arXiv subjects

P. Saracco

Publications and source records attributed to P. Saracco.

At least 55 records · Page 3Linked to original sources

arXiv2010

Colour gradients in normal and compact early-type galaxies at 1<z<2

We have derived colour gradients for a sample of 20 early-type galaxies (ETGs) at 1 < z_spec < 2 selected from the GOODS-South field. The sample includes both normal ETGs (13) having effective radii comparable to the mean radius of local ones and compact ETGs (7) having effective radii from two to six times smaller. Colour gradients have been derived in the F606W-F850LP bands (UV-U rest-frame) taking advantage of the ultradeep HST-ACS observations covering this field and providing a spatial resolution of about 0.8 kpc at the redshift of the galaxies. Despite of the narrow wavelength baseline covered (1000 Angstrom), sampling approximatively the emission dominated by the same stellar population, we detect significant radial colour variations in 50 per cent of our sample. In particular, we find five ETGs with positive colour gradients (cores bluer than the external regions), and five galaxies with negative colour gradients (cores redder than the external regions), as commonly observed in the local Universe. These results show that at 1 < z < 2, when the Universe was only 3-4 Gyr old, ETGs constituted a composite population of galaxies whose different assembly histories have generated different stellar distributions with the bluest stellar population either in the center or in the outskirts as well as throughout the whole galaxy. Moreover, we find that compact galaxies seem to preferentially show a blue cores while moving towards normal galaxies, central stellar populations become progressively redder. Nonetheless, the narrow baseline covered together with the low statistics still prevent us to be conclusive about a possible physical connection between colour gradients and the degree of compactness of high-z ETGs.

astro-ph.CO↗

arXiv2010

The number density of superdense early-type galaxies at 1<z<2 and the local cluster galaxies

Many of the early-type galaxies observed so far at z>1 turned out to have smaller radii with respect to that of a typical present-day early-type galaxy with comparable mass. This has generated the conviction that in the past early-type galaxies were more compact, hence denser, and that as a consequence, they should have increased their radius across the time to reconcile with the present-day ones. However, observations have not yet established whether the population of early-types in the early Universe was fully represented by compact galaxies nor if they were so much more numerous than in the present-day Universe to require an evolution of their sizes. Here we report the results of a study based on a complete sample of 34 early-type galaxies at 0.9 ~1.5 suggests that their build-up taken place in the first 3-4 Gyr, followed distinct paths. Also, we find that the number density of compact early-types at ~1.5 is consistent with the lower limits of the local number density of compact early-types derived from local clusters of galaxies. The similar number of compact early-types found in the early and in the present day Universe frustrates the hypothesized effective radius evolution while provides evidence that also compact ETGs were as we see them today 9-10 Gyr ago. Finally, the fact that (at least) most of the compact ETGs at high-z are accounted for by compact early-types in local cluster of galaxies implies that the former are the direct progenitors of the compact early-type cluster galaxies establishing a direct link between environment and early phases of assembly of ETGs.

astro-ph.CO↗

arXiv2010

Superdense and normal early-type galaxies at 1<z<2

We combined proprietary and archival HST observations to collect a sample of 62 early-type galaxies (ETGs) at 0.9<z<2 with spectroscopic confirmation of their redshift and spectral type. The whole sample is covered by ACS or NICMOS observations and partially by Spitzer and AKARI observations. We derived morphological parameters by fitting their HST light profiles and physical parameters by fitting their spectral energy distributions. The study of the size-mass and the size-luminosity relations of these early-types shows that a large fraction of them (~50) follows the local relations. These 'normal' ETGs are not smaller than local counterparts with comparable mass. The remaining half of the sample is composed of compact ETGs with sizes (densities) 2.5-3 (15-30) times smaller (higher) than local counterparts and, most importantly, than the other normal ETGs at the same redshift and with the same stellar mass. This suggests that normal and superdense ETGs at z~2 come from different histories of mass assembly.

