Large Dark Matter Halos Research Articles

Formation of Superthin Galaxies in IllustrisTNG

Superthin galaxies are observed to have stellar disks with extremely small minor-to-major axis ratios. In this work, we investigate the formation of superthin galaxies in the TNG100 simulation. We trace the merger history and investigate the evolution of galaxy properties of a selected sample of superthin galaxies and a control sample of galaxies that share the same joint probability distribution in the stellar-mass and color diagram. Through making comparisons between the two galaxy samples, we find that present-day superthin galaxies had similar morphologies as the control sample counterparts at higher redshifts, but have developed extended flat “superthin” morphologies since z ∼ 1. During this latter evolution stage, superthin galaxies undergo an overwhelmingly higher frequency of prograde mergers (with orbit-spin angle θ orb ≤ 40°). Accordingly the spins of their dark matter halos have grown significantly and become noticeably higher than those of their normal disk counterparts. This further results in the buildup of their stellar disks at larger distances much beyond the regimes of normal disk galaxies. We also discuss the formation scenario of those superthin galaxies that live in larger dark matter halos as satellite galaxies therein.

Derivation of a generalized Schrödinger equation for dark matter halos from the theory of scale relativity

Abstract Using Nottale’s theory of scale relativity, we derive a generalized Schrodinger equation applying to dark matter halos. This equation involves a logarithmic nonlinearity associated with an effective temperature and a source of dissipation. Fundamentally, this wave equation arises from the nondifferentiability of the trajectories of the dark matter particles whose origin may be due to ordinary quantum mechanics, classical ergodic (or almost ergodic) chaos, or to the fractal nature of spacetime at the cosmic scale. The generalized Schrodinger equation involves a coefficient D , possibly different from ħ ∕ 2 m (where ħ is the Planck constant and m the mass of the particles), whose value for dark matter halos is D = 1 . 02 × 1 0 23 m2/s. This model is similar to the Bose–Einstein condensate dark matter model except that it does not require the dark matter particle to be ultralight. It can accommodate any type of particles provided that they have nondifferentiable trajectories. We suggest that the cold dark matter crisis may be solved by the fractal (nondifferentiable) structure of spacetime at the cosmic scale, or by the chaotic motion of the particles on a very long timescale, instead of ordinary quantum mechanics. The equilibrium states of the generalized Schrodinger equation correspond to configurations with a core-halo structure. The quantumlike potential generates a solitonic core that solves the cusp problem of the classical cold dark matter model. The logarithmic nonlinearity accounts for the presence of an isothermal halo that leads to flat rotation curves (it also accounts for the isothermal core of large dark matter halos). The damping term ensures that the system relaxes towards an equilibrium state. This property is guaranteed by an H -theorem satisfied by a Boltzmann-like free energy functional. In our approach, the temperature and the friction arise from a single formalism. They correspond to the real and imaginary parts of the complex friction coefficient present in the scale covariant equation of dynamics that is at the basis of Nottale’s theory of scale relativity. They may be the manifestation of a cosmic aether or the consequence of a process of violent relaxation and gravitational cooling on a coarse-grained scale.

New dissipation mechanisms from multilevel dark matter scattering

Multilevel dark matter with diagonal and off-diagonal interactions shows a rich phenomenology in its self-scattering. If the interactions are mediated by a particle that is less massive than the dark matter, the Sommerfeld effect can lead to resonant enhancement of the scattering. For mediators lighter than the level separation, dark matter particles can upscatter to excited states and deexcite by emitting these mediators. We compute these cross sections, both above and below the kinematic threshold, in a generic two-component dark matter model and identify the large inelastic cross section as a result of maximal mixing between the two states. A new route for cooling of large dark matter halos and a new drag force between two colliding halos are identified and shown to arise purely from the inelastic scattering.

