Academic literature on the topic 'Cosmic infall'

ABSTRACT We investigate the impact of the number of filaments connected to the nodes of the cosmic web on the physical properties of their galaxies using the Sloan Digital Sky Survey. We compare these measurements to the cosmological hydrodynamical simulations H orizon-(no)AGN and Simba. We find that more massive galaxies are more connected, in qualitative agreement with theoretical predictions and measurements in dark-matter-only simulations. The star formation activity and morphology of observed galaxies both display some dependence on the connectivity of the cosmic web at a fixed stellar mass: Less star forming and less rotation supported galaxies also tend to have higher connectivity. These results qualitatively hold both for observed and for virtual galaxies, and can be understood given that the cosmic web is the main source of fuel for galaxy growth. The simulations show the same trends at a fixed halo mass, suggesting that the geometry of filamentary infall impacts galaxy properties beyond the depth of the local potential well. Based on simulations, it is also found that active galactic nucleus feedback is key to reversing the relationship between stellar mass and connectivity at a fixed halo mass. Technically, connectivity is a practical observational proxy for past and present accretion (minor mergers or diffuse infall).

AbstractWe examine the infall pattern of subhaloes onto hosts in the context of the large-scale structure. We find that the infall pattern is essentially driven by the shear tensor of the ambient velocity field. Dark matter subhaloes are preferentially accreted along the principal axis of the shear tensor which corresponds to the direction of weakest collapse. We examine the dependence of this preferential infall on subhalo mass, host halo mass and redshift. Although strongest for the most massive hosts and the most massive subhaloes at high redshift, the preferential infall of subhaloes is effectively universal in the sense that its always aligned with the axis of weakest collapse of the velocity shear tensor. It is the same shear tensor that dictates the structure of the cosmic web and hence the shear field emerges as the key factor that governs the local anisotropic pattern of structure formation. Since the small (sub-Mpc) scale is strongly correlated with the mid-range (∼ 10 Mpc) scale - a scale accessible by current surveys of peculiar velocities - it follows that findings presented here open a new window into the relation between the observed large scale structure unveiled by current surveys of peculiar velocities and the preferential infall direction of the Local Group. This may shed light on the unexpected alignments of dwarf galaxies seen in the Local Group.

ABSTRACT We use magnetohydrodynamical simulations of Milky Way-mass haloes from the Auriga project to investigate the properties of surviving and destroyed dwarf galaxies that are accreted by these haloes over cosmic time. We show that the combined luminosity function of surviving and destroyed dwarfs at infall is similar in the various Auriga haloes, and is dominated by the destroyed dwarfs. There is, however, a strong dependence on infall time: destroyed dwarfs typically have early infall times of less than 6 Gyr (since the big bang), whereas the majority of dwarfs accreted after 10 Gyr have survived to the present day. Because of their late infall, the surviving satellites have higher metallicities at infall than their destroyed counterparts of similar mass at infall; the difference is even more pronounced for the present-day metallicities of satellites, many of which continue to form stars after infall, in particular for $M_{\rm star}\gt 10^7 \, {\rm M}_\odot$. In agreement with previous work, we find that a small number of relatively massive destroyed dwarf galaxies dominate the mass of stellar haloes. However, there is a significant radial dependence: while 90 per cent of the mass in the inner regions (${\lt}20\,$ kpc) is contributed, on average, by only three massive progenitors, the outer regions (${\gt}100\,$ kpc) typically have ∼8 main progenitors of relatively lower mass. Finally, we show that a few massive progenitors dominate the metallicity distribution of accreted stars, even at the metal-poor end. Contrary to common assumptions in the literature, stars from dwarf galaxies of mass $M_{\rm star}\lt 10^7 \, {\rm M}_\odot$ make up less than 10 per cent of the accreted, metal poor stars ([Fe/H] ${\lt}-3$) in the inner $50\,$ kpc.

