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Structure And Evolution Of Simulated Dwarf Galaxies And Milky Way Satellites In Cold And Warm Dark Matter Models
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Book Synopsis Structure and Evolution of Simulated Dwarf Galaxies and Wilky Way Satellites in Cold and Warm Dark Matter Models by : Jonas Frings
Download or read book Structure and Evolution of Simulated Dwarf Galaxies and Wilky Way Satellites in Cold and Warm Dark Matter Models written by Jonas Frings and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Illuminating The Star Clusters And Dwarf Galaxies by Multi-scale Baryonic Simulations by : Moupiya Maji
Download or read book Illuminating The Star Clusters And Dwarf Galaxies by Multi-scale Baryonic Simulations written by Moupiya Maji and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Over the past decade, advances in computational architecture have made it possiblefor the first time to investigate some of the fundamental questions around the birthand the growth of the building blocks of the universe; star clusters and galaxies. Inthese stellar and star-forming systems, baryonic physics play an important role indetermining their formation and evolution. Therefore, in my research, I have exploredstar-forming systems using high resolution baryonic cosmological simulations andexplored the origin of star clusters, anisotropic spatial distribution of satellite galaxiesand the effect of reionization on the evolution of dwarf galaxies.Observations of globular clusters show that they have universal lognormal massfunctions with a characteristic peak at 2 10^5MSun , although the origin of this peakeddistribution is unclear. Here I have investigated the formation and evolution of starclusters (SCs) in interacting galaxies using high-resolution hydrodynamical simulationsperformed with two different codes. I have found that massive star clusters in therange of 10^5.5 10^7.5 MSun form preferentially in extremely high-pressure gas cloudsin highly-shocked regions produced by galaxy interactions. These findings provide thefirst simulation confirmation of the analytical theory of high pressure induced clusterformation. Furthermore, these massive star clusters have quasi-lognormal initial massfunctions with a peak around 106 M . The number of clusters declines with timedue to destructive processes, but the shape and the peak of the mass functions do notchange significantly during the course of galaxy collisions. These results suggest thatgas-rich galaxy mergers provide a favorable environment for the formation of globularclusters and that the lognormal mass functions and the unique peak may originatefrom the extreme high-pressure conditions of the birth clouds and may survive thedynamical evolution.Observations of classical Milky Way satellites suggest that they are aligned in aplane inclined to the Galactic stellar disk, a phenomenon which later became knownas the disk of satellites(DoS). However, N-body simulations of galaxies predict anisotropic distribution of subhalos around the host galaxy and this discrepancy hasbeen strongly criticized as a failure of CDM. In this thesis, I have explored this highlydebated topic by reanalyzing the observations and exploring the satellite distributions in high-resolution baryonic simulations. In particular, I have demonstrated that asmall sample size can artificially produce a highly anisotropic spatial distributionand a strong clustering of the angular momenta of the satellites and have shownthat the current Milky way DoS is transient. Furthermore, I have analyzed twocosmological simulations using the same initial conditions of a Milky-Way-sizedgalaxy, an N-body run with dark matter only, and a hydrodynamic one with bothbaryonic and dark matter, and found that the hydrodynamic simulation producesa more anisotropic distribution of satellites than the N-body one. These resultssuggest that an anisotropic distribution of satellites in galaxies can originate frombaryonic processes in the hierarchical structure formation model, but the claimedhighly flattened, coherently rotating DoS of the Milky Way may be biased by the small number selection effect. Finally, I have investigated the distribution and kinematicsof satellites around a large sample of few thousand host galaxies in the Illustrissimulation and found that the DoS become more isotropic with increasing numberof satellites and no clear coherent rotation is found in most ( 90%) of the satellitesystems. Furthermore, their overall evolution indicates that the DoS may be part oflarge scale filamentary structure. These findings can help resolve the contradictoryclaims of DoS in galaxies and show that baryonic processes may be the key to solvethe so-called small scale CDM problems.Additionally, I have also explored the effects of reionization on the star formationhistories of dwarfs galaxies, using a cosmological hydrodynamic simulation of MilkyWay and its satellite galaxies. I have found that most dwarfs are extremely old systemsand star formation is quenched earlier in lower mass galaxies. During reionization,most of the lower mass dwarfs are destroyed while the remaining massive dwarfsbecome severely baryon deficient. The dwarf galaxies play a very important role inshaping the path of cosmic history, especially in terms of reionization. Observingand studying the ultra-faint dwarfs hold the key to understanding the physics of earlyuniverse in great depth.
