arXiv subjects

A. Lapi

Publications and source records attributed to A. Lapi.

At least 109 records · Page 6Linked to original sources

arXiv2011

Herschel-ATLAS Galaxy Counts and High Redshift Luminosity Functions: The Formation of Massive Early Type Galaxies

Exploiting the Herschel-ATLAS Science Demonstration Phase (SDP) survey data, we have determined the luminosity functions (LFs) at rest-frame wavelengths of 100 and 250 micron and at several redshifts z>1, for bright sub-mm galaxies with star formation rates (SFR) >100 M_sun/yr. We find that the evolution of the comoving LF is strong up to z~2.5, and slows down at higher redshifts. From the LFs and the information on halo masses inferred from clustering analysis, we derived an average relation between SFR and halo mass (and its scatter). We also infer that the timescale of the main episode of dust-enshrouded star formation in massive halos (M_H>3*10^12 M_sun) amounts to ~7*10^8 yr. Given the SFRs, which are in the range 10^2-10^3 M_sun/yr, this timescale implies final stellar masses of order of 10^11-10^12 M_sun. The corresponding stellar mass function matches the observed mass function of passively evolving galaxies at z>1. The comparison of the statistics for sub-mm and UV selected galaxies suggests that the dust-free, UV bright phase, is >10^2 times shorter than the sub-mm bright phase, implying that the dust must form soon after the onset of star formation. Using a single reference Spectral Energy Distribution (SED; the one of the z~2.3 galaxy SMM J2135-0102), our simple physical model is able to reproduce not only the LFs at different redshifts > 1 but also the counts at wavelengths ranging from 250 micron to ~1 mm. Owing to the steepness of the counts and their relatively broad frequency range, this result suggests that the dispersion of sub-mm SEDs of z>1 galaxies around the reference one is rather small.

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arXiv2010

Non Thermal Support for the Outer Intracluster Medium

We submit that non thermalized support for the outer intracluster medium in relaxed galaxy clusters is provided by turbulence, driven by inflows of intergalactic gas across the virial accretion shocks. We expect this component to increase briskly during the cluster development for z<1/2, due to three factors. First, the accretion rates of gas and dark matter subside, when they feed on the outer wings of the initial perturbations in the accelerating Universe. Second, the infall speeds decrease across the progressively shallower gravitational potential at the shock position. Third, the shocks eventually weaken, and leave less thermal energy to feed the intracluster entropy, but relatively more bulk energy to drive turbulence into the outskirts. The overall outcome from these factors is physically modeled and analytically computed; thus we ascertain how these concur in setting the equilibrium of the outer intracluster medium, and predict how the observables in X rays and microwaves are affected, so as to probe the development of outer turbulence over wide cluster samples. By the same token, we quantify the resulting negative bias to be expected in the total mass evaluated from X-ray measurements.

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arXiv2010

Dark Matter Halos: The Dynamical Basis of Effective Empirical Models

We investigate the dynamical basis of the classic empirical models (specifically, Sersic-Einasto and generalized NFW) that are widely used to describe the distributions of collisionless matter in galaxies. We submit that such a basis is provided by our α-profiles, shown to constitute solutions of the Jeans dynamical equilibrium with physical boundary conditions. We show how to set the parameters of the empirical in terms of the dynamical models; we find the empirical models, and specifically Sersic-Einasto, to constitute a simple and close approximation to the dynamical models. Finally, we discuss how these provide an useful baseline for assessing the impact of the small-scale dynamics that may modulate the density slope in the central galaxy regions.

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arXiv2010

Cosmic Evolution of Size and Velocity Dispersion for Early Type Galaxies

[abridged] Massive, passively evolving galaxies at redshifts z>1 exhibit on the average physical sizes smaller by factors ~3 than local early type galaxies (ETGs) endowed with the same stellar mass. Small sizes are in fact expected on theoretical grounds, if dissipative collapse occurs. Recent results show that the size evolution at z<1 is limited to less than 40%, while most of the evolution occurs at z>1, where both compact and already extended galaxies are observed and the scatter in size is remarkably larger than locally. The presence at high z of a significant number of ETGs with the same size as their local counterparts as well as of ETGs with quite small size, points to a timescale to reach the new, expanded equilibrium configuration of less than the Hubble time. We demonstrate that the projected mass of compact, high-z galaxies and that of local ETGs within the *same physical radius*, the nominal half-luminosity radius of high-z ETGs, differ substantially, in that the high-z ETGs are on the average significantly denser. We propose that quasar activity, which peaks at z~2, can remove large amounts of gas from central galaxy regions on a timescale shorter than of the dynamical one, triggering a puffing up of the stellar component at constant stellar mass; in this case the size increase goes together with a decrease of the central mass. The size evolution is expected to parallel that of the quasars and the inverse hierarchy, or downsizing, seen in the quasar evolution is mirrored in the size evolution.

