Letteratura scientifica selezionata sul tema "Galaxies primitives"

AbstractWe report on a study of 9 nearby primitive galaxies observed by Hubble’s COS far-UV spectrograph that can serve as templates of high-z galaxies to be observed by JWST. By “primitive galaxies,” we mean galaxies having a low stellar mass, and low gas metallicity, , whether they are local or at high redshift. We find that far-UV spectra of these galaxies show evidence of hard radiation, including X-rays. Following Thuan et al. (2004), we identify these galaxies as massive X-ray binaries containing a massive accreting stellar black hole. We further find that the lower the metallicity, the higher the probability of extremely strong X-radiation. Following Heger et al. (2003), we suggest that the accreting black hold is produced by direct collapse of stars having initial masses greater than 50 . The X-radiation produced by black hole disk directly affects the surrounding interstellar medium, and many of these effects are observable in far-UV spectra.

Context. Galaxy evolution and the effect of the environment are most often studied using scaling relations or regression analyses around a given property. However, these approaches do not take into account the complexity of the physics of the galaxies and their diversity. Aims. We here investigate the effect of the cluster environment on the evolution of galaxies through multivariate, unsupervised classification and phylogenetic analyses applied to two relatively large samples from the Wide-field Nearby Galaxy-cluster Survey (WINGS), one of cluster members and one of field galaxies (2624 and 1476 objects, respectively). Methods. These samples are the largest ones ever analysed with a phylogenetic approach in astrophysics. To be able to use the maximum parsimony (cladistics) method, we first performed a pre-clustering in 300 clusters with a hierarchical clustering technique, before applying it to these pre-clusters. All these computations used seven parameters: B − V, log(Re), nV, ⟨μ⟩e, Hβ, D4000, and log(M*). Results. We have obtained a tree for the combined samples and do not find different evolutionary paths for cluster and field galaxies. However, the cluster galaxies seem to have accelerated evolution in the sense that they are statistically more diversified from a primitive common ancestor. The separate analyses show a hint of a slightly more regular evolution of the variables for the cluster galaxies, which may indicate they are more homogeneous compared to field galaxies in the sense that the groups of the latter appear to have more specific properties. On the tree for the cluster galaxies, there is a separate branch that gathers rejuvenated or stripped-off groups of galaxies. This branch is clearly visible on the colour-magnitude diagram, going back from the red sequence towards the blue one. On this diagram, the distribution and the evolutionary paths of galaxies are strikingly different for the two samples. Globally, we do not find any dominant variable able to explain either the groups or the tree structures. Rather, co-evolution appears everywhere, and could depend itself on environment or mass. Conclusions. This study is another demonstration that unsupervised machine learning is able to go beyond simple scaling relations by taking into account several properties together. The phylogenetic approach is invaluable in tracing the evolutionary scenarios and projecting them onto any bivariate diagram without any a priori modelling. Our WINGS galaxies are all at low redshift, and we now need to go to higher redshfits to find more primitive galaxies and complete the map of the evolutionary paths of present day galaxies.

Abstract Massive Population III stars can die as energetic supernovae that enrich the early Universe with metals and determine the properties of the first galaxies. With masses of about 109 M ⊙ at z ≳ 10, these galaxies are believed to be the ancestors of the Milky Way. This paper investigates the impact of Population III supernova remnants (SNRs) from both Salpeter-like and top-heavy initial mass functions (IMFs) on the formation of first galaxies with high-resolution radiation-hydrodynamical simulations with the ENZO code. Our findings indicate that SNRs from a top-heavy Population III IMF produce more metals, leading to more efficient gas cooling and earlier Population II star formation in the first galaxies. From a few hundred to a few thousand Population II stars can form in the central regions of these galaxies. These stars have metallicities of 10−3–10−2, Z ⊙, greater than those of extremely metal-poor (EMP) stars. Their mass function follows a power-law distribution with dN ( M * ) / dM * ∝ M * α , where M * is stellar mass, and α = 2.66–5.83 and is steeper for a top-heavy IMF. We thus find that EMP stars were not typical of most primitive galaxies.

