Faint Lyman-Break galaxies as a crucial test for galaxy formation models

It has recently been shown that galaxy formation models within the LambdaCDM cosmology predict that, compared to the observed population, small galaxies (with stellar masses < 10^{11} M_sun) form too early, are too passive since z ~ 3 and host too old stellar populations at z=0. We then expect an overproduction of small galaxies at z > 4 that should be visible as an excess of faint Lyman-break galaxies. To check whether this excess is present, we use the MORGANA galaxy formation model and GRASIL spectro-photometric + radiative transfer code to generate mock catalogues of deep fields observed with HST-ACS. We add observational noise and the effect of Lyman-alpha emission, and perform color-color selections to identify Lyman-break galaxies. The resulting mock candidates have plausible properties that closely resemble those of observed galaxies. We are able to reproduce the evolution of the bright tail of the luminosity function of Lyman-break galaxies (with a possible underestimate of the number of the brightest i-dropouts), but uncertainties and degeneracies in dust absorption parameters do not allow to give strong constraints to the model. Besides, our model shows a clear excess with respect to observations of faint Lyman-break galaxies, especially of z_{850} ~ 27 V-dropouts at z ~ 5. We quantify the properties of these "excess" galaxies and discuss the implications: these galaxies are hosted in dark matter halos with circular velocities in excess of 100 km s^{-1}, and their suppression may require a deep re-thinking of stellar feedback processes taking place in galaxy formation.Comment: 17 pages, 13 figures, 1 table; accepted for publication by MNRA

A Library of Theoretical Ultraviolet Spectra of Massive, Hot Stars for Evolutionary Synthesis

We computed a comprehensive set of theoretical ultraviolet spectra of hot, massive stars with the radiation-hydrodynamics code WM-Basic. This model atmosphere and spectral synthesis code is optimized for computing the strong P Cygni-type lines originating in the winds of hot stars, which are the strongest features in the ultraviolet spectral region. The computed set is suitable as a spectral library for inclusion in evolutionary synthesis models of star clusters and star-forming galaxies. The chosen stellar parameters cover the upper left Hertzsprung-Russell diagram at L >~ 10^2.75 Lsun and T_eff >~ 20,000 K. The adopted elemental abundances are 0.05 Zsun, 0.2 Zsun, 0.4 Zsun, Zsun, and 2 Zsun. The spectra cover the wavelength range from 900 to 3000 {\AA} and have a resolution of 0.4 {\AA}. We compared the theoretical spectra to data of individual hot stars in the Galaxy and the Magellanic Clouds obtained with the International Ultraviolet Explorer (IUE) and Far Ultraviolet Spectroscopic Explorer (FUSE) satellites and found very good agreement. We built a library with the set of spectra and implemented it into the evolutionary synthesis code Starburst99 where it complements and extends the existing empirical library towards lower chemical abundances. Comparison of population synthesis models at solar and near-solar composition demonstrates consistency between synthetic spectra generated with either library. We discuss the potential of the new library for the interpretation of the rest-frame ultraviolet spectra of star-forming galaxies. Properties that can be addressed with the models include ages, initial mass function, and heavy-element abundance. The library can be obtained both individually or as part of the Starburst99 package.Comment: ApJS (in press); 90 pages, 33 figures, 7 table

The complex physics of dusty star-forming galaxies at high redshifts as revealed by Herschel and Spitzer

We combine far-infrared photometry from Herschel (PEP/HerMES) with deep mid-infrared spectroscopy from Spitzer to investigate the nature and the mass assembly history of a sample of 31 luminous and ultraluminous infrared galaxies ((U)LIRGs) at z ∼ 1 and 2 selected in GOODS-S with 24μm fluxes between 0.2 and 0.5 mJy.We model the data with a self-consistent physical model (GRASIL) which includes a state-of-the-art treatment of dust extinction and reprocessing. We find that all of our galaxies appear to require massive populations of old (>1 Gyr) stars and, at the same time, to host a moderate ongoing activity of star formation (SFR 100M yr−1). The bulk of the stars appear to have been formed a few Gyr before the observation in essentially all cases. Only five galaxies of the sample require a recent starburst superimposed on a quiescent star formation history.We also find discrepancies between our results and those based on optical-only spectral energy distribution (SED) fitting for the same objects; by fitting their observed SEDs with our physical model we find higher extinctions (by ΔAV ∼ 0.81 and 1.14) and higher stellar masses (by Δlog(M ) ∼ 0.16 and 0.36 dex) for z ∼ 1 and z ∼ 2 (U)LIRGs, respectively. The stellar mass difference is larger for the most dust-obscured objects. We also find lower SFRs than those computed from LIR using the Kennicutt relation due to the significant contribution to the dust heating by intermediate-age stellar populations through “cirrus” emission (∼73% and ∼66% of the total LIR for z ∼ 1 and z ∼ 2 (U)LIRGs, respectively).Department of HE and Training approved lis

Motor neuron degeneration, severe myopathy and TDP-43 increase in a transgenic pig model of SOD1-linked familiar ALS

