be detectable by large area, shallow surveys at mm/sub-mm wavelengths, such as those carried out by P{\sc lanck}/HFI. Allowing for other source populations, we find that the fraction of gravitationally lensed sources at bright mm/sub-mm fluxes is expected to be up to $\simeq 40%$.

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arXiv2001

Radio Properties of FIRST Radio Sources at 1 mJy

This paper presents a detailed analysis of the radio properties for the sample of faint radio sources introduced in Magliocchetti et al. (2000). The sample comprises mainly intrinsically low-power sources whose majority ($\simgt 70$ per cent) is made of FR I radio galaxies. These objects show some degree (at 1$σ$ confidence level) of luminosity evolution, which is also needed to correctly reproduce the total number and shape of the counts distribution at 1.4 GHz. Analysis of the de-evolved local radio luminosity function shows a good agreement between data and model predictions for this class of sources. Particular care has been devoted to the issue of 'lined' galaxies (i.e. objects presenting in their spectra a continuum typical of early-type galaxies plus emission lines of different nature), which appear as an intermediate class of sources between AGN-dominated and starburst galaxies. Different evolutionary behaviours are seen between the two sub-populations of lined and non-lined low-power radio galaxies, the first class indicating a tendency for the radio luminosity to decrease with look-back time, the second one showing positive evolution. We note that different evolutionary properties also seem to characterize BL Lacs selected in different bands, so that one might envisage an association between lined FR I and the sub-class of BL Lacs selected in the X-ray band.

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arXiv2001

Clustering of SCUBA galaxies and implications for the Planck mission

The clustering properties of SCUBA-selected galaxies are investigated within the framework of a unifying scheme relating the formation of QSOs and spheroids. The theoretical angular correlation function is derived for different bias functions, corresponding to different values of the ratio $M_{halo}/M_{sph}$ between the mass of the dark halos hosting such galaxies and the mass in stars produced at the end of the major star-formation burst. SCUBA sources are predicted to be strongly clustered, with a clustering strength increasing with mass. Comparisons with the best available measurements show better fits for $M_{halo}/M_{sph}\simeq 100$. The model can also account for the clustering of Lyman-break galaxies, seen as the optical counterpart of low- to intermediate-mass primeval spheroidal galaxies. Best agreement is once again obtained for high values of the $M_{halo}/M_{sph}$ ratio. We also discuss implications for small scale fluctuations observed at different wavelengths by forthcoming experiments such as the Planck mission aimed at mapping the Cosmic Microwave Background (CMB).

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arXiv2001

Formation of Spheroidal Galaxies: The Sub-MM View

The intensity of the Cosmic Far-IR Background and the strong evolution of galaxies in the far-IR to mm wavelength range demonstrate that the bulk of starlight emitted during the early phases of galaxy evolution was reprocessed by dust. Therefore, the optical view of the galaxy formation process is highly incomplete and biased, and must be complemented with far-IR/sub-mm observations. We review the impact of sub-mm surveys on our understanding of the evolutionary history of spheroidal galaxies. A recent model, bringing into play also the inter-relationships between formation and early evolution of spheroidal galaxies and quasars, is described and some implications are outlined.