AbstractAn important factor controlling galaxy evolution is feedback from massive stars. It is believed that the nature and intensity of stellar feedback changes as a function of galaxy mass and metallicity. At low mass and metallicity, feedback from massive stars is mainly in the form of photoionizing radiation. At higher mass and metallicity, it is in stellar winds. I Zw 18 is a local blue, compact dwarf galaxy that meets the requirements for a primitive galaxy: low halo mass <109M⊙, strong photoionizing radiation, no galactic outflow, and very low metallicity, log(O/H)+12=7.2. We will describe the properties of massive stars and their role in the evolution of I Zw 18, based on analysis of ultraviolet images and spectra obtained with HST.

This article "The lunar problem is the barrier of the future time of the Earth" is a logical and rational analysis of the formation of the nuclear universe with galaxies, stars, the Sun star, the system of planets and the moons, and arrives at new original and inedited conclusions. The Big Bang of the primitive universe is a sequence programmed by the nature of thermonuclear super explosions in sidereal space. These thermonuclear super explosions swept nuclear sidereal space generating the large mass islands of galaxies like the Milky Way. The Milky Way was the first generation of the hyper-bubbles of the mixture of nuclear masses, which are: geological nuclear mass of attraction of gravity; geological nuclear mass of orbital attraction; geological nuclear mass of orbital repulsion. Because of nuclear hyper tremors, the nucleus of the Milky Way generated the second generation of the super bubbles that were repulsed from the galaxy's nucleus by the action of the geological nuclear mass of orbital repulsion, and then the super bubbles became in the many billions of celestial stars that make up the galactic disc. Because of the hyper tremors, the nucleus of these billions of stars, including the Sun, generated the third generation of super bubbles and large bubbles that were repulsed from the nuclei of the stars by the action of the geological nuclear mass of orbital repulsion, and then the super bubbles and large bubbles have become the sequence of planets, which makes up the orbital disk of the solar system. Due to a small number of nuclear super tremors, the nucleus of the sequence of planets tremble, shook and generated the fourth generation of the small bubbles that were repulsed from the nucleus of the planetary sequence by the action of the geological nuclear mass of orbital repulsion, and then, the small bubbles have become the sequence of moons, which makes up the orbital disk of the planets. And so, began the count the new time, after the sequence of Big Bang explosions.

AbstractGiant molecular clouds (GMCs) are the major reservoirs of molecular gas in galaxies, and the starting point for star formation. As such, their properties play a key role in setting the initial conditions for the formation of stars. We present a comprehensive combined inteferometric/single-dish study of the resolved GMC properties in a number of extragalactic systems, including both normal and dwarf galaxies. We find that the extragalactic GMC properties measured across a wide range of environments, characterized by the Larson relations, are to first order remarkably compatible with those in the Milky Way. Using these data to investigate trends due to galaxy metallicity, we find that: 1) these measurements are not in accord with simple expectations from photoionization-regulated star formation theory; 2) there is no trend in the virial CO-to-H2conversion factor on the spatial scales studied; and 3) there are measurable departures from the Galactic Larson relations in the Small Magellanic Cloud — the object with the lowest metallicity in the sample — where GMCs have velocity dispersions that are too small for their sizes. We will discuss the stability of these clouds in the light of our recent far-infrared analysis of this galaxy, and will contrast the results of the virial and far-infrared studies on the issue of the CO-to-H2conversion factor and what they tell us about the structure of molecular clouds in primitive galaxies.