Amyotrophic Lateral Sclerosis (ALS) is a neural disorder gradually leading to paralysis of the whole body. Alterations in superoxide dismutase SOD1 gene have been linked with several variants of familial ALS. Here, we investigated a transgenic (Tg) cloned swine model expressing the human pathological hSOD1G93A allele. As in patients, these Tg pigs transmitted the disease to the progeny with an autosomal dominant trait and showed ALS onset from about 27 months of age. Post mortem analysis revealed motor neuron (MN) degeneration, gliosis and hSOD1 protein aggregates in brainstem and spinal cord. Severe skeletal muscle pathology including necrosis and inflammation was observed at the end stage, as well. Remarkably, as in human patients, these Tg pigs showed a quite long presymptomatic phase in which gradually increasing amounts of TDP-43 were detected in peripheral blood mononuclear cells. Thus, this transgenic swine model opens the unique opportunity to investigate ALS biomarkers even before disease onset other than testing novel drugs and possible medical devices

Global burden of 369 diseases and injuries in 204 countries and territories, 1990–2019: a systematic analysis for the Global Burden of Disease Study 2019

Background: In an era of shifting global agendas and expanded emphasis on non-communicable diseases and injuries along with communicable diseases, sound evidence on trends by cause at the national level is essential. The Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) provides a systematic scientific assessment of published, publicly available, and contributed data on incidence, prevalence, and mortality for a mutually exclusive and collectively exhaustive list of diseases and injuries. Methods: GBD estimates incidence, prevalence, mortality, years of life lost (YLLs), years lived with disability (YLDs), and disability-adjusted life-years (DALYs) due to 369 diseases and injuries, for two sexes, and for 204 countries and territories. Input data were extracted from censuses, household surveys, civil registration and vital statistics, disease registries, health service use, air pollution monitors, satellite imaging, disease notifications, and other sources. Cause-specific death rates and cause fractions were calculated using the Cause of Death Ensemble model and spatiotemporal Gaussian process regression. Cause-specific deaths were adjusted to match the total all-cause deaths calculated as part of the GBD population, fertility, and mortality estimates. Deaths were multiplied by standard life expectancy at each age to calculate YLLs. A Bayesian meta-regression modelling tool, DisMod-MR 2.1, was used to ensure consistency between incidence, prevalence, remission, excess mortality, and cause-specific mortality for most causes. Prevalence estimates were multiplied by disability weights for mutually exclusive sequelae of diseases and injuries to calculate YLDs. We considered results in the context of the Socio-demographic Index (SDI), a composite indicator of income per capita, years of schooling, and fertility rate in females younger than 25 years. Uncertainty intervals (UIs) were generated for every metric using the 25th and 975th ordered 1000 draw values of the posterior distribution. Findings: Global health has steadily improved over the past 30 years as measured by age-standardised DALY rates. After taking into account population growth and ageing, the absolute number of DALYs has remained stable. Since 2010, the pace of decline in global age-standardised DALY rates has accelerated in age groups younger than 50 years compared with the 1990–2010 time period, with the greatest annualised rate of decline occurring in the 0–9-year age group. Six infectious diseases were among the top ten causes of DALYs in children younger than 10 years in 2019: lower respiratory infections (ranked second), diarrhoeal diseases (third), malaria (fifth), meningitis (sixth), whooping cough (ninth), and sexually transmitted infections (which, in this age group, is fully accounted for by congenital syphilis; ranked tenth). In adolescents aged 10–24 years, three injury causes were among the top causes of DALYs: road injuries (ranked first), self-harm (third), and interpersonal violence (fifth). Five of the causes that were in the top ten for ages 10–24 years were also in the top ten in the 25–49-year age group: road injuries (ranked first), HIV/AIDS (second), low back pain (fourth), headache disorders (fifth), and depressive disorders (sixth). In 2019, ischaemic heart disease and stroke were the top-ranked causes of DALYs in both the 50–74-year and 75-years-and-older age groups. Since 1990, there has been a marked shift towards a greater proportion of burden due to YLDs from non-communicable diseases and injuries. In 2019, there were 11 countries where non-communicable disease and injury YLDs constituted more than half of all disease burden. Decreases in age-standardised DALY rates have accelerated over the past decade in countries at the lower end of the SDI range, while improvements have started to stagnate or even reverse in countries with higher SDI. Interpretation: As disability becomes an increasingly large component of disease burden and a larger component of health expenditure, greater research and developm nt investment is needed to identify new, more effective intervention strategies. With a rapidly ageing global population, the demands on health services to deal with disabling outcomes, which increase with age, will require policy makers to anticipate these changes. The mix of universal and more geographically specific influences on health reinforces the need for regular reporting on population health in detail and by underlying cause to help decision makers to identify success stories of disease control to emulate, as well as opportunities to improve. Funding: Bill & Melinda Gates Foundation. © 2020 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 licens

Global age-sex-specific fertility, mortality, healthy life expectancy (HALE), and population estimates in 204 countries and territories, 1950-2019 : a comprehensive demographic analysis for the Global Burden of Disease Study 2019