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arXiv2010

Herschel-ATLAS: blazars in the SDP field

To investigate the poorly constrained sub-mm counts and spectral properties of blazars we searched for these in the Herschel-ATLAS (H-ATLAS) science demostration phase (SDP) survey catalog. We cross-matched 500$μ$m sources brighter than 50 mJy with the FIRST radio catalogue. We found two blazars, both previously known. Our study is among the first blind blazar searches at sub-mm wavelengths, i.e., in the spectral regime where little is still known about the blazar SEDs, but where the synchrotron peak of the most luminous blazars is expected to occur. Our early results are consistent with educated extrapolations of lower frequency counts and question indications of substantial spectral curvature downwards and of spectral upturns at mm wavelengths. One of the two blazars is identified with a Fermi/LAT $γ$-ray source and a WMAP source. The physical parameters of the two blazars are briefly discussed.These observations demonstrate that the H-ATLAS survey will provide key information about the physics of blazars and their contribution to sub-mm counts.

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arXiv2010

Probing the Astrophysics of Cluster Outskirts

In galaxy clusters the entropy distribution of the IntraCluster Plasma modulates the latter's equilibrium within the Dark Matter gravitational wells, as rendered by our Supermodel. We argue the entropy production at the boundary shocks to be reduced or terminated as the accretion rates of DM and intergalactic gas peter out; this behavior is enforced by the slowdown in the outskirt development at late times, when the Dark Energy dominates the cosmology while the outer wings of the initial perturbation drive the growth. In such conditions, we predict the ICP temperature profiles to steepen into the cluster outskirts. The detailed expectations from our simple formalism agree with the X-ray data concerning five clusters whose temperature profiles have been recently measured out to the virial radius. We predict steep temperature declines to prevail in clusters at low redshift, tempered only by rich environs including adjacent filamentary structures.

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arXiv2010

SAGACE: the Spectroscopic Active Galaxies And Clusters Explorer

The SAGACE experiment consists of a mm/sub-mm telescope with a 3-m diameter primary mirror, coupled to a cryogenic multi-beam differential spectrometer. SAGACE explores the sky in the 100-760 GHz frequency range, using four diffraction-limited bolometer arrays. The instrument is designed to perform spectroscopic surveys of the Sunyaev-Zeldovich effects of thousands of galaxy clusters, of the spectral energy distribution of active galactic nuclei, and of the [CII] line of a thousand galaxies in the redshift desert. In 2008 a full phase-A study for a national small mission was completed and delivered to the Italian Space Agency (ASI). We have shown that taking advantage of the differential operation of the Fourier Transform Spectrometer, this ambitious instrument can operate from a Molniya orbit, and can be built and operated within the tight budget of a small mission.

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arXiv2009

Gamma Rays from Annihilations at the Galactic Center in a Physical Dark Matter Distribution

We discuss the gamma-ray signal to be expected from dark matter (DM)annihilations at the Galactic Center. To describe the DM distribution in the Galactic halo we base on the Jeans equation for self-gravitating, anisotropic equilibria. In solving the Jeans equation, we adopt the specific correlation between the density ρ(r) and the velocity dispersion σ^2_r(r) expressed by the powerlaw behavior of the DM `entropy' K= σ_r^2/ρ^{2/3} ~ r^αwith α~ 1.25-1.3. Indicated (among others) by several recent N-body simulations, this correlation is privileged by the form of the radial pressure term in the Jeans equation, and yields a main body profile consistent with the classic self-similar development of DM halos. In addition, we require the Jeans solutions to satisfy regular boundary conditions both at the center (finite pressure, round gravitational potential) and in the outskirts (finite overall mass). With these building blocks we derive physical solutions, dubbed `α-profiles'. We find the one with α=1.25, suitable for the Galaxy halo, to be intrinsically flatter at the center relative to the empirical NFW formula, yet steeper than the empirical Einasto profile. So on scales of 10^{-1} deg it yields annihilation fluxes lower by a factor 5 than the former yet higher by a factor 10 than the latter; such fluxes will eventually fall within the reach of the Fermi satellite. We show the effectiveness of the α-profile in relieving the astrophysical uncertainties related to the macroscopic DM distribution, and discuss its expected performance as a tool instrumental to interpret the upcoming gamma-ray data in terms of DM annihilation.