Book Synopsis Cosmological Simulations of Galactic Disc Assembly by : Elisa House
Download or read book Cosmological Simulations of Galactic Disc Assembly written by Elisa House and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: We address the issue of kinematic heating in disc galaxies by analysing a suite of cosmological Milky Way-type disc simulations run with different particle-and grid-based hydrodynamical codes and different resolution, and compare them with observations of the Milky Way. By studying the kinematics of disc stars in these simulations, we seek to determine whether or not the existence of a fragile thin disc is possible within a cosmological framework, where multiple mergers and interactions are the essence of galaxy formation. We study the velocity dispersion-age relation for disc stars at $z=0$ and find that four of the simulations, the stellar disc appears to undergo continual/secular heating. Two other simulations suggest a "saturation" in the heating profile for young stars in the disc. None of the simulations have thin discs as old as that of the Milky Way. We also analyse the kinematics of disc stars at the time of their birth, and find that in some simulations old stars are born cold within the disc and are subsequently heated, while other simulations possess old stellar populations, which are born relatively hot. The models which are in better agreement with observations of the Milky Way's stellar disc undergo significantly lower minor-merger/assembly activity after the last major merger. By running a set of isolated Milky Way-type simulations with different resolution and different density thresholds for star formation we conclude that, on top of the effects of mergers, there exists a ``floor'' in the dispersion that is related to the underlying treatment of the heating and cooling of the interstellar medium, and the low density threshold which such codes use for star formation. A persistent issue in simulations of disc galaxies is the formation of large spheroidal components, and disc galaxies with larger bulge to disc ratios than is observed. This problem is alleviated by supernova feedback. We found that by increasing the feedback in the simulations, we decrease the amount of stars that are accreted onto the main galaxy. The star formation is quenched more efficiently in low mass satellites when stronger feedback is implemented as well as in the main halo. These effects result in a disc galaxy, which has formed less stars overall, but more importantly, contains less accreted stars. As the strong stellar feedback quenches the star formation in the small building blocks, the metallicity of the accreted stars is lower than in the case where less feedback was used. In the context of hierarchical formation, mass assembly is expected to be scale free. Yet the properties of galaxies depend strongly on their mass. We examine how baryonic physics has different effects at different mass scales by analysing three cosmological simulations using the same initial conditions that are scaled to three different masses. Despite their identical dark matter merger history, we show that the simulated galaxies have significantly different stellar accretion histories. As we go down in mass, the lowest mass progenitors are unable to form stars, resulting in a low mass galaxy with less accreted stars. The overall chemical properties are also distinct at the different mass scales, as one might expect from the mass-metallicity relation of observed galaxies. We examine gradients of chemical abundances with radius and with height above the disc, and look for properties that are retained at different mass scales and properties which change, often dramatically. We analyse the kinematic and chemical properties of their accreted and in-situ populations. Again, trends can be found that persist at all mass scales, providing signatures of hierarchical structure formation. We find that accreted populations in the high mass simulation did not resemble any of the populations in the lower mass galaxies, showing that the chemical properties of proto-galaxies, which merge at high redshift to form massive galaxies, differ from the properties of low mass galaxies that survive at z=0. We probe further the signatures of hierarchical structure formation at smaller scales, in dwarf galaxies. We analysed the morphologies, kinematics and chemical properties of two simulated dwarf galaxies with different merger histories. We again analyse the accreted and in-situ populations. Observations of dwarf galaxies have found that they are comprised of multiple components. Our simulated dwarfs indicate that such populations may indeed be a manifestation of the hierarchical formation process in action in these lower mass galaxies. In one simulated dwarf, the in-situ stellar component forms a thin disc and a thick disc. We show that the thick disc in this simulation forms from in-situ stars that are born kinematically hot in the disc from early gas-rich mergers. The thin disc is formed quiescently from the later infall of gas. The accreted stars in the simulation were found to form an extended stellar halo. Chemical signatures of the three populations are also explored. The second dwarf we analysed has different galactic components, a result found to be due to the different merger history of this galaxy. The last major merger in this simulation occurs early on in the formation process between two proto-galaxies of similar mass. The result is a dwarf galaxy comprised of a disc formed of in-situ stars and a flattened rotating stellar halo formed of accreted stars. The angular momentum of the accreted and old in-insitu stars is obtained from the last major merger. We discuss the resemblance of this flattened rotating stellar halo to fast rotating flattened elliptical galaxies, and propose that such structures may explain some of the observed extra-galactic thick discs. These studies show that galactic properties emerge through the complex inter-play between hierarchical structure formation, star formation, and feedback from supernovae. Different modelling of these processes will alter the simulated galaxy's properties, and detailed comparisons with observations can then be made to determine the dominant processes responsible for different galactic properties. We remain optimistic that further improvement in modelling will allow deeper insights into the processes of galaxy formation and evolution.
