Theory Of Structure Formation Research Articles

Theory of Interaction and Structure Formation in Liquid Crystal Colloids and Emulsion

ABSTRACT This article is semi review our result about interaction and structure formation in liquid crystal colloids and emulsion. We treat colloidal and emulsion systems in liquid crystal using effectively describing the elastic interaction. Here we consider a system whose interactions are presumably well-understood and the phase transition in which can be studied in the scale of macro particle resolution – the model system allowing to study the mechanisms of the structural phase transitions. We presented analytical results of possible structures for suspension of macro particles in liquid crystal. We have shown that cellular structure formation can occur in liquid crystals for realistic values of temperature and macroparticle concentration.

The Stromgren Sphere, the Environment, and the Reionization in the Local Universe of the Highest Redshift QSOs

In this paper we investigate the environment and reionization process around the highest redshift QSOs having Gunn-Peterson troughs (z > 6.1). Starting with the cosmic density perturbation and structure formation theory and the fact that the highest redshift QSOs are located in rare overdense regions, we show that the halo formation, gas distribution, and star formation around QSOs are biased from those of the cosmic average. We argue that a significant fraction of hydrogen in the Stromgren sphere around QSOs is ionized by photons from stars and that only about several percent to at most 10%-20% of the total hydrogen is left (e.g., in minihalos, halos, or high-density subregions) to be ionized by QSO photons. The cosmic average neutral hydrogen fraction at z ~ 6.2-6.4 should also be smaller than the upper limit of 10%-20% and may be only a few percent. We analyze the clumping property of the hydrogen ionized by QSOs and study the evolution of the Stromgren sphere. We find that the expected Stromgren radii from our models are consistent with observations if the lifetime of the highest redshift QSOs is about or longer than a few × 107 yr (as is the lifetime of the main population of QSOs, with comoving number density peaked at z ~ 2-3). With such a QSO lifetime, the ages of most of the observed QSOs are long enough that the QSO photon emission is balanced by the recombination of the hydrogen ionized by QSO photons in their Stromgren spheres, and the expected Stromgren radii from the balance are independent of the detailed values of the QSO ages. We also point out a statistical method involving a larger sample of QSOs having Gunn-Peterson troughs in future observations, which may potentially check or rule out the possibility that the highest redshift QSOs have a shorter lifetime (e.g., <107 yr), even without an accurate estimate of the hydrogen clumping property.

Black holes, cooling flows and galaxy formation

Central black holes in galaxies are now well established as a ubiquitous phenomenon, and this fact is important for theories of cosmological structure formation. Merging of galaxy haloes must preserve the proportionality between black hole mass and baryonic mass; the way in which this happens may help solve difficulties with existing ing models of galaxy formation, which suffer from excessive cooling and thus over- produce stars. Feedback from active nuclei may be the missing piece of the puzzle, regulating galaxy-scale cooling flows. Such a process now seems to be observed in cluster-scale cooling flows, where dissipation of sound waves generated by radio lobes can plausibly balance the energy lost in X-rays, at least in a time-averaged sense.

Cosmic Structure Formation at Low and High Redshifts

The currently standard theory of cosmic structure formation posits that the present-day clumpy appearance of the universe developed through gravitational amplification of the matter density fluctuations that are generated in the very early universe. The energy content of the univese and the basic statistics of the initial density field have been determined with a reasonable accuracy from recent observations of the cosmic microwave background, large-scale structure, and distant supernovae. It has become possible to make accurate predictions from the standard model. Cosmology is now at the stage where we can rigourously test the model against various observations of large- and small-scale structure. We review the latest observations and the recent progress in the theory of structure formation at low and high redshifts. Two promising methods to probe large-scale matter distribution are introduced and the future prospects are discussed. Results from state-of-the-art cosmological simulations are also presented.

