arXiv subjects

P. Santini

Publications and source records attributed to P. Santini.

At least 181 records · Page 10Linked to original sources

arXiv2009

A Comprehensive Study of Large Scale Structures in the GOODS-SOUTH Field up to z \sim 2.5

The aim of this paper is to identify and study the properties and galactic content of groups and clusters in the GOODS-South field up to z\sim2.5, and to analyse the physical properties of galaxies as a continuous function of environmental density up to high redshift. We use the deep (z850\sim26), multi-wavelength GOODS-MUSIC catalogue, which has a 15% of spectroscopic redshifts and accurate photometric redshifts for the remaining fraction. On these data, we apply a (2+1)D algorithm, previously developed by our group, that provides an adaptive estimate of the 3D density field. We support our analysis with simulations to evaluate the purity and the completeness of the cluster catalogue produced by our algorithm. We find several high density peaks embedded in larger structures in the redshift range 0.4-2.5. From the analysis of their physical properties (mass profile, M200, \sigmav, LX, U-B vs. B diagram), we derive that most of them are groups of galaxies, while two are poor clusters with masses of few times 10^14 Mo. For these two clusters we find, from the Chandra 2Ms data, an X-ray emission significantly lower than expected from their optical properties, suggesting that the two clusters are either not virialised or gas poor. We also analyse the dependance on environment of galaxy colours, luminosities, stellar masses, ages and star formations. We find that galaxies in high density regions are, on average, more luminous and massive than field galaxies up to z\sim 2. The fraction of red galaxies increases with luminosity and with density up to z\sim 1.2. At higher z this dependance on density disappears. The variation of galaxy properties as a function of redshift and density suggests that a significant change occurs at z\sim 1.5-2. (abridged)

astro-ph.CO↗

arXiv2009

Uncertainties and Systematic Effects on the estimate of stellar masses in high z galaxies

We discuss the uncertainties and the systematic effects that exist in the estimates of the stellar masses of high redshift galaxies, using broad band photometry, and how they affect the deduced galaxy stellar mass function. We use at this purpose the latest version of the GOODS-MUSIC catalog. In particular, we discuss the impact of different synthetic models, of the assumed initial mass function and of the selection band. Using Charlot & Bruzual 2007 and Maraston 2005 models we find masses lower than those obtained from Bruzual & Charlot 2003 models. In addition, we find a slight trend as a function of the mass itself comparing these two mass determinations with that from Bruzual & Charlot 2003 models. As consequence, the derived galaxy stellar mass functions show diverse shapes, and their slope depends on the assumed models. Despite these differences, the overall results and scenario remains unchanged. The masses obtained with the assumption of the Chabrier initial mass function are in average 0.24 dex lower than those from the Salpeter assumption, at all redshifts, causing a shift of galaxy stellar mass function of the same amount. Finally, using a 4.5 um-selected sample instead of a Ks-selected one, we add a new population of highly absorbed, dusty galaxies at z\simeq 2-3 of relatively low masses, yielding stronger constraints on the slope of the galaxy stellar mass function at lower masses.

astro-ph.CO↗

arXiv2008

The physical properties of Lyalpha emitting galaxies: not just primeval galaxies?

We have analyzed a sample of LBGs from z =3.5 to z=6 selected from the GOODS-S field as B,V and i-dropouts, and with spectroscopic observations showing that they have the Lyalpha line in emission. Our main aim is to investigate their physical properties and their dependence on the emission line characteristics, to shed light on the relation between galaxies with Lyalpha emission and the general LBG population.The objects were selected from their continuum colors and then spectroscopically confirmed by the GOODS collaboration and other campaigns. From the spectra we derived the line flux and EW. We then used U-band to mid-IR photometry from GOODS-MUSIC to derive the physical properties of the galaxies, such as total stellar mass, age and so on, through standard SED fitting techniques.Although most galaxies are fit by young stellar populations, a small but non negligible fraction has SEDs that require considerably older stellar component, up to 1 Gyr. There is no apparent relation between age and EW: some of the oldest galaxies have large EW, and should be also selected in narrow band surveys. Therefore not all Lyalpha emitters are primeval galaxies in the very early stages of formation,as is commonly assumed. We also find a large range of stellar populations, with masses from 5x10^8 Msol to 5x10^10 Msol and SFR from few to 60 Msol/yr. Although there is no correlation between mass and EW, we find a significant lack of massive galaxies with high EW, which could be explained if the most massive galaxies were more dusty and/or contained more neutral gas than less massive objects. Finally we find that more than half of the galaxies contain small but non negligible amounts of dust: the mean E(B-V) and the EW are well correlated, although with a large scatter, as already found at lower redshift