Book Synopsis The Milky Way's Dwarf Satellite Galaxies in [L]CDM by : Christopher Barber
Download or read book The Milky Way's Dwarf Satellite Galaxies in [L]CDM written by Christopher Barber and published by . This book was released on 2014 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Current models of cosmology and galaxy formation are possibly at odds with observations of small-scale galaxies. Such is the case for the dwarf spheroidal (dSph) galaxies of the Milky Way (MW), where tension exists in explaining their observed abundance, mass, and internal structure. Here we present an analysis of the substructure surrounding MW-sized haloes in a Lambda Cold Dark Matter (LCDM) simulation suite. Combined with a semi-analytic model of galaxy formation and evolution, we identify substructures that are expected to host dSph galaxies similar to the satellites of the MW. We subsequently use these simulations to investigate the orbital properties of dSph satellite galaxies to make contact with those orbiting the MW. After accretion into the main halo, the higher mass ``luminous'' substructure remains on highly radial orbits while the orbits of lower mass substructure, which are not expected to host stars, tend to scatter off of the luminous substructure, and thus circularize over time. The orbital ellipticity distribution of the luminous substructure shows little dependence on the mass or formation history of the main halo, making this distribution a robust prediction of LCDM. Through comparison with the ellipticity distribution computed from the positions and velocities of the nine MW dSph galaxies that currently have proper motion estimates as a function of the assumed MW mass, we present a novel means of estimating the virial mass of the Milky Way. The best match is obtained assuming a mass of 1.1 x 10^12 M_sun with 95 per cent confidence limits of (0.6 - 3.1) x 10^12 M_sun. The uncertainty in this estimate is dominated by the large uncertainties in the proper motions and small number of MW satellites used, and will improve significantly with better proper motion measurements from Gaia. We also measure the shape of the gravitational potential of subhaloes likely to host dSphs, down to radii comparable to the half-light radii of MW dSphs. Field haloes are triaxial in general, while satellite haloes become more spherical over time due to tidal interactions with the host. Thus through the determination of the shape of a MW dSph's gravitational potential via line of sight velocity measurements, one could in principle deduce the impact of past tidal interactions with the MW, and thus determine its dynamical history. Additionally, luminous subhaloes experience a radial alignment of their major axes with the direction to the host halo over time, caused by tidal torquing with the host's gravitational potential during close pericentric passages. This effect is seen at all radii, even down to the half-light radii of the satellites. Radial alignment must be taken into account when calibrating weak-lensing surveys which often assume isotropic orientations of satellite galaxies surrounding host galaxies and clusters.
Book Synopsis Evolution of Dwarf Galaxy Properties in Local Group Environments by : Kenza Sigrid Arraki
Download or read book Evolution of Dwarf Galaxy Properties in Local Group Environments written by Kenza Sigrid Arraki and published by . This book was released on 2016 with total page 312 pages. Available in PDF, EPUB and Kindle. Book excerpt: Understanding galaxy evolution depends on connecting large-scale structures determined by the [Lambda]CDM model with, at minimum, the small-scale physics of gas, star formation, and stellar feedback. Formation of galaxies within dark matter halos is sensitive to the physical phenomena occurring within and around the halo. This is especially true for dwarf galaxies, which have smaller potential wells and are more susceptible to the effects of tidal stripping and gas ionization and removal than larger galaxies. At dwarf galaxies scales comparisons of dark matter-only simulations with observations has unveiled various differences such as the core-cusp, the missing satellites, and the too big to fail problems. We have run suites of collisionless and hydrodynamical simulations of dwarf galaxies evolution in massive host environments to address these issues. We performed controlled, numerical simulations, which mimic the effects of baryons, in order to examine the assumptions implicitly made by dark matter-only simulations. The too big to fail problem is due to the overabundance of relatively massive, dense satellite galaxies found in simulations of Milky Way-like environments. We found that the removal of a small baryonic component from the central region of forming dwarf spheroidal galaxies and the inclusion of a disk component in the host galaxy can substantially reduce the central dark matter density of satellites, bringing simulations and observations of satellites into agreement. Additionally, we studied hydrodynamical simulations of massive host galaxies and their surrounding dwarf galaxy populations. The VELA simulation suite of cosmological zoom-in simulations is run with the ART code, stochastic star formation, and stellar feedback (supernovae feedback, stellar winds, radiation pressure, and photoionization pressure). The suite includes host galaxies with M[subscript vir](z = 0 ) = 1011 - 1012 M[sol] and their satellite dwarf galaxies and local isolated dwarf galaxies around each primary galaxy. We found that the inclusion of these relevant physical processes aligned the velocity functions and star formation histories of the dwarf galaxy populations closer to observations of the Local Group dwarf galaxies. By reproducing observations of dwarf galaxies we show how the inclusion of baryons in simulations relieves many of the discovered tensions between dark matter-only simulations and observations.