AbstractThe distribution of baryons beyond galaxies is descibed. The majority of the baryons, which represent 4% of the cosmic mass and energy budget, lie far from individual galaxies in the diffuse intergalactic medium (IGM). Many of these baryons are in a warm phase that can be probed by quasar absorption in the Lyman-α line of hydrogen. The mature field of quasar spectroscopy can diagnose the location, physical state, metallicity, and general geometry of this gas, which is called the “cosmic web.” The remainder of the gas is kept very hot by infall and shocks and is mostly in higher density regions such as filaments, groups and clusters. The hot gas is only detectable via X-rays and the absorption of highly ionized species of heavy elements. The baryons in low density regions of space are excellent tracers of underlying dark matter. The evolution of the cosmic web indicates where to look for the baryons in collapsed objects but the overall inefficiency of galaxy formation has conspired to keep most baryons dark.

ABSTRACT Voids may affect galaxy formation via weakening mass infall or increasing disk sizes, which could potentially play a role in the formation of giant low surface brightness galaxies (LSBGs). If a dark matter halo forms at the potential hill corresponding to a void of the cosmic web, which we denote the ‘elaphrocentre’ in contrast to a barycentre, then the elaphrocentre should weaken the infall rate to the halo when compared to infall rates towards barycentres. We investigate this hypothesis numerically. We present a complete software pipeline to simulate galaxy formation, starting from a power spectrum of initial perturbations and an N-body simulation through to merger-history-tree based mass infall histories. The pipeline is built from well-established, free-licensed cosmological software packages, and aims at highly portable long-term reproducibility. We find that the elaphrocentric accelerations tending to oppose mass infall are modest. We do not find evidence of location in a void or elaphrocentric position weakening mass infall towards a galaxy. However, we find indirect evidence of voids influencing galaxy formation: while void galaxies are of lower mass compared to galaxies in high-density environments, their spin parameters are typically higher. For a fixed mass, the implied disc scale length would be greater. Tangential accelerations in voids are found to be high and might significantly contribute to the higher spin parameters. We find significantly later formation epochs for void galaxies; this should give lower matter densities and may imply lower surface densities of disc galaxies. Thus, void galaxies have higher spin parameters and later formation epochs; both are factors that may increase the probability of forming LSBGs in voids.

Aims. We use accurate data on distances and radial velocities of galaxies around the Local Group, as well as around 14 other massive nearby groups, to estimate their radius of the zero-velocity surface, R0, which separates any group against the global cosmic expansion. Methods. Our R0 estimate was based on fitting the data to the velocity field expected from the spherical infall model, including effects of the cosmological constant. The reported uncertainties were derived by a Monte Carlo simulation. Results. Testing various assumptions about a location of the group barycentre, we found the optimal estimates of the radius to be 0.91 ± 0.05 Mpc for the Local Group, and 0.93 ± 0.02 Mpc for a synthetic group stacked from 14 other groups in the Local Volume. Under the standard Planck model parameters, these quantities correspond to the total mass of the group ~ (1.6 ± 0.2) × 1012M⊙. Thus, we are faced with the paradoxical result that the total mass estimate on the scale of R0 ≈ (3−4)Rvir is only 60% of the virial mass estimate. Anyway, we conclude that wide outskirts of the nearby groups do not contain a large amount of hidden mass outside their virial radius.

AbstractWe present results of analysis of the dark matter (DM) pairwise velocity statistics in different Cosmic Web environments. We use the DM velocity and density field from the Millennium 2 simulation together with the NEXUS+ algorithm to segment the simulation volume into voxels uniquely identifying one of the four possible environments: nodes, filaments, walls or cosmic voids. We show that the PDFs of the mean infall velocities v12 as well as its spatial dependence together with the perpendicular and parallel velocity dispersions bear a significant signal of the large-scale structure environment in which DM particle pairs are embedded. The pairwise flows are notably colder and have smaller mean magnitude in wall and voids, when compared to much denser environments of filaments and nodes. We discuss on our results, indicating that they are consistent with a simple theoretical predictions for pairwise motions as induced by gravitational instability mechanism. Our results indicate that the Cosmic Web elements are coherent dynamical entities rather than just temporal geometrical associations. In addition it should be possible to observationally test various Cosmic Web finding algorithms by segmenting available peculiar velocity data and studying resulting pairwise velocity statistics.

