Analytic modelling of the galactic fountain in the Milky Way potential

Le galassie a spirale, come la Via Lattea, sono caratterizzate dalla presenza di gas freddo e formazione stellare e vengono perciò chiamate star-forming. Per creare nuove stelle è necessaria una sufficiente riserva di gas, la cui disponibilità governa l’evoluzione della galassia stessa. Finora, non è stato individuato con certezza un meccanismo che possa alimentare la formazione di nuove stelle nelle galassie star-forming. Una delle possibili sorgenti di tale gas è l’alone galattico caldo (corona galattica) il cui raffreddamento e successivo accrescimento possono essere stimolati dal processo di fontana galattica. L’esplosione di supernovae porta nubi di gas freddo in orbita al di sopra del disco stellare; queste nubi raggiungono altezze dell’ordine del kiloparsec, interagendo con la corona di gas caldo. Il moto delle nubi all’interno di un mezzo meno denso comporta l’instaurarsi dell’instabilità di Kelvin-Helmholtz, che ’strappa’ gas dalle nubi e causa la condensazione di materia coronale. Quest’ultima viene quindi accresciuta e, ricadendo sul disco, trasferisce nuovo materiale alla galassia e ne alimenta la formazione stellare. Lo scopo di questa tesi è derivare un modello analitico di fontana galattica che consenta di ottenere una formulazione analitica per il tempo orbitale, cioè il tempo richiesto alle nubi per ricadere sul disco galattico. Infatti, più u tempo le nubi impiegano per attraversare il materiale coronale caldo e ricadere sul disco, più materiale viene accresciuto durante l’orbita. Conoscendo i tempi orbitali sarebbe possibile calcolare il tasso di accrescimento legato al fenomeno di fontana e studiarne l’andamento con il raggio del disco. Questo modello potrebbe rivelarsi utile per lo studio dell’impatto della fontana nell’evoluzione globale del disco galattico e della chimica dell’intera galassia

Star formation laws and gas turbulence in nearby galaxies

Star-forming galaxies continuously form new stars out of cold interstellar gas distributed in a disc. The star formation law, which tells how many stars form from a given amount of gas, is crucial to understand galaxy evolution. Since galaxy discs are seen in projection in the sky-plane, their gas and star formation rate densities are measured on a surface. These projected quantities have been used to derive a surface-based star formation law and it was found that it holds in the inner disc but fails in their outskirts. This thesis presents a new volume-based relation obtained by converting projected densities into volume densities using an accurate method to derive the gas disc thickness. This volumetric star formation law holds across the whole disc of star-forming galaxies and is likely a more fundamental relation. The second part of this dissertation focuses on the origin of turbulent motions of cold gas in galactic discs. Since turbulence is expected to be quickly dissipated by the gas viscosity, it needs to be maintained by some energy source. This latter has remained unknown so far, as even the prime candidates, supernova explosions of massive stars, have been considered insufficient. A crucial improvement is obtained in this thesis by realising that the thickness of galactic gas discs slows down the dissipation and reduces the energy required to sustain turbulence. This brings to the conclusion that supernova explosions can feed the turbulence of the cold gas, finally solving the conundrum

ICT and Non-ICT investments: short and long run macro dynamics

In this paper, we model business investment distinguishing between ICT (communication equipment, hardware and software) and Non-ICT (machinery and equipment, and nonresidential buildings) components and taking into account asset specific characteristics potentially affecting the reactivity of capital accumulation over the business cycle. Business investment and ICT and Non-ICT assets are estimated within a VECM model to test, in a unique framework, the assumptions of the flexible accelerator model (Clark, 1944, and Koyck, 1954) and of the neoclassical model of Hall and Jorgenson (1967), as well as how financial constraints and uncertainty influence investment behaviour (Hall and Lerner, 2010, and Bloom, 2007). Our findings suggest that the long-run relationship with standard macro determinants (output and user cost) is verified for aggregate business capital stock as well as for individual Non-ICT assets but not for ICT. In the short run, liquidity is a key determinant of investment behaviour independently of the asset type. In the long-run, uncertainty significantly affects ICT. Finally, the results of the counterfactual exercises over the latest Italian recession support the idea that ICT is a key policy variable to foster the economic recovery