Book Synopsis Constraints on Dark Matter and Milky Way Satellite Galaxies from Cosmological Simulations by : Miguel Eduardo Rocha Gaso
Download or read book Constraints on Dark Matter and Milky Way Satellite Galaxies from Cosmological Simulations written by Miguel Eduardo Rocha Gaso and published by . This book was released on 2014 with total page 181 pages. Available in PDF, EPUB and Kindle. Book excerpt: Cosmological simulations describing the non-linear evolution of dark matter structures in the Universe have become an indispensable tool to study the predictions made by our standard model of cosmology, and to confront them with observations. In this thesis I present a new idea for using cosmological simulations to infer the accretion times of Milky Way satellite galaxies from their observed positions and kinematics. We find that Carina, Ursa Minor, and Sculptor were all accreted early, more than 8 Gyr ago. Five other dwarfs, including Sextans and Segue 1, are also probable early accreters, though with larger uncertainties. On the other extreme, Leo T is just falling into the Milky Way for the first time while Leo I fell ~2 Gyr ago and is now climbing out of the Milky Way's potential after its first perigalacticon. The energies of several other dwarfs, including Fornax and Hercules, point to intermediate infall times, 2 - 8 Gyr ago. Our analysis suggests that the Large Magellanic Cloud crossed inside the Milky Way virial radius recently, within the last ~4 billion years. Also I present new constrains on how strongly dark matter particles can interact with themselves. For this we use a set cosmological simulations that implement a new self-consistent algorithm to treat dark matter self-interactions. We find that self-interacting dark matter models with cross sections in the order [sigma]/m ~ 0.5 cm2 /g ~ 1 barn/GeV would be capable of reproducing the observed core sizes and central densities of dark matter halos in a wide range of scales, from tiny dwarf galaxies to large galaxy clusters, without violating any other observational constraints. Higher resolution simulations over a wider range of masses and properly accounting for the effects of baryonic processes that are not yet included in our simulation will be required to confirm our expectations and place better constraints. I discuss our plans for achieving this goal and show some preliminary results from a new set of simulations.
Book Synopsis Probing Local Group Galactic Substructure with Cosmological Simulations by : Gregory Alan Dooley
Download or read book Probing Local Group Galactic Substructure with Cosmological Simulations written by Gregory Alan Dooley and published by . This book was released on 2017 with total page 358 pages. Available in PDF, EPUB and Kindle. Book excerpt: The Lambda cold dark matter (ACDM) model is enormously successful at predicting large scale structure in the Universe. However, some tensions still remain on small scales, specifically regarding observed satellites of the Milky Way (MW) and Andromeda. Foremost among the problems have been the missing satellite, too big to fail, and cusp/core problems, which concern the expected abundance of satellites and their inner structure. This Ph.D. thesis consists of a series of studies using dark matter only cosmological N-body simulations of MW-mass galaxies to address topics related to these issues. In light of the recent Planck mission, I investigate how changes to cosmological parameters affect dark matter halo substructure. I find that the process of continuous sub-halo accretion and destruction leads to a steady state description of most subhalo properties in a given host, unchanged by small fluctuations in cosmological parameters. Subhalo concentration, maximum circular velocity, and formation times, however, are somewhat affected. One way to reduce the central density of satellites, as needed to solve the cusp/core and too big to fail problems, is through self-interacting dark matter (SIDM). I search for new implications of SIDM and find that stars in satellites spread out to larger radii and are tidally stripped at a higher rate in SIDM than CDM, even though the mass loss rate of dark matter is unchanged. These signatures should be particularly prominent in ultrafaint dwarf galaxies for the class of otherwise difficult to constrain velocity-dependent SIDM models. I also helped carry out the Caterpillar project, a suite of 36 high mass resolution (~ 10' Mo/particle) simulations of MW-like galaxies used to study diversity in halo substructure. To these, I apply abundance matching and reionization models to make novel predictions about the abundance of satellites in isolated dwarf galaxies out to 8 Mpc to help guide future searches. Applying the same techniques to predict satellites within 50 kpc of the LMC, I discover large discrepancies with the observed stellar mass function, which may lead to new constraints on the galaxy stellar mass-halo mass relationship, and the ability of reionization to leave dark matter halos entirely dark.
Book Synopsis Cold Dark Matter Substructure and Galactic Disks I by :
Download or read book Cold Dark Matter Substructure and Galactic Disks I written by and published by . This book was released on 2007 with total page 20 pages. Available in PDF, EPUB and Kindle. Book excerpt: We conduct a series of high-resolution, fully self-consistent dissipation less N-body simulations to investigate the cumulative effect of substructure mergers onto thin disk galaxies in the context of the [Lambda]CDM paradigm of structure formation. Our simulation campaign is based on a hybrid approach combining cosmological simulations and controlled numerical experiments. Substructure mass functions, orbital distributions, internal structures, and accretion times are culled directly from cosmological simulations of galaxy-sized cold dark matter (CDM) halos. We demonstrate that accretions of massive subhalos onto the central regions of host halos, where the galactic disk resides, since z ≈ 1 should be common occurrences. In contrast, extremely few satellites in present-day CDM halos are likely to have a significant impact on the disk structure. This is due to the fact that massive subhalos with small orbital pericenters that are most capable of strongly perturbing the disk become either tidally disrupted or suffer substantial mass loss prior to z = 0. One host halo merger history is subsequently used to seed controlled N-body experiments of repeated satellite impacts on an initially-thin Milky Way-type disk galaxy. These simulations track the effects of six dark matter substructures, with initial masses in the range ≈ (0.7-2) x 101° M{sub {circle_dot}} (≈ 20-60% of the disk mass), crossing the disk in the past ≈ 8 Gyr. We show that these accretion events produce several distinctive observational signatures in the stellar disk including: a long-lived, low-surface brightness, ring-like feature in the outskirts; a significant flare; a central bar; and faint filamentary structures that (spuriously) resemble tidal streams in configuration space. The final distribution of disk stars exhibits a complex vertical structure that is well-described by a standard 'thin-thick' disk decomposition, where the 'thick' disk component has emerged primarily as a result of the interaction with the most massive subhalo. We conclude that satellite-disk encounters of the kind expected in [Lambda]CDM models can induce morphological features in galactic disks that are similar to those being discovered in the Milky Way, M31, and in other nearby and distant disk galaxies. These results highlight the significant role of CDM substructure in setting the structure of disk galaxies and driving galaxy evolution. Upcoming galactic structure surveys and astrometric satellites may be able to distinguish between competing cosmological models by testing whether the detailed structure of galactic disks is as excited as predicted by the CDM paradigm.