AbstractDisk galaxies in cosmological numerical simulations grow by accreting gas from the cosmic web. This gas reaches the external disk, and then spirals in dragged along by tidal forces and/or disk instabilities. The importance of gas infall is as clear from numerical simulations as it is obscure to observations. Extremely metal poor (XMP) galaxies seem to be the best example we have of the gas accretion process at work. They have large off-center starbursts which show significant metallicity drop compared with the host galaxy. This observation is naturally explained as a gas accretion event caught in the act. We present preliminary results of the kinematical properties of the metal poor starbursts in XMPs, which suggest that the starbursts are kinematically decoupled entities within the host galaxy.

L'observatoire Pierre Auger, situé en Argentine, combine un réseau de surface, étendu sur 3000 km ^2 et composé de détecteurs Cherenkov, avec 4 télescopes de fluorescence pour mesurer des gerbes atmosphériques initiées par les rayons cosmiques d'ultra haute énergie. Cet observatoire "hybride" (en fonctionnement depuis 2004, et dont le déploiement a été achevé en 2008) est pleinement efficace pour des énergies de rayons cosmiques primaires supérieures à 10 ^18 eV, c'est-à-dire à partir de la cheville du spectre en énergie jusqu'aux plus hautes énergies. Après l'achèvement de l'observatoire principal, la collaboration Auger a débuté le déploiement de nouveaux instruments afin d'étendre la gamme d'énergie jusqu'à 0. 1EeV. Les extensions prévues comptent deux réseaux de surface resserés, dits "infill", d'espacement de 750m et 433m, avec des possibilités de détection de muons, et trois télescopes de fluorescence supplémentaires ayant un champ de vue d'élévation plus grande. Le réseau "infill" de 750m (couvrant environ 24km ^2) et les nouveaux télescopes sont aujourd'hui en fonctionnement. Leur but est la mesure des rayons cosmiques à partir d'en dessous du deuxième genou du spectre jusqu'à la cheville où les données recouvrent celles de l'observatoire principal. L'étude de l'évolution du spectre du deuxième genou à la cheville, alliée à la composition du rayon cosmique primaire, sont cruciales dans la compréhension de la transition de l'origine galactique à extragalactique des rayons cosmiques. Cette thèse s'appuie sur les données provenant du réseau "infill" de 750 m: l'objectif est la mesure du spectre en énergie des rayons cosmiques dans la région d'énergie au-dessus de 3x10 ^17 eV, où le réseau est pleinement efficace. Pour obtenir le spectre en énergie, plusieurs étapes sont nécessaires, à partir de la reconstruction des événements, en passant par la détermination exacte de l'exposition du réseau, jusqu'à la détermination de l'énergie du primaire. La thèse traite de ces aspects, avant d'atteindre le résultat final. Le premier chapitre donne une introduction générale de la physique des rayons cosmiques et des détecteurs associés. Il résume également les résultats expérimentaux au-dessus du première genou du spectre avec un accent particulier sur ceux obtenus au-dessus de 10 ^17 eV. Les deux chapitres suivants décrivent respectivement l'observatoire Pierre Auger et le réseau "infill". Dans le chapitre 2, après une description schématique des différentes composantes de l'observatoire, les principaux résultats obtenus par l'observatoire Auger sont résumés. Le chapitre 3 prépare le terrain pour les chapitres suivants. Il présente une description plus détaillée des caractéristiques du réseau "infill", en particulier la définition du trigger, la sélection et la reconstruction des événements. Dans le chapitre 4, la qualité de la reconstruction de la distribution latérale des gerbes observées est étudiée en détail. Ceci est particulièrement important pour le spectre en énergie, puisque le signal à une distance fixe de l'axe de la gerbe est utilisé comme estimateur de l'énergie de l'événement. Ce signal est estimé au moyen de la distribution latérale de la gerbe mesurée. Le chapitre 5 présente une comparaison entre la reconstruction d'événement du réseau "infill" et celle du réseau principal. En utilisant les gerbes détectées par les deux instruments, la géométrie et l'estimation de l'énergie obtenues sont comparées, montrant une bonne correspondance. Dans le chapitre 6 le seuil d'énergie du réseau, et donc l'ensemble des événements qui seront utilisés, est défini. Les méthodes pour obtenir l'exposition du réseau et les incertitudes systématiques associées sont discutées. Enfin, dans le chapitre 7, la technique pour obtenir l'énergie du primaire pour chaque gerbe détectée est présentée. Le spectre en énergie obtenu est discuté et le flux est montré compatible avec celui mesuré par d'autres instruments dans les régions d'énergie communes