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arXiv2009

Two phase galaxy formation: The Gas Content of Normal Galaxies

We investigate the atomic (HI) and molecular (H_2) Hydrogen content of normal galaxies by combining observational studies linking galaxy stellar and gas budgets to their host dark matter (DM) properties, with a physically grounded galaxy formation model. This enables us to analyse empirical relationships between the virial, stellar, and gaseous masses of galaxies and explore their physical origins. Utilising a semi-analytic model (SAM) to study the evolution of baryonic material within evolving DM halos, we study the effects of baryonic infall and various star formation and feedback mechanisms on the properties of formed galaxies using the most up-to-date physical recipes. We find that in order to significantly improve agreement with observations of low-mass galaxies we must suppress the infall of baryonic material and exploit a two-phase interstellar medium (ISM), where the ratio of HI to H_2 is determined by the galactic disk structure. Modifying the standard Schmidt-Kennicutt star formation law, which acts upon the total cold gas in galaxy discs and includes a critical density threshold, and employing a star formation law which correlates with the H_2 gas mass results in a lower overall star formation rate. This in turn, allows us to simultaneously reproduce stellar, HI and H_2 mass functions of normal galaxies.

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arXiv2009

The Herschel ATLAS

The Herschel ATLAS is the largest open-time key project that will be carried out on the Herschel Space Observatory. It will survey 510 square degrees of the extragalactic sky, four times larger than all the other Herschel surveys combined, in five far-infrared and submillimetre bands. We describe the survey, the complementary multi-wavelength datasets that will be combined with the Herschel data, and the six major science programmes we are undertaking. Using new models based on a previous submillimetre survey of galaxies, we present predictions of the properties of the ATLAS sources in other wavebands.

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arXiv2009

Supermodel Analysis of Galaxy Clusters

[abridged] We present the analysis of the X-ray brightness and temperature profiles for six clusters belonging to both the Cool Core and Non Cool Core classes, in terms of the Supermodel (SM) developed by Cavaliere, Lapi & Fusco-Femiano (2009). Based on the gravitational wells set by the dark matter halos, the SM straightforwardly expresses the equilibrium of the IntraCluster Plasma (ICP) modulated by the entropy deposited at the boundary by standing shocks from gravitational accretion, and injected at the center by outgoing blastwaves from mergers or from outbursts of Active Galactic Nuclei. The cluster set analyzed here highlights not only how simply the SM represents the main dichotomy Cool vs. Non Cool Core clusters in terms of a few ICP parameters governing the radial entropy run, but also how accurately it fits even complex brightness and temperature profiles. For Cool Core clusters like A2199 and A2597, the SM with a low level of central entropy straightforwardly yields the characteristic peaked profile of the temperature marked by a decline toward the center, without requiring currently strong radiative cooling and high mass deposition rates. Non Cool Core clusters like A1656 require instead a central entropy floor of a substantial level, and some like A2256 and even more A644 feature structured temperature profiles that also call for a definite floor extension; in such conditions the SM accurately fits the observations, and suggests that in these clusters the ICP has been just remolded by a merger event, in the way of a remnant cool core. The SM also predicts that dark matter halos with high concentration should correlate with flatter entropy profiles and steeper brightness in the outskirts; this is indeed the case with A1689.

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arXiv2009

Two-Phase Galaxy Formation

We propose and test a scenario for the assembly and evolution of luminous matter in galaxies which substantially differs from that adopted by other semianalytic models. As for the dark matter (DM), we follow the detailed evolution of halos within the canonical LCDM cosmology using standard Montecarlo methods. However, when overlaying prescriptions for baryon evolution, we take into account an effect pointed out in the past few years by a number of studies mostly based on intensive N-body simulations, namely that typical halo growth occurs in two phases: an early, fast collapse phase featuring several major merger events, followed by a late, quiescent accretion onto the halo outskirts. We propose that the two modes of halo growth drive two distinct modes for the evolution of baryonic matter, favoring the development of the spheroidal and disc components of galaxies, respectively. We test this idea using the semianalytic technique. Our galaxy formation model envisages an early coevolution of spheroids and the central supermassive black holes, already tested in our previous works, followed by a relatively quiescent growth of discs around the preformed spheroids. In this exploratory study, we couple our model to the spectrophotometric code GRASIL, and compare our results on several properties of the local galaxy population with observations, Finding an encouraging agreement.