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arXiv2001

Optical identifications of $\sim 4000$ radio sources at the 1 mJy level

We present an analysis of the properties of optical counterparts of radio sources down to 1 mJy. Optical identifications have been obtained by matching together objects from the APM and FIRST surveys over the region $148.13^\circ\simlt {\rm RA(2000)}\simlt 218.13^\circ$, $-2.77^\circ\simlt {\rm Dec} \simlt 2.25^\circ$. Selecting radio sources down to 1 mJy, and adopting a uniform optical limit of $b_J = 21.5$ we find 3176 have a counterpart in the APM catalogue, corresponding to 13 per cent of the radio sample. For $b_J \le 20.5$ we can divide radio sources into resolved radio galaxies and stellar-like objects (principally QSO). We find the population of radio galaxies to be mainly made by early-type galaxies with very red colours ($b_J-R$ up to $\sim 4$) and a radio-to-optical ratio $10^2\simlt r\simlt 10^4$. The contribution of starbursting objects is negligible. In general QSO show $r\simgt 10^4$ and $0\simlt b_J -R \simlt 1$. On the basis of the $R$ magnitudes, we estimate the sample of radio galaxies to be complete up to $z\simeq 0.3$. We can therefore divide the whole sample of radio sources into a low-z and high-z population. The low-z one includes the objects identified as galaxies in the APM survey, the high-z one includes sources either identified as QSO or with no optical counterpart for $b_J \le 20.5$. We find that radio galaxies are strongly clustered and highly biased tracers of the underlying mass distribution. Models for the angular correlation function $w(θ)$ show good agreement with the observations if we assume a bias factor $b\simeq 2$ at $z\simeq 0.3$.

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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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arXiv2000

The Redshift Distribution of FIRST Radio Sources at 1 mJy

We present spectra for a sample of radio sources from the FIRST survey, and use them to define the form of the redshift distribution of radio sources at mJy levels.We targeted 365 sources and obtained 46 redshifts (13 per cent of the sample). We find that our sample is complete in redshift measurement to R $\sim 18.6$, corresponding to $z\sim 0.2$. Early-type galaxies represent the largest subset (45 per cent) of the sample and have redshifts $0.15\la z \la 0.5$ ; late-type galaxies make up 15 per cent of the sample and have redshifts $0.05\la z \la 0.2$; starbursting galaxies are a small fraction ($\sim 6$ per cent), and are very nearby ($z\la 0.05$). Some 9 per cent of the population have Seyfert1/quasar-type spectra, all at $z\ga 0.8$, and there are 4 per cent are Seyfert2 type galaxies at intermediate redshifts ($z\sim 0.2$). Using our measurements and data from the Phoenix survey, we obtain an estimate for $N(z)$ at $S_{1.4 \rm {GHz}}\ge 1$ mJy and compare this with model predictions. At variance with previous conclusions, we find that the population of starbursting objects makes up $\la 5 $ per cent of the radio population at S $\sim 1$ mJy.

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arXiv1999

The Observed Evolution of Galaxy Clustering Vs. Epoch-Dependent Biasing Models

We study the observed evolution of galaxy clustering as a function of redshift. We find that the clustering of galaxies decreases as we go from observations of the local Universe to $z \sim 2$. On the other hand, clustering of the Lyman break galaxies at $z \sim 3$ is very strong, comparable to the clustering of present day galaxies. However there are three major factors to take into account while comparing clustering measurements coming from various surveys: the so-called ``scale-dependence'' effect, due to measurements being made at different scales; the ``type-selection'' effect introduced by the fact that different galaxy surveys select different populations which do not have the same clustering amplitudes; and the Malmquist bias. We correct for the first two effects and discuss the implications of Malmquist bias on the interpretation of the data at different $z$. Then we compare the observed galaxy clustering with models for the evolution of clustering in some fixed cosmologies. Correcting for the scale-dependence effect significantly reduces the discrepancies amongst different measurements. We interpret the observed clustering signal at high redshift as coming from objects which are highly biased with respect to the underlying distribution of mass; this is not the case for $z\simlt 2$ where measurements are compatible with the assumption of a much lower biasing level which only shows a weak dependence on $z$. Finally we propose a model-independent test that can be used to place a lower limit on the density parameter $Ω_0$.