Context. It is thought that fossil systems are relics of structure formation in the primitive Universe. They are galaxy aggregations that have assembled their mass at high redshift with few or no subsequent accretion. Observationally these systems are selected by large magnitude gaps between their 1st and 2nd ranked galaxies (Δm12). Nevertheless, there is still debate over whether or not this observational criterium selects dynamically evolved ancient systems. Aims. We have studied the properties of the nearby fossil group RX J075243.6+455653 in order to understand the mass assembly of this system. Methods. Deep spectroscopic observations allow us to construct the galaxy luminosity function (LF) of RX J075243.6+455653 down to Mr*+6. The analysis of the faint-end of the LF in groups and clusters provides valuable information about the mass assembly of the system. In addition, we have analyzed the nearby large-scale structure around this group. Results. We identified 26 group members within r200 ~ 0.96 Mpc. These galaxies are located at Vc = 15551 ± 65 km s-1 and have a velocity dispersion of σc = 333 ± 46 km s-1. The X-ray luminosity of the group is LX = 2.2 × 1043 h70-2 erg s-1, resulting in a mass of M = 4.2 × 1013 h70-1 within 0.5r200. The group has Δm12 = 2.1 within 0.5r200, confirming the fossil nature of this system. RX J075243.6+455653 has a central brightest group galaxy (BGG) with Mr = −22.67, one of the faintest BGGs observed in fossil systems. The LF of the group shows a flat faint-end slope (α = −1.08 ± 0.33). This low density of dwarf galaxies is confirmed by the low value of the dwarf-to-giant ratio (DGR = 0.99 ± 0.49) for this system. Both the lack of dwarf galaxies and the low luminosity of the BGG suggests that RX J075243.6+455653 still has to accrete mass from its nearby environment. This mass accretion will be achieved because it is the dominant structure of a rich environment formed by several groups of galaxies (15) within ~ 7 Mpc from the group center and with ± 1000 km s-1. Conclusions. RX J075243.6+455653 is a group of galaxies that has not yet completed the process of its mass assembly. This new mass accretion will change the fossil state of the group. This group is an example of a galaxy aggregation selected by a large magnitude gap but still in the process of the accretion of its mass.