Evidence for enhanced star formation rates in z~0.35 cluster galaxies undergoing ram pressure stripping

Ram pressure stripping (RPS) is one of the most invoked mechanisms to explain the observed differences between cluster and field galaxies. In the local Universe, its effect on the galaxy star forming properties has been largely tackled and the general consensus is that this process first compresses the gas available in the galaxy disks, boosting the star formation for a limited amount of time, and then removes the remaining gas leading to quenching. Much less is known on the effect and preponderance of RPS at higher redshift, due to the lack of statistical samples. Exploiting VLT/MUSE observations of galaxies at 0.2<z<0.55 and the catalog of ram pressure stripped galaxies by Moretti et al., we compare the global star formation rate-mass (SFR-M*) relation of 29 cluster galaxies undergoing RPS to that of 26 field and cluster undisturbed galaxies that constitute our control sample. Stripping galaxies occupy the upper envelope of the control sample SFR-M* relation, showing a systematic enhancement of the SFR at any given mass. The boost is >3sigma when considering the SFR occurring in both the tail and disk of galaxies. The enhancement is retrieved also on local scales: considering spatially resolved data, ram pressure stripped galaxies overall have large {\Sigma}SFR values, especially for Sigma_*>10^7.5M_sun kpc~2. RPS seems to leave the same imprint on the SFR-M* and Sigma_SFR-Sigma_* relations both in the Local Universe and at z~0.35.Comment: A&A in pres

The evolution of the cold gas fraction in nearby clusters ram-pressure stripped galaxies

Cluster galaxies are affected by the surrounding environment, which influences, in particular, their gas, stellar content and morphology. In particular, the ram-pressure exerted by the intracluster medium promotes the formation of multi-phase tails of stripped gas detectable both at optical wavelengths and in the sub-mm and radio regimes, tracing the cold molecular and atomic gas components, respectively. In this work we analyze a sample of sixteen galaxies belonging to clusters at redshift $\sim 0.05$ showing evidence of an asymmetric HI morphology (based on MeerKAT observations) with and without a star forming tail. To this sample we add three galaxies with evidence of a star forming tail and no HI detection. Here we present the galaxies $\rm H_{2}$ gas content from APEX observations of the CO(2-1) emission. We find that in most galaxies with a star forming tail the $\rm H_{2}$ global content is enhanced with respect to undisturbed field galaxies with similar stellar masses, suggesting an evolutionary path driven by the ram-pressure stripping. As galaxies enter into the clusters their HI is displaced but also partially converted into $\rm H_{2}$, so that they are $\rm H_{2}$ enriched when they pass close to the pericenter, i. e. when they develop the star forming tails that are visible in UV/B broad bands and in H$\alpha$ emission. An inspection of the phase-space diagram for our sample suggests an anticorrelation between the HI and $\rm H_{2}$ gas phases as galaxies fall into the cluster potential. This peculiar behaviour is a key signature of the ram-pressure stripping in action.Comment: 19 pages, 8 figures, accepted for publication in Ap

GASP and MaNGA Surveys Shed Light on the Enigma of the Gas Metallicity Gradients in Disk Galaxies

Making use of both MUSE observations of 85 galaxies from the survey GASP (GAs Stripping Phenomena in galaxies with MUSE) and a large sample from MaNGA (Mapping Nearby Galaxies at Apache Point Observatory survey), we investigate the distribution of gas metallicity gradients as a function of stellar mass for local cluster and field galaxies. Overall, metallicity profiles steepen with increasing stellar mass up to 1010.3 M o˙ and flatten out at higher masses. Combining the results from the metallicity profiles and the stellar mass surface density gradients, we propose that the observed steepening is a consequence of local metal enrichment due to in situ star formation during the inside-out formation of disk galaxies. The metallicity gradient-stellar mass relation is characterized by a rather large scatter, especially for 109.8 < M ∗/M o˙ < 1010.5, and we demonstrate that metallicity gradients anti-correlate with the galaxy gas fraction. Focusing on the galaxy environment, at any given stellar mass, cluster galaxies have systematically flatter metallicity profiles than their field counterparts. Many subpopulations coexist in clusters: galaxies with shallower metallicity profiles appear to have fallen into their present host halo sooner and have experienced the environmental effects for a longer time than cluster galaxies with steeper metallicity profiles. Recent galaxy infallers, like galaxies currently undergoing ram pressure stripping, show metallicity gradients more similar to those of field galaxies, suggesting they have not felt the effect of the cluster yet