astro-ph↗

arXiv2008

Chasing highly obscured QSOs in the COSMOS field

(abridged) We take advantage of the deep Chandra and Spitzer coverage of a large area (more than 10 times the area covered by the Chandra deep fields, CDFs in the COSMOS field, to extend the search of highly obscured, Compton-thick active nuclei to higher luminosity. These sources have low surface density and large samples can be provided only through large area surveys, like the COSMOS survey. We analyze the X-ray properties of COSMOS MIPS sources with 24$μ$m fluxes higher than 550$μ$Jy. For the MIPS sources not directly detected in the Chandra images we produce stacked images in soft and hard X-rays bands. To estimate the fraction of Compton-thick AGN in the MIPS source population we compare the observed stacked count rates and hardness ratios to those predicted by detailed Monte Carlo simulations including both obscured AGN and star-forming galaxies. The density of lower luminosity Compton-thick AGN (logL(2-10keV)=43.5-44) at z=0.7--1.2 is $(3.7\pm1.1) \times10^{-5}$ Mpc$^{-3}$, corresponding to $\sim67%$ of that of X-ray selected AGN. The comparison between the fraction of infrared selected, Compton thick AGN to the X-ray selected, unobscured and moderately obscured AGN at high and low luminosity suggests that Compton-thick AGN follow a luminosity dependence similar to that discovered for Compton-thin AGN, becoming relatively rarer at high luminosities. We estimate that the fraction of AGN (unobscured, moderately obscured and Compton thick) to the total MIPS source population is $49\pm10%$, a value significantly higher than that previously estimated at similar 24$μ$m fluxes. We discuss how our findings can constrain AGN feedback models.

astro-ph↗

arXiv2008

Magnetization and spin dynamics of a Cr-based magnetic cluster: Cr$_{7}$Ni

We study the magnetization and the spin dynamics of the Cr$_7$Ni ring-shaped magnetic cluster. Measurements of the magnetization at high pulsed fields and low temperature are compared to calculations and show that the spin Hamiltonian approach provides a good description of Cr$_7$Ni magnetic molecule. In addition, the phonon-induced relaxation dynamics of molecular observables has been investigated. By assuming the spin-phonon coupling to take place through the modulation of the local crystal fields, it is possible to evaluate the decay of fluctuations of two generic molecular observables. The nuclear spin-lattice relaxation rate $1/T_1$ directly probes such fluctuations, and allows to determine the magnetoelastic coupling strength.

cond-mat.mtrl-sci↗

arXiv2008

Neutron spectroscopy and magnetic relaxation of the Mn$_6$ nanomagnets

Inelastic neutron scattering has been used to determine the microscopic Hamiltonian describing two high-spin variants of the high-anisotropy Mn$_6$ nanomagnet. The energy spectrum of both systems is characterized by the presence of several excited total-spin multiplets partially overlapping the S=12 ground multiplet. This implies that the relaxation processes of these molecules are different from those occurring in prototype giant-spin nanomagnets. In particular, we show that both the height of the energy barrier and resonant tunnelling processes are greatly influenced by low-lying excited total-spin multiplets.

cond-mat.mtrl-sci↗

arXiv2008

Towards a Theory of Chaos Explained as Travel on Riemann Surfaces

This paper presents a more complete version than hitherto published of our explanation of a transition from regular to irregular motions and more generally of the nature of a certain kind of deterministic chaos. To this end we introduced a simple model analogous to a three-body problem in the plane, whose general solution is obtained via quadratures all performed in terms of elementary functions. For some values of the coupling constants the system is isochronous and explicit formulas for the period of the solutions can be given. For other values, the motions are confined but feature aperiodic (in some sense chaotic) motions. This rich phenomenology can be understood in remarkable, quantitative detail in terms of travel on a certain (circular) path on the Riemann surfaces defined by the solutions of a related model considered as functions of a complex time. This model is meant to provide a paradigmatic first step towards a somewhat novel understanding of a certain kind of chaotic phenomena.

nlin.SI↗

arXiv2008

Large Scale Structures at High Redshift in the GOODS Field

We present a catalogue of overdensities in the GOODS-South field. We find various high density peaks that are embedded in structures diffused on the entire field, up to z ~ 2.5. The slope of their colour-magnitude relation does not show significative evolution with z. We find evidence that galaxies forming these structures are more massive than galaxies located in low density regions. We also analyse the variation of galaxy properties with the associated environmental density and we find that the segregation of red galaxies with density is stronger at low redshift and at high luminosities while it gets much weaker for increasing z.