Book Synopsis The Tumultuous Lives of Galactic Dwarfs and the Missing Satellites Problem by : Andrey V. Kravtsov
Download or read book The Tumultuous Lives of Galactic Dwarfs and the Missing Satellites Problem written by Andrey V. Kravtsov and published by . This book was released on 2004 with total page 16 pages. Available in PDF, EPUB and Kindle. Book excerpt: Hierarchical Cold Dark Matter (CDM) models predict that Milky Way sized halos contain several hundred dense low-mass dark matter satellites (the substructure), an order of magnitude more than the number of observed satellites in the Local Group. If the CDM paradigm is correct, this prediction implies that the Milky Way and Andromeda are filled with numerous dark halos. To understand why these halos failed to form stars and become galaxies, we need to understand their history. We analyze the dynamical evolution of the substructure halos in a high-resolution cosmological simulation of Milky Way sized halos in the ACDM cosmology.
Book Synopsis Dark Matter And Cosmic Web Story (Second Edition) by : Jaan Einasto
Download or read book Dark Matter And Cosmic Web Story (Second Edition) written by Jaan Einasto and published by World Scientific. This book was released on 2024-04-29 with total page 412 pages. Available in PDF, EPUB and Kindle. Book excerpt: The concepts of dark matter and the cosmic web are some of the most significant developments in cosmology in the past century. They have decisively changed the classical cosmological paradigm, which was first elaborated upon during the first half of the 20th century but ran into serious problems in the second half. Today, they are integral parts of modern cosmology, which explains everything from the Big Bang to inflation to the large-scale structure of the Universe.Dark Matter and Cosmic Web Story describes the contributions that led to a paradigm shift from the Eastern point of view. It describes the problems with the classical view, the attempts to solve them, the difficulties encountered by those solutions, and the conferences where the merits of the new concepts were debated. Amidst the science, the story of scientific work in a small country occupied by the Soviet Union and the tumultuous events that led to its breakup are detailed as well.The development of cosmology has often treated as a West-East conflict between the American school led by Jim Peebles in Princeton and the Soviet team led by Yakov Zeldovich in Moscow. Actually, the development of ideas was broader, and a certain role played the Tartu team. The Tartu cosmology school was founded by Ernst Öpik and has its own traditions and attitude to science. In the new edition of the book the interplay between three cosmology schools is written in more detail. The recent development of dark matter and cosmic web studies is described, as well as the evolution of global properties of the cosmic web.This book is accompanied by a website which contains additional material: copies of the originals of some crucial papers, astronomical movies, and movies which showcase the private life of the author. In this second edition, two chapters on the statistical description of the cosmic web and its development were added, as well as chapter on the sociology of science. To keep the length of this book reasonable, a lot of reorganisation of the text has been done as well.