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arXiv2009

Galaxy Clusters, a Novel Look at Diffuse Baryons Withstanding Dark Matter Gravity

[abridged] The equilibria of the intracluster plasma (ICP) and of the gravitationally dominant dark matter (DM) are governed by the hydrostatic and the Jeans equation. Jeans, with the DM `entropy' set to K ~ r^αand α~ 1.25 - 1.3 applying from groups to rich clusters, yields our radial α-profiles. In the ICP the entropy run k(r) is mainly shaped by shocks, as steadily set by supersonic accretion of gas at the cluster boundary, and intermittently driven from the center by merging events or by AGNs; the resulting equilibrium is described by the exact yet simple formalism constituting our ICP Supermodel. With a few parameters, this accurately represents the runs of density n(r) and temperature T(r) as required by recent X-ray data on surface brightness and spectroscopy for both cool core (CC) and non cool core (NCC) clusters; the former are marked by a middle temperature peak, whose location is predicted from rich clusters to groups. The Supermodel inversely links the inner runs of n(r) and T(r), and highlights their central scaling with entropy n_c ~ k_c^-1 and T_c ~ k_c^0.35, to yield radiative cooling times t_c ~ 0.3 (k_c/15 keV cm^2)^1.2 Gyr. We discuss the stability of the central values so focused either in CC and NCC clusters. From the Supermodel we derive as limiting cases the classic polytropic β-models, and the `mirror' model with T(r) ~ σ^2(r) suitable for NCC and CC clusters, respectively; these highlight how the ICP temperature T(r) tends to mirror the DM velocity dispersion σ^2(r) away from entropy injections. Finally, we discuss how the Supermodel connects information derived from X-ray and gravitational lensing observations.

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arXiv2009

Structure and History of Dark Matter Halos Probed with Gravitational Lensing

We test with gravitational lensing data the dark matter (DM) halos embedding the luminous baryonic component of galaxy clusters; our benchmark is provided by their two-stage cosmogonical development that we compute with its variance, and by the related 'α-profiles' we derive. The latter solve the Jeans equation for the self-gravitating, anisotropic DM equilibria, and yield the radial runs of the density ρ(r) and the velocity dispersion σ_r^2(r) in terms of the DM 'entropy' K = σ_r^2/ρ^(2/3) ~ r^(α) highlighted by recent N-body simulations; the former constrains the slope to the narrow range α~ 1.25 - 1.3. These physically based α-profiles meet the overall requirements from gravitational lensing observations, being intrinsically flatter at the center and steeper in the outskirts relative to the empirical NFW formula. Specifically, we project them along the l.o.s. and compare with a recent extensive dataset from strong and weak lensing observations in and around the cluster A1689. We find an optimal fit at both small and large scales in terms of a halo constituted by an early body with α~ 1.25 and by recent extensive outskirts, that make up an overall mass 10^15 M_sun with a concentration parameter c ~ 10 consistent with the variance we compute in the \LambdaCDM cosmogony. The resulting structure corresponds to a potential well shallow in the outskirts as that inferred from the X rays radiated from the hot electrons and baryons constituting the intracluster plasma.

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arXiv2009

Dark Matter Equilibria in Galaxies and Galaxy Systems

[abridged] In the dark matter (DM) halos embedding galaxies and galaxy systems the `entropy' K = σ^2 / ρ^{2/3} (a quantity that combines the radial velocity dispersion σwith the density ρ) is found from intensive N-body simulations to follow a powerlaw run K ~ r^α throughout the halos' bulk, with αaround 1.25. Taking up from phenomenology just that α~ const applies, we cut through the rich analytic contents of the Jeans equation describing the self-gravitating equilibria of the DM; we specifically focus on computing and discussing a set of novel physical solutions that we name α-profiles, marked by the entropy slope αitself, and by the maximal gravitational pull κ_crit required for a viable equilibrium to hold. We then use an advanced semianalytic description for the cosmological buildup of halos to constrain the values of αto within the narrow range 1.25-1.29 from galaxies to galaxy systems. Our range of αapplies since the transition time that - both in oursemianalytic description and in state-of-the-art numerical simulations - separates two development stages: an early violent collapse that comprises a few major mergers and enforces dynamical mixing, followed by smoother mass addition through slow accretion. We also give an accurate analytic representation of the α-profiles with parameters derived from the Jeans equation. We finally stress how our findings and predictions as to αand κ_crit contribute to understand hitherto unsolved issues concerning the fundamental structure of DM halos.