astro-ph.CO↗

arXiv2010

New models for PIXE simulation with Geant4

Particle induced X-ray emission (PIXE) is a physical effect that is not yet adequately modelled in Geant4. The current status as in Geant4 9.2 release is reviewed and new developments are described. The capabilities of the software prototype are illustrated in application to the shielding of the X-ray detectors of the eROSITA telescope on the upcoming Spectrum-X-Gamma space mission.

physics.comp-ph↗

arXiv2010

R&D on co-working transport schemes in Geant4

A research and development (R&D) project related to the extension of the Geant4 toolkit has been recently launched to address fundamental methods in radiation transport simulation. The project focuses on simulation at different scales in the same experimental environment; this problem requires new methods across the current boundaries of condensed-random-walk and discrete transport schemes. The new developments have been motivated by experimental requirements in various domains, including nanodosimetry, astronomy and detector developments for high energy physics applications.

physics.comp-ph↗

arXiv2010

Design and performance evaluations of generic programming techniques in a R&D prototype of Geant4 physics

A R&D project has been recently launched to investigate Geant4 architectural design in view of addressing new experimental issues in HEP and other related physics disciplines. In the context of this project the use of generic programming techniques besides the conventional object oriented is investigated. Software design features and preliminary results from a new prototype implementation of Geant4 electromagnetic physics are illustrated. Performance evaluations are presented. Issues related to quality assurance in Geant4 physics modelling are discussed.

physics.comp-ph↗

arXiv2009

Research in Geant4 electromagnetic physics design, and its effects on computational performance and quality assurance

The Geant4 toolkit offers a rich variety of electromagnetic physics models; so far the evaluation of this Geant4 domain has been mostly focused on its physics functionality, while the features of its design and their impact on simulation accuracy, computational performance and facilities for verification and validation have not been the object of comparable attention yet, despite the critical role they play in many experimental applications. A new project is in progress to study the application of new design concepts and software techniques in Geant4 electromagnetic physics, and to evaluate how they can improve on the current simulation capabilities. The application of a policy-based class design is investigated as a means to achieve the objective of granular decomposition of processes; this design technique offers various advantages in terms of flexibility of configuration and computational performance. The current Geant4 physics models have been re-implemented according to the new design as a pilot project. The main features of the new design and first results of performance improvement and testing simplification are presented; they are relevant to many Geant4 applications, where computational speed and the containment of resources invested in simulation production and quality assurance play a critical role.

physics.comp-ph↗

arXiv2009

Geant4-related R&D for new particle transport methods

A R&D project has been launched in 2009 to address fundamental methods in radiation transport simulation and revisit Geant4 kernel design to cope with new experimental requirements. The project focuses on simulation at different scales in the same experimental environment: this set of problems requires new methods across the current boundaries of condensed-random-walk and discrete transport schemes. An exploration is also foreseen about exploiting and extending already existing Geant4 features to apply Monte Carlo and deterministic transport methods in the same simulation environment. An overview of this new R&D associated with Geant4 is presented, together with the first developments in progress.

physics.comp-ph↗

arXiv2009

Recent Developments on PIXE Simulation with Geant4

Particle induced X-ray emission (PIXE) is an important physical effect that is not yet adequately modelled in Geant4. This paper provides a critical analysis of the problem domain associated with PIXE simulation and describes a set of software developments to improve PIXE simulation with Geant4. The capabilities of the developed software prototype are illustrated and applied to a study of the passive shielding of the X-ray detectors of the German eROSITA telescope on the upcoming Russian Spectrum-X-Gamma space mission.

physics.comp-ph↗

arXiv2009

Inter-Comparison and Validation of Geant4 Photon Interaction Models

A R&D project, named Nano5, has been recently launched to study an architectural design in view of addressing new experimental issues related to particle transport in high energy physics and other related physics disciplines with Geant4. In this frame, the first step has involved the redesign of the photon interaction models currently available in Geant4; this task has motivated a thorough investigation of the physics and computational features of these models, whose first results are presented here.