Book Synopsis The Local Group and Its Dwarf Galaxy Members in the Standard Model of Cosmology by : Azadeh Fattahi
Download or read book The Local Group and Its Dwarf Galaxy Members in the Standard Model of Cosmology written by Azadeh Fattahi and published by . This book was released on 2017 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: According to the current cosmological paradigm, ``Lambda Cold Dark Matter'' (LambdaCDM), only ~20% of the gravitating matter in the universe is made up of ordinary (i.e. baryonic) matter, while the rest consists of invisible dark matter (DM) particles, which existence can be inferred from their gravitational influence on baryonic matter and light. Despite the large success of the LambdaCDM model in explaining the large scale structure of the Universe and the conditions of the early Universe, there has been debate on whether this model can fully explain the observations of low mass (dwarf) galaxies. The Local Group (LG), which hosts most of the known dwarf galaxies, is a unique laboratory to test the predictions of the LambdaCDM model on small scales. I analyze the kinematics of LG members, including the Milky~Way-Andromeda (MW-M31) pair and dwarf galaxies, in order to constrain the mass of the LG. I construct samples of LG analogs from large cosmological N-body simulations, according to the following kinematics constraints: (a) the separation and relative velocity of the MW-M31 pair; (b) the receding velocity of dwarf galaxies in the outskirts of the LG. I find that these constraints yield a median total mass of 2*10^#x12; solar masses for the MW and M31, but with a large uncertainty. Based on the mass and the kinematics constraints, I select twelve LG candidates for the APOSTLE simulations project. The APOSTLE project consists of high-resolution cosmological hydrodynamical simulations of the LG candidates, using the EAGLE galaxy formation model. I show that dwarf satellites of MW and M31 analogs in APOSTLE are in good agreement with observations, in terms of number, luminosity and kinematics. There have been tensions between the observed masses of LG dwarf spheroidals and the predictions of N-body simulations within the LambdaCDM framework; simulations tend to over-predict the mass of dwarfs. This problem is known as the ``too-big-to-fail'' problem. I find that the enclosed mass within the half-light radii of Galactic classical dwarf spheroidals, is in excellent agreement with the simulated satellites in APOSTLE, and that there is no too-big-to-fail problem in APOSTLE simulations. A few factors contribute in solving the problem: (a) the mass of haloes in hydrodynamical simulations are lower compared to their N-body counterparts; (b) stellar mass-halo mass relation in APOSTLE is different than the ones used to argue for the too-big-to-fail problem; (c) number of massive satellites correlates with the virial mass of the host, i.e. MW analogs with virial masses above ~ 3*10^#x12; solar masses would have faced too-big-to-fail problems; (d) uncertainties in observations were underestimated in previous works. Stellar mass-halo mass relation in APOSTLE predicts that all isolated dwarf galaxies should live in haloes with maximum circular velocity (V_max) above 20 km/s. Satellite galaxies, however, can inhabit lower mass haloes due to tidal stripping which removes mass from the inner regions of satellites as they orbit their hosts. I examine all satellites of the MW and M31, and find that many of them live in haloes less massive than V_max=20 km/s. I additionally show that the low mass population is following a different trend in stellar mass-size relation compared to the rest of the satellites or field dwarfs. I use stellar mass-halo mass relation of APOSTLE field galaxies, along with tidal stripping trajectories derived in Penarrubia et al., in order to predict the properties of the progenitors of the LG satellites. According to this prediction, some satellites have lost a significant amount of dark matter as well as stellar mass. Cra~II, And~XIX, XXI, and XXV have lost 99 per-cent of their stellar mass in the past. I show that the mass discrepancy-acceleration relation of dwarf galaxies in the LG is at odds with MOdified Newtonian Dynamics (MOND) predictions, whereas tidal stripping can explain the observations very well. I compare observed velocity dispersion of LG satellites with the predicted values by MOND. The observations and MOND predictions are inconsistent, in particular in the regime of ultra faint dwarf galaxies.
Book Synopsis Faint Galaxies and Small Halos by : Ethan Oliver Nadler
Download or read book Faint Galaxies and Small Halos written by Ethan Oliver Nadler and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: This thesis addresses the intersection of dark matter and galaxy formation physics by modeling the faintest galaxies in the Universe in a cosmological context. In the first part of this work, I present an empirical model for the connection between faint galaxies and small dark matter halos. Combining this model with tailored cosmological simulations of Milky Way analogs and state-of-the-art satellite galaxy observations yields new insights into dwarf galaxy formation and hierarchical structure formation. Next, I show that these same dwarf galaxy observations place stringent constraints on microphysical dark matter properties including its warmth, self-interactions, Standard Model interactions, minimum particle mass, and formation epoch. Finally, I describe a unified framework that combines dark matter constraints from dwarf galaxies and strong gravitational lensing, paving the way for next-generation surveys of small-scale cosmic structure to deliver unprecedented insights into dark matter physics.
Book Synopsis Galactic Buiilding Blocks by : Andrew Lipnicky
Download or read book Galactic Buiilding Blocks written by Andrew Lipnicky and published by . This book was released on 2017 with total page 314 pages. Available in PDF, EPUB and Kindle. Book excerpt: "The prevailing model of structure formation that describes how matter is distributed throughout the Universe is known as the Lambda Cold Dark Matter paradigm. A key component of this paradigm is dark matter, which has so far gone undetected in laboratory experiments but is inferred from a wide variety of astrophysical observations. Although the Cold Dark Matter paradigm is extremely successful on large scales, there are significant differences between what computer simulations predict and what we observe on galaxy scales. The purpose of the work presented in this Dissertation is to address some of the issues surrounding the current structure formation paradigm and further develop some tools for investigating small scale structure. An issue that has caused recent controversy is known as the Planes of Dwarf Galaxies problem which describes the curious alignment of the Milky Way's dwarf galaxies into a thin planar structure. We have investigated this structure through time by integrating their orbits using the latest proper motion data as well as compared the distribution with current cosmological simulations and found no significant difference between the Milky Way distribution and simulations. Through analysis of observations of the disturbances in the extended neutral hydrogen disks of spiral galaxies, one can characterize dark matter substructure and the dark matter halo of a host galaxy. This process is called Tidal Analysis. Using a simple test particle code to model satellite interactions with a gas disk, we have developed a scaling relation to relate the observed density response of the disk to the mass and pericenter of a satellite. With this relation, observers can now immediately infer the recent interaction history of a spiral galaxy from neutral hydrogen studies. Changing gears to observational studies of small scale structure, we report observations of Cepheid variables in a putative dwarf galaxy located along the line of sight of the galactic plane that was first predicted through the use of Tidal Analysis. Observations are still ongoing; however, preliminary results indicate that the Cepheids are part of structure that is moving independently of the Milky Way. Finally, in an effort to use Tidal Analysis on other galaxies to constrain substructure, we have begun a 21-cm radio observing campaign of a set of spiral galaxies at redshift z=0.1 to obtain their total mass in neutral hydrogen. This unique set of galaxies also act as strong gravitational lenses, thus allowing us to use both Tidal Analysis and gravitational lensing together for the first time. We report a secure detection and mass measurement for one of our sources and six upper mass limits."--Abstract.