Wave propagation in modified gravity

We investigate the propagation of scalar waves induced by matter sources in the context of scalar-tensor theories of gravity which include screening mechanisms for the scalar degree of freedom. The usual approach when studying these theories in the non-linear regime of cosmological perturbations is based on the assumption that scalar waves travel at the speed of light. Within General Relativity such approximation is good and leads to no loss of accuracy in the estimation of observables. We find, however, that mass terms and non-linearities in the equations of motion lead to propagation and dispersion velocities significantly different from the speed of light. As the group velocity is the one associated to the propagation of signals, a reduction of its value has direct impact on the behavior and dynamics of nonlinear structures within modified gravity theories with screening. For instance, the internal dynamics of galaxies and satellites submerged in large dark matter halos could be affected by the fact that the group velocity is smaller than the speed of light. It is therefore important, within such framework, to take into account the fact that different part of a galaxy will see changes in the environment at different times. A full non-static analysis may be necessary under those conditions.

Models of dark matter halos based on statistical mechanics: The fermionic King model

We discuss the nature of phase transitions in the fermionic King model which describes tidally truncated quantum self-gravitating systems. This distribution function takes into account the escape of high energy particles and has a finite mass. On the other hand, the Pauli exclusion principle puts an upper bound on the phase space density of the system and stabilizes it against gravitational collapse. As a result, there exists a statistical equilibrium state for any accessible values of energy and temperature. We plot the caloric curves and investigate the nature of phase transitions as a function of the degeneracy parameter in both microcanonical and canonical ensembles. We consider stable and metastable states and emphasize the importance of the latter for systems with long-range interactions. Phase transitions can take place between a "gaseous" phase unaffected by quantum mechanics and a "condensed" phase dominated by quantum mechanics. The phase diagram exhibits two critical points, one in each ensemble, beyond which the phase transitions disappear. There also exist a region of negative specific heats and a situation of ensemble inequivalence for sufficiently large systems. We apply the fermionic King model to the case of dark matter halos made of massive neutrinos. The gaseous phase describes large halos and the condensed phase describes dwarf halos. Partially degenerate configurations describe intermediate size halos. We argue that large dark matter halos cannot harbor a fermion ball because these nucleus-halo configurations are thermodynamically unstable (saddle points of entropy). Large dark matter halos may rather contain a central black hole resulting from a dynamical instability of relativistic origin occurring during the gravothermal catastrophe.

Models of dark matter halos based on statistical mechanics: The classical King model

We consider the possibility that dark matter halos are described by the Fermi-Dirac distribution at finite temperature. This is the case if dark matter is a self-gravitating quantum gas made of massive neutrinos at statistical equilibrium. This is also the case if dark matter can be treated as a self-gravitating collisionless gas experiencing Lynden-Bell's type of violent relaxation. In order to avoid the infinite mass problem and carry out a rigorous stability analysis, we consider the fermionic King model. In this paper, we study the non-degenerate limit leading to the classical King model. This model was initially introduced to describe globular clusters. We propose to apply it also to large dark matter halos where quantum effects are negligible. We determine the caloric curve and study the thermodynamical stability of the different configurations. Equilibrium states exist only above a critical energy $E_c$ in the microcanonical ensemble and only above a critical temperature $T_c$ in the canonical ensemble. For $E<E_c$, the system undergoes a gravothermal catastrophe and, for $T<T_c$, it undergoes an isothermal collapse. We compute the profiles of density, circular velocity, and velocity dispersion. We compare the prediction of the classical King model to the observations of large dark matter halos. Because of collisions and evaporation, the central density increases while the slope of the halo density profile decreases until an instability takes place. We show that large dark matter halos are relatively well-described by the King model at, or close to, the point of marginal microcanonical stability. At that point, the King model generates a density profile that can be approximated by the modified Hubble profile. This profile has a flat core and decreases as $r^{-3}$ at large distances, like the observational Burkert profile. Less steep halos are unstable.