CMB Analysis of Boomerang &amp; Maxima &amp; the Cosmic Parameters {Ωtot,Ωbh2, Ωcdmh2, ΩΛ, ns}

We show how estimates of parameters characterizing inflation-based theories of structure formation localized over the past year when large scale structure (LSS) information from galaxy and cluster surveys was combined with the rapidly developing cosmic microwave background (CMB) data, especially from the recent Boomerang and Maxima balloon experiments. All current CMB data plus a relatively weak prior probability on the Hubble constant, age and LSS points to little mean curvature (Ωtot = 1.08±0.06) and nearly scale invariant initial fluctuations (ns = 1.03±0.08), both predictions of (non-baroque) inflation theory. We emphasize the role that degeneracy among parameters in the Lpk = 212 ± 7 position of the (first acoustic) peak plays in defining the Ωtot range upon marginalization over other variables. Though the CDM density is in the expected range (Ωcdmh2 = 0.17 ± 0.02), the baryon density Ωbh2 = 0.030 ± 0.005 is somewhat above the independent 0.019 ± 0.002 nucleosynthesis estimates. CMB+LSS gives independent evidence for dark energy (ΩΛ = 0.66 ± 0.06) at the same level as from supernova (SN1) observations, with a phenomenological quintessence equation of state limited by SN1+CMB+LSS to wQ &lt; −0.7 cf. the wQ=−1 cosmological constant case.

A search for low surface brightness dwarf galaxies in different environments

Current theories of large scale structure and galaxy formation predict the existence of numerous low mass dark matter haloes in the Universe today. If these haloes contain sufficient stars they should be detectable as low luminosity stellar systems or dwarf galaxies. We have searched for these objects in four regions of increasing density - the general field, the area around a giant spiral galaxy, the low density Ursa Major cluster, and the high density Virgo cluster. Using identical deep optical data covering a total of 602 and probing fainter magnitudes than has been done previously, we used identical selection and detection methods to compare the dwarf galaxy populations in these different environments. We found substantially more dwarfs per giant galaxy in the Virgo cluster (20:1) compared to the field environment (6:1 max). A comparison of the HI properties and (B-I) colours for the objects for which we had additional data also showed that in general, the cluster objects are redder and gas poor compared to the objects in the field. We discuss the possible mechanisms which may have resulted in creating a population of cluster dwarf galaxies, which would explain the high number density which we found in our data. It is likely that a combination of tidal interactions and transformation of infalling dlrrs into dEs will result in the large population of cluster dwarfs. Conclusive evidence regarding their formation must now be obtained by a more detailed investigation of their stellar populations. The lack of dwarf galaxies in the field region is likely to be due to the effect of inefficient star formation in the field environment compared to the cluster. Thus the low mass dark matter haloes predicted by CDM models must still be 'dark' and can only be identified by further deep HI studies of the field environment, and future gravitational lensing studies of substructure.

Interacting Dark Matter and Dark Energy

We discuss models for the cosmological dark sector in which the energy density of a scalar field approximates Einstein's cosmological constant and the scalar field value determines the dark matter particle mass by a Yukawa coupling. A model with one dark matter family can be adjusted so the observational constraints on the cosmological parameters are close to but different from what is predicted by the ΛCDM model. This may be a useful aid to judging how tightly the cosmological parameters are constrained by the new generation of cosmological tests that depend on the theory of structure formation. In a model with two families of dark matter particles the scalar field may be locked to near zero mass for one family. This can suppress the long-range scalar force in the dark sector and eliminate evolution of the effective cosmological constant and the mass of the nonrelativistic dark matter particles, making the model close to ΛCDM, until the particle number density becomes low enough to allow the scalar field to evolve. This is a useful example of the possibility for complexity in the dark sector.

Radio, optical and infrared observations of CLASS B0128+437

We present new observations of the gravitational lens system CLASS B0128+437. HST observations detect a very faint, extended object in I-band with no emission from the lensed images visible; no detection at all is made in V-band. The lens system is detected with much higher signal to noise with UKIRT in K-band, but the resolution is not sufficient to allow the lensed images and the lens galaxy to be separated. A careful astrometric calibration, however, suggests that the peak of the infrared emission corresponds to the two merging images A and B and therefore that the lensed images dominate at infrared wavelengths. The new radio data consist of VLBI radio images at three frequencies, 2.3, 5 and 8.4GHz, made with the VLBA and the 100-m Effelsberg telescope. The lensed source consists of three well-defined sub-components embedded in a more extended jet. Due to the fact that the sub-components have different spectral indices it is possible to determine which part of each image corresponds to the same source sub-component. Our main finding is that one of the images, B, looks very different to the others, there being no obvious division into separate sub-components and the image being apparently both broader and smoother. This is a consequence we believe of scatter-broadening in the ISM of the lensing galaxy. The large number of multiply-imaged source sub-components also provide an abundance of modelling constraints and we have attempted to fit an SIE+external shear model to the data, as well as utilising the novel method of Evans & Witt. It proves difficult in both cases, however, to obtain a satisfactory fit which strongly suggests the presence of sub-structure in the mass distribution of the lensing galaxy, perhaps of the kind that is predicted by CDM theories of structure formation.