Book Synopsis The Formation and Survival of Disk Galaxies by : James E. Taylor
Download or read book The Formation and Survival of Disk Galaxies written by James E. Taylor and published by . This book was released on 2001 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The dynamical evolution of substructure within dark matter halos is of central importance in determining many aspects of galaxy formation and galaxy evolution in cold dark matter cosmologies. The overall sequence in which the different stellar components of galaxies are assembled, the survival of galactic disks, the number of dwarf satellites orbiting giant galaxies, and the nature of stellar material in galactic halos all depend on the dynamics of halo substructure. In this thesis, I develop an analytic description of the evolution of substructure within a dark matter halo, and use it to construct a semi-analytic model of the formation and evolution of disk galaxies. Substructure within an individual halo is modelled as a set of distinct subhalos, orbiting in a smooth background. These subhalos evolve through three main processes: dynamical friction, tidal mass loss, and tidal heating. By including analytic descriptions of these three processes explicitly in a simple orbital integration scheme, it is possible to reproduce the results of high-resolution numerical simulations at a fraction of the computational expense. The properties of a subhalo can be estimated with an accuracy of 20%, until it has lost most of its mass or been disrupted. Using this description of satellite dynamics, I construct a semi-analytic model for the evolution of a galaxy or cluster halo. I show that this model reproduces the basic features of numerical simulations, and use it to investigate two major problems in current galaxy formation scenarios: the prediction of excessive substructure in galaxy halos, and the survival of galactic disks in halos filled with substructure. I show that the small number of dwarf galaxies observed in the Local Group can be explained by considering the effects of reionisation on star formation in small halos. The stellar luminosities predicted in this case match the observed luminosities of local satellites. The predicted spatial distribution, sizes and characteristic velocities of dwarf galaxies are also consistent with those observed locally. Many of these satellite galaxies are disrupted by tidal stripping or encounters. I investigate the properties of their debris, and show that its total mass and spatial distribution are similar to those of the stellar halo of the Milky Way. Furthermore, the stars in this debris are mainly old, satisfying another observational constraint on models of galaxy formation. Some satellites have been disrupted fairly recently, however, suggesting that coherent tidal streams may still be visible at the present day. Finally, I investigate the effects of encounters on the central disk within the main halo. I find that the rate of disruptive encounters drops off sharply after the galaxy is assembled, such that the typical disk has remained undisturbed for the past 8-10 billion years. Less disruptive encounters are more common, and disks are often heated as they re-form after their last disruption, producing components like the thick disk of the Milky Way. These results may resolve the long-standing uncertainty about disk ages in hierarchical, cold dark matter cosmologies. It is less clear whether the bulge-to-disk mass ratios predicted by the model, for the currently favoured LCDM cosmology, are consistent with observations. The relative mass of the bulge in typical disk galaxies may place an upper limit on the age of their stellar contents.
Book Synopsis Galaxy Formation and Evolution by : Houjun Mo
Download or read book Galaxy Formation and Evolution written by Houjun Mo and published by Cambridge University Press. This book was released on 2010-05-20 with total page 841 pages. Available in PDF, EPUB and Kindle. Book excerpt: A coherent introduction for researchers in astronomy, particle physics, and cosmology on the formation and evolution of galaxies.