3.55 keV photon line and its morphology from a 3.55 keV axionlike particle line

Galaxy clusters can efficiently convert axion-like particles (ALPs) to photons. We propose that the recently claimed detection of a 3.55--3.57 keV line in the stacked spectra of a large number of galaxy clusters and the Andromeda galaxy may originate from the decay of either a scalar or fermionic $7.1$ keV dark matter species into an axion-like particle (ALP) of mass $m_{a} \lesssim 6\cdot 10^{-11}~{\rm eV}$, which subsequently converts to a photon in the cluster magnetic field. In contrast to models in which the photon line arises directly from dark matter decay or annihilation, this can explain the anomalous line strength in the Perseus cluster. As axion-photon conversion scales as $B^2$ and cool core clusters have high central magnetic fields, this model can also explains the observed peaking of the line emission in the cool cores of the Perseus, Ophiuchus and Centaurus clusters, as opposed to the much larger dark matter halos. We describe distinctive predictions of this scenario for future observations.

Evolution of star clusters in a cosmological tidal field

We present a method to couple N-body star cluster simulations to a cosmological tidal field, using the Astrophysical Multipurpose Software Environment. We apply this method to star clusters embedded in the CosmoGrid dark matter-only LambdaCDM simulation. Our star clusters are born at z = 10 (corresponding to an age of the Universe of about 500 Myr) by selecting a dark matter particle and initializing a star cluster with 32,000 stars on its location. We then follow the dynamical evolution of the star cluster within the cosmological environment. We compare the evolution of star clusters in two Milky-Way size haloes with a different accretion history. The mass loss of the star clusters is continuous irrespective of the tidal history of the host halo, but major merger events tend to increase the rate of mass loss. From the selected two dark matter haloes, the halo that experienced the larger number of mergers tends to drive a smaller mass loss rate from the embedded star clusters, even though the final masses of both haloes are similar. We identify two families of star clusters: native clusters, which become part of the main halo before its final major merger event, and the immigrant clusters, which are accreted upon or after this event; native clusters tend to evaporate more quickly than immigrant clusters. Accounting for the evolution of the dark matter halo causes immigrant star clusters to retain more mass than when the z=0 tidal field is taken as a static potential. The reason for this is the weaker tidal field experienced by immigrant star clusters before merging with the larger dark matter halo.

The H ii Region of a Primordial Star

The concordance model of cosmology and structure formation predicts the formation of isolated very massive stars at high redshifts in dark matter dominated halos of 10{sup 5} to 10{sup 6} Msun. These stars photo-ionize their host primordial molecular clouds, expelling all the baryons from their halos. When the stars die, a relic H II region is formed within which large amounts of molecular hydrogen form which will allow the gas to cool efficiently when gravity assembles it into larger dark matter halos. The filaments surrounding the first star hosting halo are largely shielded and provide the pathway for gas to stream into the halo when the star has died. We present the first fully three dimensional cosmological radiation hydrodynamical simulations that follow all these effects. A novel adaptive ray casting technique incorporates the time dependent radiative transfer around point sources. This approach is fast enough so that radiation transport, kinetic rate equations, and hydrodynamics are solved self-consistently. It retains the time derivative of the transfer equation and is explicitly photon conserving. This method is integrated with the cosmological adaptive mesh refinement code enzo, and runs on distributed and shared memory parallel architectures. Where applicable the three dimensional calculation not onlymore » confirm expectations from earlier one dimensional results but also illustrate the multi-fold hydrodynamic complexities of H II regions. In the absence of stellar winds the circumstellar environments of the first supernovae and putative early gamma-ray bursts will be of low density {approx}1 cm{sup -3}. Albeit marginally resolved, ionization front instabilities lead to cometary and elephant trunk like small scale structures reminiscent of nearby star forming regions.« less