Building the core of the Istat system of models for forecasting the Italian economy: MeMo-It

10noopenThis paper provides a description of Istat’s new Macroeconometric Model MeMo-It. This is the tool used by Istat to produce bi-annual medium-term forecasts of the Italian economy since May 2012. The key model’s features are illustrated and compared with other modeling approaches used in the literature. The paper describes the theoretical foundations of the model along with the set of equations and their estimation. It also presents an assessment of model’s performance by focusing on the key multipliers.openBacchini, Fabio; Brandimarte, Cristina; Crivelli, Piero; De Santis, Roberta; Fioramanti, Marco; Girardi, Alessandro; Golinelli, Roberto; Jona-Lasinio, Cecilia; Mancini, Massimo; Pappalardo, CarmineBacchini, Fabio; Brandimarte, Cristina; Crivelli, Piero; DE SANTIS, Roberta; Fioramanti, Marco; Girardi, Alessandro; Golinelli, Roberto; JONA-LASINIO, CECILIA SUSANNA; Mancini, Massimo; Pappalardo, Carmin

The Smallest Scale of Hierarchy Survey (SSH). II. Extended star formation and bar-like features in the dwarf galaxy NGC 3741: recent merger or ongoing gas accretion?

Using Large Binocular Telescope deep imaging data from the Smallest Scale of Hierarchy Survey (SSH) and archival Hubble Space Telescope data, we reveal the presence of two elongated stellar features contiguous to a bar-like stellar structure in the inner regions of the dwarf irregular galaxy NGC 3741. These structures are dominated by stars younger than a few hundred Myr and collectively are about twice as extended as the old stellar component. These properties are very unusual for dwarf galaxies in the nearby Universe and difficult to explain by hydro-dynamical simulations. From the analysis of archival 21-cm observations, we find that the young stellar "bar" coincides with an HI high-density region proposed by previous studies to be a purely gaseous bar; we furthermore confirm radial motions of a few km/s, compatible with an inflow/outflow, and derive a steeply-rising rotation curve and high HI surface density at the center, indicating a very concentrated mass distribution. We propose that the peculiar properties of the stellar and gaseous components of NGC 3741 may be explained by a recent merger or ongoing gas accretion from the intergalactic medium, which caused gas inflows towards the galaxy center and triggered star formation a few hundred Myr ago. This event may explain the young and extended stellar features, the bar-like structure, the very extended HI disc and the central HI spiral arms. The high central HI density and the steeply rising rotation curve suggest that NGC 3741 may be the progenitor or the descendant of a starburst dwarf

Meccanismi di produzione dell'energia in astrofisica

Lo scopo di questo elaborato è descrivere alcuni dei meccanismi di produzione dell’energia studiati nel campo astrofisico. Essendo questi piuttosto numerosi, sono stati trascurati i processi ritenuti di sola conversione di energia da una forma ad un’altra, come, per esempio,l’emissione da parte di una particella accelerata. In questo modo si è potuto dedicare più spazio ad altri fenomeni, molto comuni ed efficienti, che saranno qui anticipatamente elencati. Nel Capitolo 1 vengono descritti i processi di fusione nucleare che alimentano le stelle; per ognuno sono state riportate la quantità di energia prodotta e i tempi scala. Si è scelto inoltre di dare maggiore importanza a quei fenomeni che caratterizzano le fasi principali dell’evoluzione stellare, essendo questi anche i più efficienti, mentre le reazioni secondarie sono state solamente accennate. Nella Sezione 1.4 vengono descritti i meccanismi alla base dell’esplosione di supernova, essendo un’importante fase evolutiva nella quale la quantità di energia in gioco è considerevole. Come conclusione dell’argomento vengono riportare le equazioni che descrivono la produzione energetica nei processi di fusione descritti precedentemente. Nella seconda parte dell’elaborato, viene descritto il fenomeno dell’accrescimento gravitazionale utilizzando come oggetto compatto di riferimento un buco nero. Si è scelto di porre l’accento sull’efficienza della produzione energetica e sul limite di luminosità di Eddington