Book Synopsis Models of the Galaxy and the Massive Spectroscopic Stellar Survey RAVE by : Tilmann Piffl
Download or read book Models of the Galaxy and the Massive Spectroscopic Stellar Survey RAVE written by Tilmann Piffl and published by . This book was released on 2013 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Numerical simulations of galaxy formation and observational Galactic Astronomy are two fields of research that study the same objects from different perspectives. Simulations try to understand galaxies like our Milky Way from an evolutionary point of view while observers try to disentangle the current structure and the building blocks of our Galaxy. Due to great advances in computational power as well as in massive stellar surveys we are now able to compare resolved stellar populations in simulations and in observations. In this thesis we use a number of approaches to relate the results of the two fields to each other. The major observational data set we refer to for this work comes from the Radial Velocity Experiment (RAVE), a massive spectroscopic stellar survey that observed almost half a million stars in the Galaxy. In a first study we use three different models of the Galaxy to generate synthetic stellar surveys that can be directly compared to the RAVE data. To do this we evaluate the RAVE selection function to great detail. Among the Galaxy models is the widely used Besancon model that performs well when individual parameter distribution are considered, but fails when we study chemodynamic correlations. The other two models are based on distributions of mass particles instead of analytical distribution functions. This is the first time that such models are converted to the space of observables and are compared to a stellar survey. We show that these models can be competitive and in some aspects superior to analytic models, because of their self-consistent dynamic history. In the case of a full cosmological simulation of disk galaxy formation we can recover features in the synthetic survey that relate to the known issues of the model and hence proof that our technique is sensitive to the global structure of the model. We argue that the next generation of cosmological galaxy formation simulations will deliver valuable models for our Galaxy. Testing these models with our approach will provide a direct connection between stellar Galactic astronomy and physical cosmology. In the second part of the thesis we use a sample of high-velocity halo stars from the RAVE data to estimate the Galactic escape speed and the virial mass of the Milky Way. In the course of this study cosmological simulations of galaxy formation also play a crucial role. Here we use them to calibrate and extensively test our analysis technique. We find the local Galactic escape speed to be 533 (+54/-41) km/s (90% confidence). With this result in combination with a simple mass model of the Galaxy we then construct an estimate of the virial mass of the Galaxy. For the mass profile of the dark matter halo we use two extreme models, a pure Navarro, Frenk & White (NFW) profile and an adiabatically contracted NFW profile. When we use statistics on the concentration parameter of these profile taken from large dissipationless cosmological simulations we obtain an estimate of the virial mass that is almost independent of the choice of the halo profile. For the mass M_340 enclosed within R_340 = 180 kpc we find 1.3 (+0.4/-0.3) x 10^12 M_sun. This value is in very good agreement with a number of other mass estimates in the literature that are based on independent data sets and analysis techniques. In the last part of this thesis we investigate a new possible channel to generate a population of Hypervelocity stars (HVSs) that is observed in the stellar halo. Commonly, it is assumed that the velocities of these stars originate from an interaction with the super-massive black hole in the Galactic center. It was suggested recently that stars stripped-off a disrupted satellite galaxy could reach similar velocities and leave the Galaxy. Here we study in detail the kinematics of tidal debris stars to investigate the probability that the observed sample of HVSs could partly originate from such a galaxy collision. We use a suite of $N$-body simulations following the encounter of a satellite galaxy with its Milky Way-type host galaxy. We quantify the typical pattern in angular and phase space formed by the debris stars and develop a simple model that predicts the kinematics of stripped-off stars. We show that the distribution of orbital energies in the tidal debris has a typical form that can be described quite accurately by a simple function. The main parameters determining the maximum energy kick a tidal debris star can get is the initial mass of the satellite and only to a lower extent its orbit. Main contributors to an unbound stellar population created in this way are massive satellites (M_sat > 10^9 M_sun). The probability that the observed HVS population is significantly contaminated by tidal debris stars appears small in the light of our results
Book Synopsis The Role of Halo Substructure in Gamma-Ray Dark Matter Searches by : Miguel A. Sánchez-Conde
Download or read book The Role of Halo Substructure in Gamma-Ray Dark Matter Searches written by Miguel A. Sánchez-Conde and published by MDPI. This book was released on 2020-05-28 with total page 220 pages. Available in PDF, EPUB and Kindle. Book excerpt: An important, open research topic today is to understand the relevance that dark matter halo substructure may have for dark matter searches. In the standard cosmological model, halo substructure or subhalos are predicted to be largely abundant inside larger halos, for example, galaxies such as ours, and are thought to form first and later merge to form larger structures. Dwarf satellite galaxies—the most massive exponents of halo substructure in our own galaxy—are already known to be excellent targets for dark matter searches, and indeed, they are constantly scrutinized by current gamma-ray experiments in the search for dark matter signals. Lighter subhalos not massive enough to have a visible counterpart of stars and gas may be good targets as well, given their typical abundances and distances. In addition, the clumpy distribution of subhalos residing in larger halos may boost the dark matter signals considerably. In an era in which gamma-ray experiments possess, for the first time, the exciting potential to put to test the preferred dark matter particle theories, a profound knowledge of dark matter astrophysical targets and scenarios is mandatory should we aim for accurate predictions of dark matter-induced fluxes for investing significant telescope observing time on selected targets and for deriving robust conclusions from our dark matter search efforts. In this regard, a precise characterization of the statistical and structural properties of subhalos becomes critical. In this Special Issue, we aim to summarize where we stand today on our knowledge of the different aspects of the dark matter halo substructure; to identify what are the remaining big questions, and how we could address these; and, by doing so, to find new avenues for research.
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