physics.comp-ph↗

arXiv2009

Validation of fluorescence transition probability calculations

A systematic and quantitative validation of the K and L shell X-ray transition probability calculations according to different theoretical methods has been performed against experimental data. This study is relevant to the optimization of data libraries used by software systems, namely Monte Carlo codes, dealing with X-ray fluorescence. The results support the adoption of transition probabilities calculated according to the Hartree-Fock approach, which manifest better agreement with experimental measurements than calculations based on the Hartree-Slater method.

physics.comp-ph↗

arXiv2008

The population of early-type galaxies at 1<z<2 - New clues on their formation and evolution

[Abridged]We present the morphological analysis based on HST-NICMOS observations in the F160W filter of a sample of 32 early-type galaxies (ETGs) at 1<z<2 with spectroscopic confirmation of their redshift and spectral type. We find that ETGs at are composed of two distinct populations, an older population (oETGs) and a younger population (yETGs) whose mean ages differ by about 1.5-2 Gyr. Young ETGs are not denser than local ones since they follow the size-mass relation of local ETGs and luminosity evolution brings them onto the local Kormendy and size-luminosity relations. On the constrary, old ETGs do not follow the size-mass relation of local ETGs and luminosity evolution does not account for the discrepancy they show with respect to the local size-luminosity and Kormendy relations. An increase of their effective radius R_e by a factor 2.5-3 (a density decrease by a factor 15-30) from z~1.5-2 to z~0 is required to bring these galaxies onto the local scaling relations. The different behaviour shown by the two populations implies different formation and evolution scenarios. The older population must have formed at higher-z in a sort of dissipative gas-rich collapse ables to produce remnants which at z~2 are old and compact. The size evolution they must experience from z~2 to z~0 has to leave unchanged their mass to not exceed the local number of high-mass ETGs. Thus, major merging cannot fit this requirement. The younger population of ETGs can be formed later through subsequent episodes of merging which increased progressively their size and assembled their mass down to z~2. At z<2 they evolve purely in luminosity since episodes of major merging would bring them far from the local scaling relations.

astro-ph↗

arXiv2008

Probing the Nature of EROs through ASTRO-F/AKARI observations

We present a preliminary analysis of ASTRO-F data of a complete sample of ~150 EROs (R-K>5) down to K(Vega)<19, for which reliable photometric redshifts are available, in the range 0.8<z<2, selected over two fields (S7 and S2) of the MUNICS survey. The area covered is about 420 arcmin^2. We have imaged this area with AKARI telescope in N3 (3.4 micron), N60 (65 micron) and WL (150 micron) down to 12 microJy in the N3 filter, in order to detect the rest frame H or K-band emission, thus providing an excellent sampling of the SED of our EROs. From a first analysis we have an identification rate of ~63% in the N3 filter over the S7 field. These data allow us to distinguish starburst from passive early type phenomena, to meseaure the SFR of the starburst component and to constrain the mass assembly of early type galaxies.

astro-ph↗

arXiv2008

The evolution of the morphological scale of early-type galaxies since z=2

We present the morphological analysis based on HST-NICMOS observations in the F160W filter of a sample of 30 early-type galaxies spectroscopically confirmed at 1.2<z<2. We derive the effective radius R_e and the mean surface brightness mu_e of galaxies in the rest-frame R-band. We find that early-types at z~1.5 are characterized by a surface brightness (SB) much higher then their local counterparts with comparable R_e. In particular, we find that the mean SB of these early-types should get fainter by ~2.5 mag from z~1.5 to z=0 to match the SB of the local early-types with comparable R_e. This evolution exceeds by a factor two the luminosity evolution expected for early-types in this redshift range and more than a factor three the one derived from the observed luminosity function of galaxies. Consequently, an evolution of the effective radius R_e from the epoch of their formation towards z=0 has to be invoked and the hypothesis of fixed size rejected.

astro-ph↗

arXiv2006

The Kormendy relation of massive elliptical galaxies at z~1.5. Evidence for size evolution ?