Le télescope spatial James Webb, lancé en décembre 2021, est considéré comme l'observatoire majeur de la décennie en cours. S'il doit explorer toutes les phases de l'histoire cosmique, l'un de ses principaux buts scientifiques est de dévoiler les premières étoiles et galaxies formées dans l'obscurité de l'Univers primitif, qui ont conduit à sa réionisation avant d'évoluer vers la population de galaxies observable aujourd'hui. Pour interpréter les données spectroscopiques recueillies par le JWST, il est nécessaire de développer des modèles qui contraignent efficacement la physique de l'émission des galaxies primitives. Cette démarche peut être guidée par l'observation de galaxies proches aux propriétés similaires à celles attendues pour les galaxies primitives, c'est-à-dire extrêmement pauvres en métaux et formant activement des étoiles. De telles galaxies dans l'Univers local présentent des champs de rayonnement étonnamment durs, se traduisant notamment par d'intenses raies d'émission à haute ionisation, qui ne peuvent être pleinement expliquées par aucun modèle existant. De récentes études pointent vers le rôle essentiel que pourraient jouer les étoiles binaires massives dans cette émission. Le but de cette thèse est d'abord d'ouvrir une nouvelle brèche dans la modélisation de l'émission des galaxies primitives, en explorant les signatures spectrales d'étoiles binaires massives. Pour cela, j'étudie l'émission nébulaire de populations d'étoiles jeunes générées grâce au nouveau modèle GALSEVN, qui combine le code de synthèse de population SEVN, qui inclut les interactions entre étoiles binaires, et le code d'évolution spectrale GALAXEV. Cette démarche confirme que les interactions entre les étoiles des systèmes binaires influencent fortement les propriétés d'émission des galaxies jeunes. Je montre en particulier que GALSEVN reproduit les rapports d'intensités HeII/Hb élevés communément observés dans les galaxies sous métalliques à formation stellaire active, difficiles à reproduire avec les modèles actuels. Je montre également comment la considération de sursauts de formation stellaire successifs peut améliorer l'accord avec les observations, tandis que les valeurs les plus extrêmes des caractéristiques spectrales de HeII peuvent être reproduites par des populations stellaires dominées par les étoiles massives. GALSEVN permet aussi de calculer de manière inédite l'émission des disques d'accrétion dans les binaires X et des chocs radiatifs issus des vents stellaires et des supernovae, en utilisant une approche auto-cohérente construite à partir des caractéristiques des populations stellaires. Ce travail montre que ces contributions n'affectent que peu les rapports tels que HeII/Hb, contrairement à ce qui a pu être prédit par d'autres modèles, qui semblent surestimer les luminosités X de galaxies proches par rapport à la fonction de luminosité moyenne observée pour les binaires X. GALSEVN peut également prédire l'émission de populations stellaires extrêmement sous-métalliques, représentatives des premières générations d'étoiles de notre Univers. L'étude de différents paramètres caractérisant l'émission de ces étoiles conforte l'idée que les premières étoiles ont pu jouer un rôle majeur dans la réionisation de l'Univers et la formation des structures observées aujourd'hui. L'ensemble des résultats présentés dans cette thèse permettent finalement d'établir une solide base pour l'investigation plus poussée des propriétés des galaxies primitives. En construisant une grille étendue de modèles aux propriétés stellaires et nébulaires variées, il est possible d'appliquer des méthodes statistiques pour interpréter les données collectées par le JWST et contraindre les propriétés physiques des galaxies observées. En conclusion, cette thèse a conduit au développement et à la publication de nouveaux modèles permettant une interprétation inédite des propriétés physiques de galaxies à l'émission dominée par leur jeune population stellaire
The James Webb Space Telescope, launched in December 2021, is heralded as the major observatory of the coming decade. While it will explore all phases of cosmic history, one of its main scientific goals is to reveal the first stars and galaxies formed in the darkness of the early Universe, which led to its reionisation before evolving into the population of galaxies observable today. To interpret the spectroscopic data collected by JWST, it is necessary to develop models which can effectively constrain the physics of primeval-galaxy emission. This approach can be guided by observations of nearby galaxies with properties similar to those expected of primeval galaxies, i.e. extremely metal-poor and actively star-forming. Such galaxies in the local Universe exhibit surprisingly hard radiation fields, resulting notably in intense high-ionization emission lines, which cannot be fully explained by any existing model. Recent studies point to the essential role that massive binary stars could play in this emission.The aim of this thesis is to open up a new gap in the modelling of the emission from primeval galaxies, by exploring the spectral signatures of massive binary stars. To this end, I study the nebular emission of populations of young stars generated using the new GALSEVN model, combining the population synthesis code SEVN, which includes interactions between stars in binary systems, with the spectral evolution code GALAXEV. This approach confirms that interactions between stars in binary systems strongly influence the emission properties of young galaxies. In particular, I show that GALSEVN is able to account for the high HeII/Hb intensity ratios commonly observed in metal-poor galaxies with active stellar formation, which are difficult to reproduce with current models. I also demonstrate how successive bursts of star formation can improve agreement with observations, while the most extreme HeII spectral features can be reproduced by stellar populations dominated by massive stars. GALSEVN also makes it possible to evaluate the emission from accretion discs of X-ray binaries and radiative shocks from stellar winds and supernovae in an innovative way, using a self-consistent approach built from the characteristics of the modelled stellar populations. This work shows that these contributions likely have little effect on ratios such as HeII/Hb, contrary to predictions from other models, which appear to overestimate the X-ray luminosities of nearby galaxies relative to the observed mean luminosity function of X-ray binaries. GALSEVN can also predict the emission of extremely metal-poor stellar populations, representative of the first generations of stars in our Universe. A study of different parameters characterising the emission of these stars supports the view that the first stars may have played a major role in the reionization of the Universe and the subsequent formation of the structures observed today. Overall, the results presented in this thesis provide a solid basis for further investigation of the properties of early galaxies. By building a grid of models spanning a wide range of stellar and nebular properties, it is possible to apply statistical methods to interpret data collected by JWST in terms of constraints on the physical properties of the observed galaxies. In conclusion, this thesis has led to the development and publication of new models enabling a novel interpretation of the physical properties of galaxies whose emission is dominated by young stellar populations