The Pierre Auger Observatory, in Argentina, combines a 3000 $$\mathrm{km^2}$$ surface array of water Cherenkov detectors with fluorescence telescopes to measure extensive air showers initiated by ultra-high energy cosmic rays. This "hybrid" observatory (in operation since 2004, and completed in 2008) is fully efficient for cosmic rays energies above $$10^{18}$$ eV, that is, from just below the "ankle" of the energy spectrum up to the highest energies. After the completion of the main observatory, the Auger collaboration has started to deploy new instruments to extend the energy range down to about 0. 1 EeV. The planned extensions include two infill surface arrays with 750 and 433 m spacing, with muon detection capabilities, and three additional fluorescence telescopes with a more elevated field of view. The 750 m infill array (covering about 24 $$\mathrm{km^2}$$) and the new telescopes are now operational. Their aim is the measurement of cosmic rays from below the second knee of the spectrum up to the ankle, where data from the extensions overlap those from the main observatory. The study of the evolutior of the spectrum through the second knee and the ankle, together with the primary mass composition, are crucial to the understanding of the transition from a galactic cosmic ray origin to an extragalactic one. This thesis makes use of data from the 750 m infill array: the objective is the measurement of the cosmic ray energy spectrum in the energy region above $$3 \times 10^{17}$$ eV, where the array is fully efficient. To get to the energy spectrum, several steps are needed, from the reconstruction of events, through the precise determination of the exposure of the array, up to the determination of the primary energy. The thesis deals with these aspects, before reaching the final result. The first chapter gives a general introduction to cosmic ray physics and detectors. It also summarizes experimental results above the first knee of the spectrum with particular emphasis on those obtained above $$10^{17}$$ eV. The next two chapters describe the Pierre Auger Observatory and the infill array, respectively. In chapter 2, the main Auger results are summarized too, after a schematic description of th different components of the observatory. Chapter 3 sets the stage for the following chapters. It presents a more detailed description of the characteristics of the infill array, in particular the trigger definitions, event selection and reconstruction. In chapter 4 the performance of the reconstruction of the lateral distribution of observed showers is studied in detail. This is particularly important for the energy spectrum, since the signal at a fixed distance from the shower axis is used as the energy estimator of the event. This signal is estimated by means c the measured lateral distribution of the shower. Chapter 5 presents a comparison between the event reconstruction of the infill and main arrays. Using the set of showers detected by both instruments, the derived geometry and energy estimation are compared, showing a good agreement. In chapter 6, the energy threshold of the array, and hence the set of events to be used, is defined. The methods to obtain the exposure of the array are discussed, as well as related systematic uncertainties. Finally, in chapter 7, the technique to derive the primary energy for each detected shower is presented. The derived energy spectrum is discussed, and the flux is shown to be consistent with that measured by other instruments in the overlapping energy regions

Radiation in the first days of supernova explosions contains rich information about physical properties of the exploding stars. In the past three years, I used the intermediate Palomar Transient Factory to conduct one-day cadence surveys, in order to systematically search for infant supernovae. I show that the one-day cadences in these surveys were strictly controlled, that the realtime image subtraction pipeline managed to deliver transient candidates within ten minutes of images being taken, and that we were able to undertake follow-up observations with a variety of telescopes within hours of transients being discovered. So far iPTF has discovered over a hundred supernovae within a few days of explosions, forty-nine of which were spectroscopically classified within twenty-four hours of discovery.