astro-ph↗

arXiv2007

Searching for non Gaussian signals in the BOOMERanG 2003 CMB maps

We analyze the BOOMERanG 2003 (B03) 145 GHz temperature map to constrain the amplitude of a non Gaussian, primordial contribution to CMB fluctuations. We perform a pixel space analysis restricted to a portion of the map chosen in view of high sensitivity, very low foreground contamination and tight control of systematic effects. We set up an estimator based on the three Minkowski functionals which relies on high quality simulated data, including non Gaussian CMB maps. We find good agreement with the Gaussian hypothesis and derive the first limits based on BOOMERanG data for the non linear coupling parameter f_NL as -300<f_NL<650 at 68% CL and -800<f_NL<1050 at 95% CL.

astro-ph↗

arXiv2007

Unveiling obscured accretion in the Chandra Deep Field South

A large population of heavily obscured, Compton Thick AGNs is predicted by models of galaxy formation, models of Cosmic X-ray Background and by the ``relic'' super-massive black-hole mass function measured from local bulges. However, so far only a handful of Compton thick AGNs have been possibly detected using even the deepest Chandra and XMM surveys. Compton-thick AGNs can be recovered thanks to the reprocessing of the AGN UV emission in the infrared by selecting sources with AGN luminosity's in the mid-infrared and faint near-infrared and optical emission. To this purpose, we make use of deep HST, VLT, Spitzer and Chandra data on the Chandra Deep Field South to constrain the number of Compton thick AGN in this field. We show that sources with high 24$μ$m to optical flux ratios and red colors form a distinct source population, and that their infrared luminosity is dominated by AGN emission. Analysis of the X-ray properties of these extreme sources shows that most of them (80$\pm15%$) are indeed likely to be highly obscured, Compton thick AGNs. The number of infrared selected, Compton thick AGNs with 5.8$μ$m luminosity higher than $10^{44.2}$ erg s$^{-1}$ turns out to be similar to that of X-ray selected, unobscured and moderately obscured AGNs with 2-10 keV luminosity higher than $10^{43}$ erg s$^{-1}$ in the redshift bin 1.2-2.6. This ``factor of 2'' source population is exactly what it is needed to solve the discrepancies between model predictions and X-ray AGN selection.

astro-ph↗

arXiv2007

A Photometrically Detected Forming Cluster of Galaxies at Redshift 1.6 in the GOODS Field

We report the discovery of a localized overdensity at z~1.6 in the GOODS-South Field, presumably a poor cluster in the process of formation. The three-dimensional galaxy density has been estimated on the basis of well calibrated photometric redshifts from the multiband photometric GOODS-MUSIC catalog using the (2+1)D technique. The density peak is embedded in the larger scale overdensity of galaxies known to exist at z=1.61 in the area. The properties of the member galaxies are compared to those of the surrounding field and we found that the two populations are significantly different supporting the reality of the structure. The reddest galaxies, once evolved according to their best fit models, have colors consistent with the red sequence of lower redshift clusters. The estimated M_200 total mass of the cluster is in the range 1.3 x 10^14 - 5.7x 10^14 Msun, depending on the assumed bias factor b. An upper limit for the 2-10 keV X-ray luminosity, based on the 1Ms Chandra observations, is L_X=0.5 x 10^43 erg s^-1, suggesting that the cluster has not yet reached the virial equilibrium.

astro-ph↗

arXiv2007

Physical properties of z~4 LBGs: differences between galaxies with and without Ly-alpha emission

We have analysed the physical properties of z~4 Lyman Break Galaxies observed in the GOODS-S survey, in order to investigate the possible differences between galaxies where the Ly-alpha is present in emission, and those where the line is absent or in absorption. The objects have been selected from their optical color and then spectroscopically confirmed by Vanzella et al. (2005). From the public spectra we assessed the nature of the Ly-alpha emission and divided the sample into galaxies with Ly-alpha in emission and objects without Ly-alpha line (i.e. either absent or in absorption). We have then used the complete photometry, from U band to mid infrared from the GOODS-MUSIC database, to study the observational properties of the galaxies, such as UV spectral slopes and optical to mid-infrared colors, and the possible differences between the two samples. Finally through standard spectral fitting tecniques we have determined the physical properties of the galaxies, such as total stellar mass, stellar ages and so on, and again we have studied the possible differences between the two samples. Our results indicate that LBG with Ly-alpha in emission are on average a much younger and less massive population than the LBGs without Ly-alpha emission. Both populations are forming stars very actively and are relatively dust free, although those with line emission seem to be even less dusty on average. We briefly discuss these results in the context of recent models for the evolution of Lyman break galaxies and Ly-alpha emitters.