We present the morphological analysis based on HST-NIC2 (0.075 arcsec/pixel) images in the F160W filter of a sample of 9 massive field (> 10^{11} M_\odot) galaxies spectroscopically classified as early-types at 1.2 within R_e of our galaxies and we compared them with those of early-types at lower redshifts. We find that the surface brightness of our galaxies should get fainter by 2.5 mag from z~1.5 to z~0 to match the surface brightness of the local ellipticals with comparable R_e, i.e. the local Kormendy relation. Luminosity evolution without morphological changes can only explain half of this effect, as the maximum dimming expected for an elliptical galaxy is ~1.6 mag in this redshift range. Thus, other parameters, possibly structural, may undergo evolution and play an important role in reconciling models and observations. Hypothesizing an evolution of the effective radius of galaxies we find that R_e should increase by a factor 1.5 from z~1.5 to z~0.

astro-ph↗

arXiv2006

GRB 050223: A dark GRB in a dusty starburst galaxy

Aims: We aim at detecting and determining the properties of the host galaxy of the dark GRB 050223. Methods: We use VLT optical/NIR images coupled to Swift X-ray positioning, and optical spectra of the host galaxy to measure its properties. Results: We find a single galaxy within the Swift error box of GRB 050223. It is located at z = 0.584 and its luminosity is L ~ 0.4 L*. Emission lines in the galaxy spectrum imply an intrinsic SFR > 7 Msun/yr, and a large extinction A_V > 2 mag within it. We also detect absorption lines, which reveal an underlying stellar population with an age between 40 Myr and 1.5 Gyr. Conclusions: The identification of a host galaxy with atypical properties using only the X-ray transient suggests that a bias may be present in the former sample of host galaxies. Dust obscuration together with intrinsic faintness are the most probable causes for the darkness of this burst.

astro-ph↗

arXiv2006

Extremely compact massive galaxies at z~1.4

The optical rest-frame sizes of 10 of the most massive (~5x10^{11}h_{70}^{-2}M_sun) galaxies found in the near-infrared MUNICS survey at 1.2<z<1.7 are analysed. Sizes were estimated both in the J and K' filters. These massive galaxies are at least a factor of 4_{-1.0}^{+1.9} (+-1 sigma) smaller in the rest-frame V-band than local counterparts of the same stellar mass. Consequently, the stellar mass density of these objects is (at least) 60 times larger than massive ellipticals today. Although the stellar populations of these objects are passively fading, their structural properties are rapidly changing since that redshift. This observational fact disagrees with a scenario where the more massive and passive galaxies are fully assembled at z~1.4 (i.e. a monolithic scenario) and points towards a dry merger scenario as the responsible mechanism for the subsequent evolution of these galaxies.

astro-ph↗

arXiv2006

An X-ray bright ERO hosting a type 2 QSO

We present the XMM-Newton and the optical-VLT spectra along with the optical and the near-infrared photometric data of one of the brightest X-ray (F(2-10 keV)~1e-13 erg/s cm^2) extremely red objects (R-K>=5) discovered so far. The source, XBSJ0216-0435, belongs to the XMM-Newton Bright Serendipitous Survey and it has extreme X-ray-to-optical (~220) and X-ray-to-near-infrared (~60) flux ratios. Thanks to its brightness, the X-ray statistics are good enough for an accurate spectral analysis by which the presence of an X-ray obscured (NH>1e22 cm^-2) QSO (L(2-10 keV)=4e45 erg/s) is determined. A statistically significant (~99%) excess around 2 keV in the observed-frame suggests the presence of an emission line. By assuming that this feature corresponds to the iron Kalpha line at 6.4 keV, a first estimate of the redshift of the source is derived (z_x~2). The presence of a high redshift QSO2 has been finally confirmed through dedicated VLT optical spectroscopic observations (z_o=1.985+/-0.002). This result yields to an optical validation of a new X-ray Line Emitting Object (XLEO) for which the redshift has been firstly derived from the X-ray data. XBSJ0216-0435 can be considered one of the few examples of X-ray obscured QSO2 at high redshift for which a detailed X-ray and optical spectral analysis has been possible. The spectral energy distribution from radio to X-rays is also presented. Finally from the near-infrared data the luminosity and the stellar mass of the host galaxy has been estimated finding a new example of the coexistence at high-z between massive galaxies and powerful QSOs.

astro-ph↗