Background: Accurate and up-to-date assessment of demographic metrics is crucial for understanding a wide range of social, economic, and public health issues that affect populations worldwide. The Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2019 produced updated and comprehensive demographic assessments of the key indicators of fertility, mortality, migration, and population for 204 countries and territories and selected subnational locations from 1950 to 2019. Methods: 8078 country-years of vital registration and sample registration data, 938 surveys, 349 censuses, and 238 other sources were identified and used to estimate age-specific fertility. Spatiotemporal Gaussian process regression (ST-GPR) was used to generate age-specific fertility rates for 5-year age groups between ages 15 and 49 years. With extensions to age groups 10–14 and 50–54 years, the total fertility rate (TFR) was then aggregated using the estimated age-specific fertility between ages 10 and 54 years. 7417 sources were used for under-5 mortality estimation and 7355 for adult mortality. ST-GPR was used to synthesise data sources after correction for known biases. Adult mortality was measured as the probability of death between ages 15 and 60 years based on vital registration, sample registration, and sibling histories, and was also estimated using ST-GPR. HIV-free life tables were then estimated using estimates of under-5 and adult mortality rates using a relational model life table system created for GBD, which closely tracks observed age-specific mortality rates from complete vital registration when available. Independent estimates of HIV-specific mortality generated by an epidemiological analysis of HIV prevalence surveys and antenatal clinic serosurveillance and other sources were incorporated into the estimates in countries with large epidemics. Annual and single-year age estimates of net migration and population for each country and territory were generated using a Bayesian hierarchical cohort component model that analysed estimated age-specific fertility and mortality rates along with 1250 censuses and 747 population registry years. We classified location-years into seven categories on the basis of the natural rate of increase in population (calculated by subtracting the crude death rate from the crude birth rate) and the net migration rate. We computed healthy life expectancy (HALE) using years lived with disability (YLDs) per capita, life tables, and standard demographic methods. Uncertainty was propagated throughout the demographic estimation process, including fertility, mortality, and population, with 1000 draw-level estimates produced for each metric. Findings: The global TFR decreased from 2·72 (95% uncertainty interval [UI] 2·66–2·79) in 2000 to 2·31 (2·17–2·46) in 2019. Global annual livebirths increased from 134·5 million (131·5–137·8) in 2000 to a peak of 139·6 million (133·0–146·9) in 2016. Global livebirths then declined to 135·3 million (127·2–144·1) in 2019. Of the 204 countries and territories included in this study, in 2019, 102 had a TFR lower than 2·1, which is considered a good approximation of replacement-level fertility. All countries in sub-Saharan Africa had TFRs above replacement level in 2019 and accounted for 27·1% (95% UI 26·4–27·8) of global livebirths. Global life expectancy at birth increased from 67·2 years (95% UI 66·8–67·6) in 2000 to 73·5 years (72·8–74·3) in 2019. The total number of deaths increased from 50·7 million (49·5–51·9) in 2000 to 56·5 million (53·7–59·2) in 2019. Under-5 deaths declined from 9·6 million (9·1–10·3) in 2000 to 5·0 million (4·3–6·0) in 2019. Global population increased by 25·7%, from 6·2 billion (6·0–6·3) in 2000 to 7·7 billion (7·5–8·0) in 2019. In 2019, 34 countries had negative natural rates of increase; in 17 of these, the population declined because immigration was not sufficient to counteract the negative rate of decline. Globally, HALE increased from 58·6 years (56·1–60·8) in 2000 to 63·5 years (60·8–66·1) in 2019. HALE increased in 202 of 204 countries and territories between 2000 and 2019

Global, regional, and national burden of disorders affecting the nervous system, 1990–2021: a systematic analysis for the Global Burden of Disease Study 2021

BackgroundDisorders affecting the nervous system are diverse and include neurodevelopmental disorders, late-life neurodegeneration, and newly emergent conditions, such as cognitive impairment following COVID-19. Previous publications from the Global Burden of Disease, Injuries, and Risk Factor Study estimated the burden of 15 neurological conditions in 2015 and 2016, but these analyses did not include neurodevelopmental disorders, as defined by the International Classification of Diseases (ICD)-11, or a subset of cases of congenital, neonatal, and infectious conditions that cause neurological damage. Here, we estimate nervous system health loss caused by 37 unique conditions and their associated risk factors globally, regionally, and nationally from 1990 to 2021.MethodsWe estimated mortality, prevalence, years lived with disability (YLDs), years of life lost (YLLs), and disability-adjusted life-years (DALYs), with corresponding 95% uncertainty intervals (UIs), by age and sex in 204 countries and territories, from 1990 to 2021. We included morbidity and deaths due to neurological conditions, for which health loss is directly due to damage to the CNS or peripheral nervous system. We also isolated neurological health loss from conditions for which nervous system morbidity is a consequence, but not the primary feature, including a subset of congenital conditions (ie, chromosomal anomalies and congenital birth defects), neonatal conditions (ie, jaundice, preterm birth, and sepsis), infectious diseases (ie, COVID-19, cystic echinococcosis, malaria, syphilis, and Zika virus disease), and diabetic neuropathy. By conducting a sequela-level analysis of the health outcomes for these conditions, only cases where nervous system damage occurred were included, and YLDs were recalculated to isolate the non-fatal burden directly attributable to nervous system health loss. A comorbidity correction was used to calculate total prevalence of all conditions that affect the nervous system combined.FindingsGlobally, the 37 conditions affecting the nervous system were collectively ranked as the leading group cause of DALYs in 2021 (443 million, 95% UI 378–521), affecting 3·40 billion (3·20–3·62) individuals (43·1%, 40·5–45·9 of the global population); global DALY counts attributed to these conditions increased by 18·2% (8·7–26·7) between 1990 and 2021. Age-standardised rates of deaths per 100 000 people attributed to these conditions decreased from 1990 to 2021 by 33·6% (27·6–38·8), and age-standardised rates of DALYs attributed to these conditions decreased by 27·0% (21·5–32·4). Age-standardised prevalence was almost stable, with a change of 1·5% (0·7–2·4). The ten conditions with the highest age-standardised DALYs in 2021 were stroke, neonatal encephalopathy, migraine, Alzheimer's disease and other dementias, diabetic neuropathy, meningitis, epilepsy, neurological complications due to preterm birth, autism spectrum disorder, and nervous system cancer.InterpretationAs the leading cause of overall disease burden in the world, with increasing global DALY counts, effective prevention, treatment, and rehabilitation strategies for disorders affecting the nervous system are needed