The z ~4 Lyman Break Galaxies: Colors and Theoretical Predictions

We investigate several fundamental properties of z ~ 4 Lyman break galaxies by comparing observations with the predictions of a semianalytic model based on the cold dark matter theory of hierarchical structure formation. We use a sample of B435-dropouts from the Great Observatories Origins Deep Survey and complement the Advanced Camera for Surveys optical B435, V606, i775, and z850 data with the Very Large Telescope Infrared Spectrometer and Array Camera J, H, and Ks observations. We extract B435-dropouts from our semianalytic mock catalog using the same color criteria and magnitude limits that were applied to the observed sample. We find that the i775-Ks colors of the model-derived and observed B435-dropouts are in good agreement. However, we find that the i775-z850 colors differ significantly, indicating perhaps that either too little dust or an incorrect extinction curve has been used. Motivated by the reasonably good agreement between the model and observed data, we present predictions for the stellar masses, star formation rates, and ages for the z ~ 4 Lyman break sample. We find that according to our model, the color selection criteria used to select our z ~ 4 sample surveys 67% of all galaxies at this epoch down to z850 < 26.5. We find that our model predicts a ~40% mass buildup between the z ~ 4 and z ~ 3 epochs for the UV rest-frame L* galaxies. Furthermore, according to our model, at least 50% of the total stellar mass resides in relatively massive UV-faint objects that fall below our observational detection limit.

Low Luminosity Galaxies

AbstractLow luminosity (dwarf) galaxies play a crucial role in our current theories of galaxy and large scale structure formation. In the hierarchical picture they are the building blocks from which other structures form. These theories in their basic form overpredict the numbers of small dark matter halos (dwarf galaxies?) unless some form of star formation supression is invoked. In this paper we describe observations of dwarf galaxies in a range of different environments. We find that there are far too few dwarf galaxies in low density environments to be compatible with the theories. These observations are not consistent with an environment-independent mechanism suppressing dwarf galaxy formation. It is also not clear how these mechanisms can supress star formation if dwarf galaxies have large mass-to-light ratios (≈100). Either the whole idea of hierarchical galaxy formation has to be rejected or other environmentally dependent physical processes have to be invoked. We suggest that small, gas-rich dI galaxies have their evolution rapidly advanced as they move into the dense cluster environment.

Gravitational Lens Statistics as a Probe of Halo Profiles

Recent development of the structure formation theory based on the cold dark matter scenario implies that a number of larger separation lensed quasars, for which a confirmed detection has not yet been achieved, will be observed in the ongoing large-scale surveys such as the 2dF survey and SDSS. We show that statistics of such large separation lenses can be a powerful probe of the density profile of dark halos. After we summarize the current status of the lens surveys in the 2dF and SDSS, we focus our discussion on what information can be extracted from these lens surveys. in addition, we also propose statistics of differential time delays between multiple images as an alternative probe of the density profile of dark halos.

Lensing Diagnostics of Halo Substructure

The Cold Dark Matter (CDM) hierarchical structure formation theory predicts substructures in dark matter halos. the number of predicted subhalos seems to exceed the observed number of luminous satellite galaxies. Gravitational lenses can be used to probe luminous or dark substructures. Image positions and flux ratios in broad-band (including radio and optical) and emission lines can all be used to probe substructures on different mass scales. the observed gravitational lenses appear to require a few percent of the mass surface density in substructures within the mass range of 104M⊙ − 109M⊙. Numerical simulations predict roughly the same mass fraction in substructures within the virialised region. But at typical image positions (a few percent of the virial radius), the predicted surface mass density in substructures appears to be lower than required. Both observations and numerical simulations are somewhat uncertain at present so it is not yet clear whether the discrepancy is severe.