Growth of Structure Seeded by Primordial Black Holes

We discuss the possibilities for primordial black holes (PBHs) to grow via the accretion of dark matter. In agreement with previous works, we find that accretion during the radiation-dominated era does not lead to a significant mass increase. However, during matter-domination, PBHs may grow by up to two orders of magnitude in mass through the acquisition of large dark matter halos. We discuss the possibility of PBHs being an important component in dark matter halos of galaxies as well as their potential to explain the ultra-luminous x-ray sources (ULXs) observed in nearby galactic disks. We point out that although PBHs are ruled out as the dominant component of dark matter, there is still a great deal of parameter space open to them playing a role in the modern-day universe. For example, a primordial halo population of PBHs each at $10^{2.5} M_\odot$ making up 0.1% of the dark matter grow to $10^{4.5} M_\odot$ via the accumulation of dark matter halos to account for $\sim 10%$ of the dark matter mass by a redshift of $z \approx 30$. These intermediate mass black holes may then ``light up'' when passing through molecular clouds, becoming visible as ULXs at the present day, or they may form the seeds for supermassive black holes at the centers of galaxies.

Cosmological Implications of Dwarf Spheroidal Chemical Evolution

The chemical properties of dwarf spheroidals in the local group are shown to be inconsistent with star formation being truncated after the reionization epoch (z �8). Enhanced levels of [Ba/Y] in stars in dwarf spheroidals like Sculptor indicate strong s-process production from low-mass stars whose lifetimes are comparable with the duration of the pre-reionization epoch. The chemical evolution of Sculptor is followed using a model with SNe II and SNe Ia feedback and mass- and metallicitydependent nucleosynthetic yields for elements from H to Pb. We are unable to reproduce the Ba/Y ratio unless stars formed over an interval long enough for the low-mass stars to pollute the interstellar medium with s-elements. This robust result challenges the suggestion that most of the local group dwarf spheroidals are fossils of reionization and supports the case for large initial dark matter halos. Subject headings: galaxies: abundance ratios — galaxies: dwarf — galaxies: Local Group — galaxies: evolution — stars: abundances — stars: AGB — nucleosynthesis

Radiation Hydrodynamic Evolution of Primordial HiiRegions

We simulate the ionization environment of z ~ 20 luminous objects formed within the framework of the current CDM (cold dark matter) cosmology and compute their UV escape fraction. These objects are likely single, very massive stars that are copious UV emitters. We present analytical estimates as well as one-dimensional radiation hydrodynamic calculations of the evolution of these first H II regions in the universe. The initially D-type ionization front (I-front) evolves to become R-type within 105 yr at a distance of ~1 pc. This ionization front then completely overruns the halo, accelerating an expanding shell of gas outward to velocities in excess of 30 km s-1, about 10 times the escape velocity of the confining dark matter halo. We find that the evolution of the H II region depends only weakly on the assumed stellar ionizing luminosities. Consequently, most of the gas surrounding the first stars will leave the dark halo whether or not the stars produce supernovae. If they form the first massive seed black holes, these are unlikely to accrete within a Hubble time after they formed until they are incorporated into larger dark matter halos that contain more gas. Because these I-fronts exit the halo on timescales much shorter than the stars' main-sequence lifetimes, their host halos have UV escape fractions of 0.95, fixing an important parameter for theoretical studies of cosmological hydrogen reionization.

Giant Lyα Nebulae Associated with High‐Redshift Radio Galaxies

We report deep, Keck narrowband Lyα images of the luminous z > 3 radio galaxies 4C 41.17, 4C 60.07, and B2 0902+34. The images show giant, 100-200 kpc scale, emission-line nebulae, centered on these galaxies, that exhibit a wealth of morphological structure, including extended low surface brightness emission in the outer regions, radially directed filaments, cone-shaped structures and (indirect) evidence for extended Lyα absorption. We discuss these features within a general scenario in which the nebular gas cools gravitationally in large cold dark matter halos, forming stars and multiple stellar systems. Merging of these building blocks triggers large-scale starbursts, forming the stellar bulges of massive radio galaxy hosts, and feeds supermassive black holes, which produce the powerful radio jets and lobes. The radio sources, starburst superwinds, and radiation pressure from active galactic nucleus then disrupt the accretion process, limiting galaxy and black hole growth, and imprint the observed filamentary and cone-shaped structures of the Lyα nebulae.