Physical modelling of high redshift galaxy spectra: a new multiwavelenght view on galaxy formation and evolution

A major step forward in our understanding of the formation mechanism and evolution with cosmic time of galaxies is given by the recent development of powerful astronomical tools able to detect exceedingly faint signals from remote distances. One of the latter developments of particular signicance was the opening of wavelengths longwards the classical visual band to the astronomical observation from space. From the mid '80 with the pioneering IRAS survey, until today, with the operation of the Herschel Space Observatory in the far-infrared and the Large Atacama Millimetre Array (ALMA) in the millimetre, a variety of initiatives have been put in operation to observe the Cosmo at long wavelengths. Among the many novelties emerging from all this, it became more and more evident the role of diuse media in galaxies in shaping their spectral emission and modifying the flux emerging from stars. A particular eect is that due to the mixing of dust and gas in the galaxy interstellar media (ISM): as dust particles have a large cross section with respect to the optical and ultraviolet photons typically emitted by stars, a large fraction of stellar emission is absorbed by dust and re-emitted at long wavelengths in the IR and millimeter bands. According to observational evidence and theoretical expectation, this re-shaping of galaxy emission spectra by the dust content of their ISM becomes more and more important the richer the galaxy content is of primeval gas and dust. This happens particularly when we consider the early phases of their formation and evolution. Important discoveries in this sense have been made using large millimetric telescopes on ground (like SCUBA, IRAM, APEX among others), revealing the existence of a population of extremely luminous galaxies at high redshift, with luminosities comparable to those of quasars. Most of the light emitted by these objects falls in the far-infrared and sub-mm and is visible strongly redshifted at millimeter wavelengths. In the optical these galaxies appear as extremely faint and red, due to the combined eect of dust extinction and redshift. This discovery, later conrmed by space infrared observatories, proved the existence of phases of enormous star formation activity already at high redshift, very likely related to the formation of massive spheroidal galaxies (ellipticals, S0's). Altogether, all these recent developments require new approaches to investigate the physical properties of high-redshift galaxies, hence constraining the history of their assembly and early evolution. At the time when only visual (optical-UV) data were available on moderate-redshift galaxies the modellistic requirements to interprete their spectro-photometric data were most simply to add linearly the contributions of all stars present in the galaxy, quite independently on the geometry of the stellar distribution or dust opacities. Now the new data on dust re-radiations oered by infrared observatories showed that, particularly during major episodes of stellar formation, large or even dominant fractions of the emission by young stars are absorbed by dust and re-emitted as large far-infrared bumps. There is thus a clear association of star formation and dust extinction, because high star formation rates require highly opaque media for gas to collapse and form stars. In a real star-forming galaxy stars, gas and dust are mixed in a very complicated way, and dust extinction is a strong function of the age of the stellar population in the galaxy. New data then force us to consider a new generation of models of galaxy synthetic spectra, with radical complications with respect to previous classical modelling, occurring at two levels: on one side dust extinction can not be neglected and should be considered as a function of the age of the stellar population in the galaxy. In addition, the eect of dierential extinction entails that geometrical eects in the distribution of stars and dust play a fundamental role and have to be carefully modelled. Various attempts have been made during the last decade to provide simplied approaches to the above problems. The simplest widely used is to calculate the summed spectrum of all stars and extinguish it with an average and possibly representative extinction law. Virtually all results of physical analyses of galaxy populations in the universe are based on simplied numerical codes based on this approach. This can provide sensible results in relation to galaxy populations characterized by the presence of negligible amounts of interstellar gas and modest extinction, like in early type galaxies in the local universe or moderately starforming spirals. But we already know that this is an improper approach when considering more actively star-forming galaxies or objects detected during early phases of gas collapse hence in the presence of a very rich interstellar medium. These correspond to the most important and interesting phases of the formation and evolution of galaxies. The aim of the present work is to contribute to overcome the diculties and limitations described above, with a new eort of modelling and physical analysis of populations of galaxies, both in the local universe and at high redshifts. More specically, the main focus of my PhD research is to investigate the nature and mass assembly history of dusty star forming galaxies at high redshift (0.8< z <2.5), observed with Herschel, through a self-consistent modelling of their main physical properties like stellar, gas and dust mass, SFR and SFH, and dust attenuation. With total infrared luminosities between 1011 1012 L and 1012 L, respectively, Luminous and Ultra luminous infrared galaxies [(U)LIRGs hereafter], are among the most luminous and complex extragalactic objects we can conceive, including all varieties of young and old stellar populations, dust absorption, scattering, grain thermal re-radiation, and AGN emission (Lonsdale et al., 2006). Although they are quite rare in the local universe, they dominate the cosmic star formation rate and the FIR background at z > 1. Therefore they are suitable laboratories to study the main physical processes which drive galaxy formation and evolution. The physical characterization of the ULIRG phenomenon requires a multi-wavelength approach and a detailed treatment of dust eects. Galaxy Evolution Synthesis Technique seems to be a powerful tool to interpret galaxy spectra. The spectral energy distribution (SED) of a galaxy contains valuable