astro-ph↗

arXiv2007

ConvPhot: A Profile-Matching Algorithm for Precision Photometry

We describe in this paper a new, public software for accurate "PSF-matched" multiband photometry for images of different resolution and depth, that we have named ConvPhot, of which we analyse performances and limitations. It is designed to work when a high resolution image is available to identify and extract the objects, and colours or variations in luminosity are to be measured in another image of lower resolution but comparable depth. To maximise the usability of this software, we explicitly use the outputs of the popular SExtractor code, that is used to extract all objects from the high resolution "detection" image. The technique adopted by the code is essentially to convolve each object to the PSF of the lower resolution "measure" image, and to obtain the flux of each object by a global chi2 minimisation on such "measure" image. We remark that no a priori assumption is done on the shape of the objects. In this paper we provide a full description of the algorithm, a discussion of the possible systematic effects involved and the results of a set of simulations and validation tests that we have performed on real as well as simulated images. The source code of ConvPhot, written in C language under the GNU Public License, is released worldwide.

astro-ph↗

arXiv2005

Cosmological Parameters from the 2003 flight of BOOMERANG

We present the cosmological parameters from the CMB intensity and polarization power spectra of the 2003 Antarctic flight of the BOOMERANG telescope. The BOOMERANG data alone constrains the parameters of the $Λ$CDM model remarkably well and is consistent with constraints from a multi-experiment combined CMB data set. We add LSS data from the 2dF and SDSS redshift surveys to the combined CMB data set and test several extensions to the standard model including: running of the spectral index, curvature, tensor modes, the effect of massive neutrinos, and an effective equation of state for dark energy. We also include an analysis of constraints to a model which allows a CDM isocurvature admixture.

astro-ph↗

arXiv2005

A Measurement of the CMB Spectrum from the 2003 Flight of BOOMERANG

We report measurements of the CMB polarization power spectra from the January 2003 Antarctic flight of BOOMERANG. The primary results come from six days of observation of a patch covering 0.22% of the sky centered near R.A. = 82.5 deg., Dec= -45 deg. The observations were made using four pairs of polarization sensitive bolometers operating in bands centered at 145 GHz. Using two independent analysis pipelines, we measure a non-zero signal in the range 100< l <1000 with a significance 4.8-sigma, a 2-sigma upper limit of 8.6 uK^2 for any contribution, and a 2-sigma upper limit of 7.0 uK^2 for the spectrum. Estimates of foreground intensity fluctuations and the non-detection of and signals rule out any significant contribution from galactic foregrounds. The results are consistent with a Lambda-CDM cosmology seeded by adiabatic perturbations. We note that this is the first detection of CMB polarization with bolometric detectors.

astro-ph↗

arXiv2005

Molecular engineering of antiferromagnetic rings for quantum computation

The substitution of one metal ion in a Cr-based molecular ring with dominant antiferromagnetic couplings allows to engineer its level structure and ground-state degeneracy. Here we characterize a Cr7Ni molecular ring by means of low-temperature specific-heat and torque-magnetometry measurements, thus determining the microscopic parameters of the corresponding spin Hamiltonian. The energy spectrum and the suppression of the leakage-inducing S-mixing render the Cr7Ni molecule a suitable candidate for the qubit implementation, as further substantiated by our quantum-gate simulations.

cond-mat.mes-hall↗

arXiv2005

Quantum-gate implementation in permanently coupled AF spin rings without need of local fields

We propose a scheme for the implementation of quantum gates which is based on the qubit encoding in antiferromagnetic molecular rings. We show that a proper engineering of the intercluster link would result in an effective coupling that vanishes as far as the system is kept in the computational space, while it is turned on by a selective excitation of specific auxiliary states. These are also shown to allow the performing of single- and two-qubit gates without an individual addressing of the rings by means of local magnetic fields.

cond-mat.other↗

arXiv2005

NMR as a probe of the relaxation of the magnetization in magnetic molecules

We investigate the time autocorrelation of the molecular magnetization $M(t)$ for three classes of magnetic molecules (antiferromagnetic rings, grids and nanomagnets), in contact with the phonon heat bath. For all three classes, we find that the exponential decay of the fluctuations of $M(t)$, associated with the irreversible exchange of energy with the heat bath, is characterized by a single characteristic time $τ(T,B)$ for not too high temperature $T$ and field $B$. This is reflected in a nearly single-lorentzian shape of the spectral density of the fluctuations. We show that such fluctuations are effectively probed by NMR, and that our theory explains the recent phenomenological observation by Baek et al. (PRB70, 134434) that the Larmor-frequency dependence of $1/T_1$ data in a large number of AFM rings fits to a single-lorentzian form.

cond-mat.mtrl-sci↗