Our observations of infant Type Ia supernovae provide evidence for both the single-degenerate and double-degenerate progenitor channels. On the one hand, a low-velocity Type Ia supernova iPTF14atg revealed a strong ultraviolet pulse within four days of its explosion. I show that the pulse is consistent with the expected emission produced by collision between the supernova ejecta and a companion star, providing direct evidence for the single degenerate channel. By comparing the distinct early-phase light curves of iPTF14atg to an otherwise similar event iPTF14dpk, I show that the viewing angle dependence of the supernova-companion collision signature is probably responsible to the difference of the early light curves. I also show evidence for a dark period between the supernova explosion and the first light of the radioactively-powered light curve. On the other hand, a peculiar Type Ia supernova iPTF13asv revealed strong near-UV emission and absence of iron in the spectra within the first two weeks of explosion, suggesting a stratified ejecta structure with iron group elements confined to the slow-moving part of the ejecta. With its total ejecta mass estimated to exceed the Chandrasekhar limit, I show that the stratification and large mass of the ejecta favor the double-degenerate channel.

In a separate approach, iPTF found the first progenitor system of a Type Ib supernova iPTF13bvn in the pre-explosion HST archival mages. Independently, I used the early-phase optical observations of this supernova to constrain its progenitor radius to be no larger than several solar radii. I also used its early radio detections to derive a mass loss rate of 3e-5 solar mass per year for the progenitor right before the supernova explosion. These constraints on the physical properties of the iPTF13bvn progenitor provide a comprehensive data set to test Type Ib supernova theories. A recent HST revisit to the iPTF13bvn site two years after the supernova explosion has confirmed the progenitor system.

Moving forward, the next frontier in this area is to extend these single-object analyses to a large sample of infant supernovae. The upcoming Zwicky Transient Facility with its fast survey speed, which is expected to find one infant supernova every night, is well positioned to carry out this task.

Of the approximately 33.2 million people living with human immunodeficiency virus (HIV) infection or acquired immune deficiency syndrome (AIDS) globally, it is estimated that about 2.1 million are children under the age of 15 (UNAIDS Annual Report 2007), with the vast majority of cases found in the developing world and particularly Sub-Saharan Africa. Over 90% of these infections occur through vertical transmission; that is, the virus is passed from mother to infant during the preor perinatal period, or through breast-feeding (UNAIDS 2007). Although medical management of the virus continues to improve both in the United States and around the world, the negative effects of this disease continue to impact these youngest and most vulnerable members of society in significant ways. In particular, neurocognitive deficits associated with HIV and AIDS in children are common, and vertically infected children are at increased risk for a host of neurocognitive consequences due to both the direct and indirect impact of HIV on the developing central nervous system (CNS) (Armstrong, Seidel, and Swales 1993; Mintz 1996; Knight et al., 2000). Such disruptions to the normal neurodevelopmental course may yield a variety of outcomes, from relatively mild symptoms, to severe and global cognitive compromise. These disparate outcomes appear related to multiple factors including specific disease factors, such as level of immunosuppression and the nature of the insult to the brain (Mintz 1999), the occurrence of severe infection or comorbid conditions during sensitive developmental periods (Armstrong et al., 1993; Armstrong and Mulhern, 1999; Mintz, 1999; Mitchell, 2001), as well as the timing of highly active antiretroviral therapy (HAART) initiation (Hazra et al., 2007; Mintz, 1999; Mitchell, 2001). Moreover, HIV is associated with a host of other stressors that may profoundly affect developmental trajectories above and beyond the impact of the virus alone. These include extreme poverty, malnutrition, prenatal drug exposure, and loss of primary care giver through illness or death (Armstrong et al., 1993; Coscia et al., 2001).