information about its physical properties, including the stellar, gas and dust content, the age and abundance distribution of the stellar populations resulting from the SFH, and their interaction with the interstellar medium (ISM). The study of the SED therefore oers the most direct way to investigate galaxy formation and evolution, both through direct observations and corresponding theoretical modelling. To model the emission from stars and dust consistently and get reliable estimates for the main galaxy physical parameters we need to solve the radiative transfer equation for idealised but realistic geometrical distributions of stars and dust, as well as take advantage of the full SED coverage from UV to sub-mm. The power of our approach, here, lies in the combination of a full multi-wavelength coverage for our selected galaxies, including the FIR from 70 to 500 micron by Herschel PACS and SPIRE and the IRS Spitzer spectra where available, with a self-consistent spectral synthesis code GRASIL (Silva et al., 1998) used to interpret galaxy SEDs. This code computes the SEDs of galaxies from far-UV to radio including a state-of-the-art treatment of dust extinction and reprocessing based on a full radiative transfer solution. The characteristics of this model (e.g. accounting of diuse and clumped dust and stars, realistic geometry, both giving rise to an age-dependent dust attenuation, and a full computation of dust temperature as a function of grain size and composition) together with the large spectral coverage, allows a thorough physical analysis of the observed SEDs. Our analysis demonstrates that a correct and self-consistent treatment of dust extinction and reprocessing together with a full multiwavelength coverage (from far-UV to radio), is essential to get reliable estimates of the main physical parameters like stellar mass, dust extinction and SFRs. We show that such a physical approach can have strong impact on the claimed relation between the SFR and stellar mass. This is due to the uncertainties related to the interpretation of the optical to far-IR emission depending on the age of stars responsible for the dust heating and reprocessing. We observe that the addition of radio emission to the spectral multi-band tting oers a tight constraint on the current SFR, considering that only stars younger than about 10 Myr produce the galactic cosmic rays responsible for the non-thermal radio emission. Moreover as the radio emission probes the SFH on dierent timescales with respect to the IR emission, our analysis also allows us to better understand and constrain the source's past SFHs, in particular the number of massive stars contributing to the NT component of radio emission through core-collapse SNe.Un importante passo avanti nella comprensione del meccanismo di formazione ed evoluzione delle galassie e dato dal recente sviluppo di potenti strumenti astronomici in grado di rilevare segnali estremamente deboli provenienti da distanze remote. Uno degli ultimi sviluppi di particolare rilevanza e stata l'apertura di lunghezze d'onda maggiori della banda visibile all'osservazione astronomica dallo spazio. Dalla meta degli anni '80 con la pionieristica survey IRAS, no ad oggi, con il telescopio spaziale Herschel operante nel lontano infrarosso e l'Atacama Large Millimetre Array (ALMA) nel millimetrico, una serie di iniziative sono state messe a punto per osservare il Cosmo a lunghezze d'onda lunghe. Tra le tante novita che emergono da tutto questo, e diventato sempre piu evidente il ruolo del mezzo diuso nelle galassie nel `modellare' la loro emissione spettrale modicando il flusso emergente dalle stelle. Un eetto particolare e quello dovuto alla combinazione di polveri e gas nel mezzo interstellare della galassia (ISM): poiche le particelle di polvere hanno una elevata sezione d'urto rispetto ai fotoni ottici e UV tipicamente emessi dalle stelle, una frazione signicativa dell' emissione stellare e assorbita dalla polvere e riemessa a lunghezze d'onda nella banda IR e millimetrica. Secondo l'evidenza osservativa e le previsioni teoriche, questo `re-shaping' degli spettri di emissione delle galassie ad opera del contenuto di polvere del loro ISM diventa sempre piu importante quanto piu ricco e il contenuto di gas e polvere primordiale della galassia. Cio accade soprattutto quando si considerano le prime fasi della loro formazione ed evoluzione. Scoperte importanti in questo senso sono state fatte utilizzando grandi telescopi millimetrici da terra (come SCUBA, IRAM, APEX tra gli altri), rivelando l'esistenza di una popolazione di galassie ad alto redshift estremamente luminose, con luminosita confrontabili a quelle dei quasar. La maggior parte della radiazione emessa da questi oggetti cade nel lontano infrarosso e sub-mm ed e visibile fortemente spostata verso il rosso alle lunghezze d'onda millimetriche. Nell'ottico queste galassie appaiono come estremamente deboli e rosse, per l'effetto combinato di estinzione da polvere e redshift. Questa scoperta, in seguito confermata da osservatori spaziali infrarossi, ha dimostrato l'esistenza di fasi di intensa attivita di formazione stellare gia ad alto redshift, molto probabilmente legate alla formazione di galassie sferoidali massive (ellittiche, S0). Complessivamente, tutti questi recenti sviluppi richiedono nuovi approcci per studiare le proprieta fisiche delle galassie ad alto redshift, quindi vincolare la storia del loro assemblaggio e l'evoluzione iniziale. Al tempo in cui erano disponibili solo i dati ottici e UV, per galassie a basso redshift, i requisiti modellistici per interpretare i loro dati spettro-fotometrici consistevano semplicemente nel sommare linearmente i contributi di tutte le stelle presenti nella galassia, indipendentemente dalla geometria della distribuzione stellare o opacita della polvere. Ora i nuovi dati a disposizione sulla ri-emissione della polvere forniti da osservatori infrarossi hanno dimostrato che, in particolare durante i principali episodi di formazione stellare, grandi o addirittura dominanti frazioni della emissione da stelle giovani vengono assorbite dalla polvere e riemesse come grandi picchi nel lontano infrarosso. Vi è quindi una chiara associazione tra la formazione stellare e l'estinzione della polvere, in quanto elevati tassi di formazione stellare richiedono mezzi altamente opachi anche il gas possa collassare e formare stelle. In una galassia `star forming' reale stelle, gas e polvere sono mescolati in modo molto complesso, e l'estinzione della polvere e una forte funzione dell' eta della popolazione stellare