The mechanics of dark matter halo formation

We study the survival of sub-structures (clumps) within larger self-gravitating galaxies or dark matter halos. Building on scaling relations obtained from N -body calculations of violent relaxation, we argue that the tidal field of galaxies and halos can only destroy sub-structures if spherical symmetry is imposed at formation. Unless the larger halo is built up from a few large clumps, we find that clump-clump encounters are unlikely to homogenise the halo on a dynamical timescale. Phase-mixing would proceed faster in the inner parts and allow for the secular evolution of a stellar disc.

Globular Cluster Systems. II. On the Formation of Old Globular Clusters and Their Sites of Formation

We studied the metal-poor globular cluster populations of a large variety of galaxies and compared their mean metallicity with the properties of the host galaxies. For this purpose, we constructed a comprehensive database of old metal-poor globular cluster populations, hosted by 47 galaxies, spanning about 10 mag in absolute brightness. The mean metallicities of the systems are found to be very similar and lie in the -1.65 < [Fe/H] ≤ -1.20 range (74% of the population). Using only globular cluster systems with more than six objects detected, we find that 85% of the population are within -1.65 < [Fe/H] ≤ -1.20. The relation between the mean metallicity of the metal-poor globular cluster systems and the absolute V magnitude of their host galaxies presents a very low slope that includes zero. An analysis of the correlation of the mean metallicity of the populations with other galaxy properties (such as velocity dispersion, metallicity, and environment density) also leads to the conclusion that no strong correlation exists. The lack of correlation with galaxy properties suggests a formation of all metal-poor globular clusters in very similar gas fragments. A weak correlation (to be confirmed) might exist between the mean metallicity of the metal-poor clusters and the host galaxy metallicity. This would imply that at least some fragments in which metal-poor globular clusters formed were already embedded in the larger dark matter halo of the final galaxy (as opposed to being independent satellites that were accreted later). Our result suggests a homogeneous formation of metal-poor globular clusters in all galaxies in typical fragments of masses around 109–1010 M⊙ with very similar metallicities, compatible with hierarchical formation scenarios for galaxies. We further compare the mean metallicities of the metal-poor globular cluster populations with the typical metallicities of high-redshift objects. If we add the constraint that globular clusters need a high column density of gas to form, damped Lyα systems are the most likely sites among the known high-redshift objects for the formation of metal-poor globular cluster populations.

Black hole models for the dark matter in Draco and Ursa Minor

view Abstract Citations (8) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Black Hole Models for the Dark Matter in Draco and Ursa Minor Strobel, Nicolas V. ; Lake, George Abstract The stellar velocity dispersions of Draco and Ursa Minor have been used to infer large dark matter halos with central mass densities that are two orders of magnitude larger than found in brighter galaxies. We offer a different dark matter model. We fit the star count profiles using massive central black holes (1-2 x 10^7^ M_sun_) and investigate the agreement between this model and the latest velocity data. Such black holes produce the large observed velocity dispersions and create a density and distinctive velocity cusp close to the center. Star counts reaching the main sequence will lead to more accurate centering of the isopleths and may reveal a more prominent cusp. Publication: The Astrophysical Journal Pub Date: April 1994 DOI: 10.1086/187280 Bibcode: 1994ApJ...424L..83S Keywords: Black Holes (Astronomy); Cusps (Mathematics); Dark Matter; Halos; Milky Way Galaxy; Nomographs; Velocity; Ellipticity; Globular Clusters; Space Density; Astrophysics; COSMOLOGY: DARK MATTER; GALAXIES: LOCAL GROUP full text sources ADS | data products SIMBAD (2) NED (2)