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arXiv2008

The dramatic size evolution of elliptical galaxies and the quasar feedback

Observations have evidenced that passively evolving massive galaxies at high redshift are much more compact than local galaxies with the same stellar mass. We argue that the observed strong evolution in size is directly related to the quasar feedback, which removes huge amounts of cold gas from the central regions in a Salpeter time, inducing an expansion of the stellar distribution. The new equilibrium configuration, with a size increased by a factor >~ 3, is attained after ~ 40 dynamical times, corresponding to ~ 2 Gyr. This means that massive galaxies observed at z >~ 1 will settle on the Fundamental Plane by z ~ 0.8-1. In less massive galaxies (M_star <~ 2 10^10 M_sun), the nuclear feedback is subdominant, and the mass loss is mainly due to stellar winds. In this case, the mass loss timescale is longer than the dynamical time and results in adiabatic expansion that may increase the effective radius by a factor of up to ~ 2 in 10 Gyr, although a growth by a factor of ~ 1.6 occurs within the first 0.5 Gyr. Since observations are focused on relatively old galaxies, with ages >~ 1 Gyr, the evolution for smaller galaxies is more difficult to perceive. Significant evolution of velocity dispersion is predicted for both small and large galaxies.

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arXiv2008

Long Gamma-Ray Bursts and Their Host Galaxies at High Redshift

Motivated by the recent observational and theoretical evidence that long Gamma-Ray Bursts (GRBs) are likely associated with low metallicity, rapidly rotating massive stars, we examine the cosmological star formation rate (SFR) below a critical metallicity Z_crit Z_sun/10 - Z_sun/5, to estimate the event rate of high-redshift long GRB progenitors. To this purpose, we exploit a galaxy formation scenario already successfully tested on a wealth of observational data on (proto)spheroids, Lyman break galaxies, Lyman alpha emitters, submm galaxies, quasars, and local early-type galaxies. We find that the predicted rate of long GRBs amounts to about 300 events/yr/sr, of which about 30 per cent occur at z>~6. Correspondingly, the GRB number counts well agree with the bright SWIFT data, without the need for an intrinsic luminosity evolution. Moreover, the above framework enables us to predict properties of the GRB host galaxies. Most GRBs are associated with low mass galaxy halos M_H<~10^11 M_sun, and effectively trace the formation of small galaxies in such halos. The hosts are young, with age smaller than 5*10^7 yr, gas rich, but poorly extincted (A_V<~0.1) because of their chemical immaturity; this also implies high specific SFR and quite extreme alpha-enhancement. Only the minority of hosts residing in large halos with M_H>~10^12 M_sun have larger extinction (A_V~0.7-1), SFRs exceeding 100 M_sun/yr and can be detected at submm wavelengths. Most of the hosts have UV magnitudes in the range -20 <~M_1350<~ -16, and Lyman alpha luminosity in the range 2*10^40 <~L_Lya<~2*10^42 erg/s. GRB hosts are thus tracing the faint end of the luminosity function of Lyman break galaxies and Lyman alpha emitters.

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arXiv2007

Missing Baryons, from Clusters to Groups of Galaxies

From clusters to groups of galaxies, the powerful bremsstrahlung radiation L_X emitted in X rays by the intracluster plasma is observed to decline sharply with lowering virial temperatures T (i.e., at shallower depths of the gravitational wells) after a steep local L_X-T correlation; this implies increasing scarcity of diffuse baryons relative to dark matter, well under the cosmic fraction. We show how the widely debated issue concerning these `missing baryons' is solved in terms of the thermal and/or dynamical effects of the kinetic (at low redshifts z) and radiative (at high z) energy inputs from central active galactic nuclei, of which independent evidence is being observed. From these inputs we compute shape and z-evolution expected for L_X-T correlation which agree with the existing data, and provide a predictive pattern for future observations.

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