nella galassia. I nuovi dati a disposizione ci obbligano a considerare una nuova generazione di modelli di spettri sintetici di galassie, con radicali complicazioni rispetto al precedente modelling classico, che si vericano a due livelli: da un lato l'eetto di estinzione della non puo essere trascurato e deve essere considerato in funzione dell'eta della popolazione stellare nella galassia. Inoltre, l'eetto di estinzione dierenziale fa s che gli eetti geometrici nella distribuzione di stelle e polvere svolgano un ruolo fondamentale e debbano essere accuratamente modellati. Vari tentativi sono stati fatti nel corso degli ultimi dieci anni per fornire approcci semplicati ai problemi di cui sopra. Il piu semplice ampiamente utilizzato consiste nel calcolare lo spettro totale sommando i contributi di tutte le stelle estinguendolo poi con una legge di estinzione media possibilmente rappresentativa. Praticamente tutti i risultati delle analisi siche di popolazioni di galassie nell'universo si basano su codici numerici semplicati basati su questo approccio. Questo puo fornire risultati ragionevoli per popolazioni di galassie caratterizzate da quantita trascurabili di gas interstellare e moderata estinzione, come nelle galassie del primo tipo nell'universo locale o nelle spirali `moderately star forming'. Ma sappiamo gia che si tratta di un approccio non corretto quando si considerano galassie caratterizzate da una piu attiva formazione stellare o oggetti osservati durante le prime fasi di collasso del gas e quindi in presenza di un mezzo interstellare molto ricco. Queste corrispondono alle fasi piu importanti ed interessanti della formazione ed evoluzione delle galassie. Lo scopo del presente lavoro e quello di contribuire a superare le dicolta e le limitazioni appena descritte, attraverso l'implementazione di una nuova modellistica e analisi sica delle popolazioni di galassie sia nell' universo locale che ad alto redshift. Piu nello specico il fulcro di questo progetto di ricerca di dottorato e quello di studiare la natura sica e la storia di assemblaggio in massa di galassie oscurate dalla polvere, ad alto redshift (0.8< z <2.5) e con attivita recente di formazione stellare, osservate con Herschel, attraverso una modellizzazione auto-consistente delle loro principali proprieta siche tra cui massa stellare, del gas e della polvere, SFR e SFH e l'oscuramento da polvere. Con tipiche luminosita infrarosse negli intervalli 1011 1012 L e 1012 L, le galassie luminose e ultraluminose nell'IR [(U)LIRGs nel testo], rispettivamente, sono tra gli oggetti extra-galattici piu luminosi e complessi che si possano concepire, comprendenti un'ampia varieta di popolazioni stellare giovani e vecchie, assorbimento da polvere, scattering, riemissione termica da parte dei grani e emissione AGN (Lonsdale et al., 2006). Sebbene questi oggetti siano abbastanza rari nell'Universo locale, essi dominano la rate di formazione stellare cosmica e il FIR background a z > 1. Percio' essi rappresentano i laboratori piu adatti ove studiare i principali processi sici che regolano la formazione ed evoluzione delle galassie. La caratterizzazione sica della fenomenologia delle (U)LIRGs richiede un approccio multi lunghezza d'onda e una trattazione dettagliata degli eetti della polvere. La tecnica di sintesi evolutiva delle galassie costituisce un potente strumento per interpretare gli spettri delle galassie. La distribuzione spettrale d'energia di una galassia (SED) contiene preziose informazioni sulle sue proprieta siche, che includono il contenuto in gas e in stelle, la distribuzione di eta e di abbondanze della popolazione stellare che viene dalla storia di formazione stellare e la loro interazione con il mezzo interstellare. Lo studio della SED quindi costituisce il metodo piu diretto per investigare la formazione ed evoluzione delle galassie, sia attraverso osservazioni dirette che attraverso corrispondenti modelli teorici. Per modellare l'emissione da stelle e polvere in maniera consistente e ottenere stime af-dabili dei principali parametri sici della galassia e necessario risolvere le equazioni del trasferimento radiativo per distribuzioni geometriche idealizzate ma realistiche di stelle e polvere, cos come trarre vantaggio da una copertura in lunghezza d'onda completa dal far-UV al radio. La forza del nostro approccio sta nella combinazione di una completa copertura multilunghezza d'onda per la nostra selezione di galassie, che include il FIR 70-500 micron da Herschel PACS e SPIRE e gli spettri IRS di Spitzer, ove disponibili, con un codice di sintesi spettrale auto-consistente, GRASIL citep Silva1998, utilizzato per interpretare le SED delle galassie. Questo codice calcola le SED delle galassie, dal lontano UV al radio, includendo un trattamento dettagliato degli eetti di estinzione e ri-emissione della polvere basato sulla risoluzione delle equazioni del trasferimento radiativo. Le caratteristiche di questo modello, prima tra tutte l'attenuazione da polvere dipendente dall'eta delle popolazioni stellari, diverse geometrie per la distribuzione di stelle e polvere nella galassia, calcolo della distribuzione delle temperature della polvere in funzione della tipologia dei grani insieme con la completa copertura spettrale, consentono un' approfondita analisi sica delle SED osservate. La nostra analisi dimostra quindi che un trattamento corretto e auto-consistente dell' estinzione e riemissione da polvere assieme ad una copertura Multiwavelength completa (dal lontano-UV al radio), e essenziale per ottenere stime adabili dei principali parametri sici come la massa stellare, l'estinzione da polvere e la SFR. Mostriamo che un approccio sico di questo tipo puo avere un forte impatto sulla rivendicata relazione tra SFR e la massa stellare. Cio e dovuto alle incertezze legate all'interpretazione dell' emissione dall' ottico al lontano IR in funzione dell'eta delle popolazioni stellari responsabili del riscaldamento della polvere e ri-emissione. Osserviamo che l'aggiunta dell' emissione radio al t spettrale multibanda fornisce uno stretto vincolo alla SFR recente, dato che soltanto le stelle piu giovani di circa 10 milioni di anni producono i raggi cosmici galattici responsabili dell'emissione radio non termica. Inoltre, poiche l'emissione radio sonda la SFH su tempi scala diversi rispetto all'emissione IR, la nostra analisi ci permette anche di capire meglio e vincolare SFH della sorgente, in particolare il numero di stelle massicce che contribuiscono alla componente non termica di emissione radio attraverso le `core-collapse SNe'