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arXiv1999

Constraints on the Clustering, Biasing and Redshift Distribution of Radio Sources

We discuss how different theoretical predictions for the variance $σ^2$ of the distribution of radio sources can be matched to measurements from the FIRST survey at different flux limits. The predictions are given by the integration of models for the angular correlation function $w(θ)$ for three different functional forms of the redshift distribution $N(z)$, different spatial correlation functions and by different evolutions of the bias $b(z)$ with redshift. We also consider the two cases of open and flat Universes. Although the predicted $w(θ)$ show substantial differences due to differences in the $N(z)$'s, these differences are not significant compared to the uncertainties in the current observations. It turns out that the best fit is provided by models with constant biasing at all times, although the difference between models with epoch-independent bias and models with bias that evolves linearly with redshift is not very large. All models with strong evolution of bias with epoch are ruled out. As a further step we directly calculated $w_{obs}(θ)$ at 3mJy from the catalogue and matched it with our models for the angular correlation function in the hypothesis that the clustering signal comes from two different populations, namely AGN-powered sources and starbursting galaxies. The results are consistent with a scenario for hierarchical clustering where the fainter starbursting galaxies trace the mass at all epochs, while brighter AGN's are strongly biased, with $b(z)$ evolving linearly with redshift, as suggested by some theories of galaxy formation and evolution.

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arXiv1999

The Redshift Evolution of Clustering in the HDF

We present a correlation function analysis for the catalogue of photometric redshifts obtained from the Hubble Deep Field image by Fernandez-Soto et al., 1998. By dividing the catalogue into redshift bins of width $Δz=0.4$ we measured the angular correlation function $w(θ)$ as a function of redshift up to $z\sim 4.8$. From these measurements we derive the trend of the correlation length $r_0$. We find that $r_0(z)$ is roughly constant with look-back time up to $z \simeq 2$, and then increases to higher values at $z\simgt 2.4$. We estimate the values of $r_0$, assuming $ξ(r,z)=(r/r_0(z))^{-γ}$, $γ=1.8$ and different geometries. For $Ω_0=1$ we find $r_0(z=3)\simeq 7.00\pm 4.87 h^{-1}$ Mpc, in good agreement with the values obtained from analysis of the Lyman Break Galaxies.

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arXiv1998

Gravothermal Catastrophe in Anisotropic Spherical Systems

In this paper we investigate the gravothermal instability of spherical stellar systems endowed with a radially anisotropic velocity distribution. We focus our attention on the effects of anisotropy on the conditions for the onset of the instability and in particular we study the dependence of the spatial structure of critical models on the amount of anisotropy present in a system. The investigation has been carried out by the method of linear series which has already been used in the past to study the gravothermal instability of isotropic systems. We consider models described by King, Wilson and Woolley-Dickens distribution functions. In the case of King and Woolley-Dickens models, our results show that, for quite a wide range of amount of anisotropy in the system, the critical value of the concentration of the system (defined as the ratio of the tidal to the King core radius of the system) is approximately constant and equal to the corresponding value for isotropic systems. Only for very anisotropic systems the critical value of the concentration starts to change and it decreases significantly as the anisotropy increases and penetrates the inner parts of the system. For Wilson models the decrease of the concentration of critical models is preceded by an intermediate regime in which critical concentration increases, it reaches a maximum and then it starts to decrease. The critical value of the central potential always decreases as the anisotropy increases.

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arXiv1998

Variance and Skewness in the FIRST survey

We investigate the large-scale clustering of radio sources in the FIRST 1.4-GHz survey by analysing the distribution function (counts in cells). We select a reliable sample from the the FIRST catalogue, paying particular attention to the problem of how to define single radio sources from the multiple components listed. We also consider the incompleteness of the catalogue. We estimate the angular two-point correlation function $w(θ)$, the variance $Ψ_2$, and skewness $Ψ_3$ of the distribution for the various sub-samples chosen on different criteria. Both $w(θ)$ and $Ψ_2$ show power-law behaviour with an amplitude corresponding a spatial correlation length of $r_0 \sim 10 h^{-1}$Mpc. We detect significant skewness in the distribution, the first such detection in radio surveys. This skewness is found to be related to the variance through $Ψ_3=S_3(Ψ_2)^α$, with $α=1.9\pm 0.1$, consistent with the non-linear gravitational growth of perturbations from primordial Gaussian initial conditions. We show that the amplitude of variance and skewness are consistent with realistic models of galaxy clustering.

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