Physical modelling of high redshift galaxy spectra: a new multiwavelenght view on galaxy formation and evolution

A major step forward in our understanding of the formation mechanism and evolution with cosmic time of galaxies is given by the recent development of powerful astronomical tools able to detect exceedingly faint signals from remote distances. One of the latter developments of particular signicance was the opening of wavelengths longwards the classical visual band to the astronomical observation from space. From the mid '80 with the pioneering IRAS survey, until today, with the operation of the Herschel Space Observatory in the far-infrared and the Large Atacama Millimetre Array (ALMA) in the millimetre, a variety of initiatives have been put in operation to observe the Cosmo at long wavelengths. Among the many novelties emerging from all this, it became more and more evident the role of diuse media in galaxies in shaping their spectral emission and modifying the flux emerging from stars. A particular eect is that due to the mixing of dust and gas in the galaxy interstellar media (ISM): as dust particles have a large cross section with respect to the optical and ultraviolet photons typically emitted by stars, a large fraction of stellar emission is absorbed by dust and re-emitted at long wavelengths in the IR and millimeter bands. According to observational evidence and theoretical expectation, this re-shaping of galaxy emission spectra by the dust content of their ISM becomes more and more important the richer the galaxy content is of primeval gas and dust. This happens particularly when we consider the early phases of their formation and evolution. Important discoveries in this sense have been made using large millimetric telescopes on ground (like SCUBA, IRAM, APEX among others), revealing the existence of a population of extremely luminous galaxies at high redshift, with luminosities comparable to those of quasars. Most of the light emitted by these objects falls in the far-infrared and sub-mm and is visible strongly redshifted at millimeter wavelengths. In the optical these galaxies appear as extremely faint and red, due to the combined eect of dust extinction and redshift. This discovery, later conrmed by space infrared observatories, proved the existence of phases of enormous star formation activity already at high redshift, very likely related to the formation of massive spheroidal galaxies (ellipticals, S0's). Altogether, all these recent developments require new approaches to investigate the physical properties of high-redshift galaxies, hence constraining the history of their assembly and early evolution. At the time when only visual (optical-UV) data were available on moderate-redshift galaxies the modellistic requirements to interprete their spectro-photometric data were most simply to add linearly the contributions of all stars present in the galaxy, quite independently on the geometry of the stellar distribution or dust opacities. Now the new data on dust re-radiations oered by infrared observatories showed that, particularly during major episodes of stellar formation, large or even dominant fractions of the emission by young stars are absorbed by dust and re-emitted as large far-infrared bumps. There is thus a clear association of star formation and dust extinction, because high star formation rates require highly opaque media for gas to collapse and form stars. In a real star-forming galaxy stars, gas and dust are mixed in a very complicated way, and dust extinction is a strong function of the age of the stellar population in the galaxy. New data then force us to consider a new generation of models of galaxy synthetic spectra, with radical complications with respect to previous classical modelling, occurring at two levels: on one side dust extinction can not be neglected and should be considered as a function of the age of the stellar population in the galaxy. In addition, the eect of dierential extinction entails that geometrical eects in the distribution of stars and dust play a fundamental role and have to be carefully modelled. Various attempts have been made during the last decade to provide simplied approaches to the above problems. The simplest widely used is to calculate the summed spectrum of all stars and extinguish it with an average and possibly representative extinction law. Virtually all results of physical analyses of galaxy populations in the universe are based on simplied numerical codes based on this approach. This can provide sensible results in relation to galaxy populations characterized by the presence of negligible amounts of interstellar gas and modest extinction, like in early type galaxies in the local universe or moderately starforming spirals. But we already know that this is an improper approach when considering more actively star-forming galaxies or objects detected during early phases of gas collapse hence in the presence of a very rich interstellar medium. These correspond to the most important and interesting phases of the formation and evolution of galaxies. The aim of the present work is to contribute to overcome the diculties and limitations described above, with a new eort of modelling and physical analysis of populations of galaxies, both in the local universe and at high redshifts. More specically, the main focus of my PhD research is to investigate the nature and mass assembly history of dusty star forming galaxies at high redshift (0.8< z <2.5), observed with Herschel, through a self-consistent modelling of their main physical properties like stellar, gas and dust mass, SFR and SFH, and dust attenuation. With total infrared luminosities between 1011 1012 L and 1012 L, respectively, Luminous and Ultra luminous infrared galaxies [(U)LIRGs hereafter], are among the most luminous and complex extragalactic objects we can conceive, including all varieties of young and old stellar populations, dust absorption, scattering, grain thermal re-radiation, and AGN emission (Lonsdale et al., 2006). Although they are quite rare in the local universe, they dominate the cosmic star formation rate and the FIR background at z > 1. Therefore they are suitable laboratories to study the main physical processes which drive galaxy formation and evolution. The physical characterization of the ULIRG phenomenon requires a multi-wavelength approach and a detailed treatment of dust eects. Galaxy Evolution Synthesis Technique seems to be a powerful tool to interpret galaxy spectra. The spectral energy distribution (SED) of a galaxy contains valuable information about its physical properties, including the stellar, gas and dust content, the age and abundance distribution of the stellar populations resulting from the SFH, and their interaction with the interstellar medium (ISM). The study of the SED therefore oers the most direct way to investigate galaxy formation and evolution, both through direct observations and corresponding theoretical modelling. To model the emission from stars and dust consistently and get reliable estimates for the main galaxy physical parameters we need to solve the radiative transfer equation for idealised but realistic geometrical distributions of stars and dust, as well as take advantage of the full SED coverage from UV to sub-mm. The power of our approach, here, lies in the combination of a full multi-wavelength coverage for our selected galaxies, including the FIR from 70 to 500 micron by Herschel PACS and SPIRE and the IRS Spitzer spectra where available, with a self-consistent spectral synthesis code GRASIL (Silva et al., 1998) used to interpret galaxy SEDs. This code computes the SEDs of galaxies from far-UV to radio including a state-of-the-art treatment of dust extinction and reprocessing based on a full radiative transfer solution. The characteristics of this model (e.g. accounting of diuse and clumped dust and stars, realistic geometry, both giving rise to an age-dependent dust attenuation, and a full computation of dust temperature as a function of grain size and composition) together with the large spectral coverage, allows a thorough physical analysis of the observed SEDs. Our analysis demonstrates that a correct and self-consistent treatment of dust extinction and reprocessing together with a full multiwavelength coverage (from far-UV to radio), is essential to get reliable estimates of the main physical parameters like stellar mass, dust extinction and SFRs. We show that such a physical approach can have strong impact on the claimed relation between the SFR and stellar mass. This is due to the uncertainties related to the interpretation of the optical to far-IR emission depending on the age of stars responsible for the dust heating and reprocessing. We observe that the addition of radio emission to the spectral multi-band tting oers a tight constraint on the current SFR, considering that only stars younger than about 10 Myr produce the galactic cosmic rays responsible for the non-thermal radio emission. Moreover as the radio emission probes the SFH on dierent timescales with respect to the IR emission, our analysis also allows us to better understand and constrain the source's past SFHs, in particular the number of massive stars contributing to the NT component of radio emission through core-collapse SNe