Using radio galaxies to find super-structures

Radio galaxies are excellent at tracing large-scale structure due to their high bias. We present new results from the TONS08 radio galaxy redshift survey. We find unequivocal evidence for a huge (at least 80×80×100 Mpc 3) super-structure at redshift z=0.27, confirming tentative evidence for such a structure from the 7C redshift survey (7CRS). A second, newly discovered super-structure is also tentatively found at redshift 0.35 (of dimensions at least 100×100×100 Mpc 3). Out of the total sample size of 84 radio galaxies, at least 25 are associated with the two super-structures. We use quasi-linear structure formation theory to estimate the number of such structures expected in the TONS08 volume if the canonical value for radio galaxy bias is assumed. Under this assumption, the structures represent ≈4–5 σ peaks in the primordial density field and their expected number is low (∼10 −2−10 −4). Fortunately, there seems no shortage of plausible explanations (many of which are testable) for these low probabilities in the form of potential mechanisms for boosting the bias on large scales. These include: the association of radio galaxies with highly biased rich clusters in super-structures, enhanced triggering by group/group mergers, and enhanced triggering and/or redshift space distortion in collapsing systems as the growth of super-structures moves into the non-linear regime. Similar structures could have been missed in previous surveys because of the effects of Poisson-sampling fluctuations.

Status of cold dark matter cosmology

Cold Dark Matter (CDM) has become the standard modern theory of cosmological structure formation. Its predictions appear to be in good agreement with data on large scales, and it naturally accounts for many properties of galaxies. But despite its many successes, there has been concern about CDM on small scales because of the possible contradiction between the linearly rising rotation curves observed in some dark-matter-dominated galaxies vs. the 1/ r density cusps at the centers of simulated CDM halos. Other CDM issues on small scales include the very large number of small satellite halos in simulations, far more than the number of small galaxies observed locally, and problems concerning the angular momentum of the baryons in dark matter halos. The latest data and simulations have lessened, although not entirely resolved, these concerns. Meanwhile, the main alternatives to CDM that have been considered to solve these problems, self-interacting dark matter (SIDM) and warm dark matter (WDM), have been found to have serious drawbacks.

Two 100-Mpc-scale structures in the three-dimensional distribution of radio galaxies and their implications

We present unequivocal evidence for a huge (80 x 100 x 100 Mpc 3 ) superstructure at redshift z = 0.27 in the three-dimensional distribution of radio galaxies from the Texas-Oxford NVSS Structure 08 h region (TONS08) sample, confirming tentative evidence for such a structure from the 7C redshift survey (7CRS). A second, newly discovered superstructure is also found less securely at redshift 0.35 (of dimensions 100 x 100 x 100 Mpc 3 ). We present full observational details on the TONS08 sample which was constructed to probe structures in the redshift range 0? z? 0.5 by matching NVSS sources with objects in APM catalogues to obtain a sample of optically bright (E R ≤ 19.83), radio-faint (1.4-GHz flux density S 1 . 4 ≥ 3 mJy) radio galaxies in the same 25 deg 2 area as part II of the 7CRS. Out of the total sample size of 84 radio galaxies, at least 25 are associated with the two 100 Mpc-scale superstructures. We use quasi-linear structure formation theory to estimate the number of such structures expected in the TONS08 volume if the canonical value for radio galaxy bias is assumed. Under this assumption, the structures represent 14-5σ peaks in the primordial density field and their expected number is low (∼10 - 2 -10 - 4 ). Because the TONS08 survey was designed to follow up a previously, if tentatively, identified superstructure in the 7CRS, the probability of finding two superstructures in TONS08 is uncertain but, assuming that the tentative detection of the z = 0.35 superstructure is real, must lie between ∼10 - 4 and ∼10 - 5 , depending on how representative the TONS08 region proves to be. We show that similar structures (with similarly low probabilities) are also found in previous radio galaxy redshift surveys, but their significance has not been fully appreciated because they have been traced by very small numbers of radio galaxies. Fortunately, there are several plausible explanations (many of which are testable) for these low probabilities in the form of potential mechanisms for boosting the bias on large scales. These include the association of radio galaxies with highly biased rich clusters in superstructures, enhanced triggering by group-group mergers and enhanced triggering and/or redshift space distortion in collapsing systems as the growth of superstructures moves into the non-linear regime.

Disk‐Bulge‐Halo Models for the Andromeda Galaxy

We present a suite of semianalytic disk-bulge-halo models for the Andromeda galaxy (M31) that satisfy three fundamental conditions: (1) internal self-consistency, (2) consistency with observational data, and (3) stability of the disk against the formation of a central bar. The models are chosen from a set first constructed by Kuijken & Dubinski. We develop an algorithm to search the parameter space for this set in order to best match observations of the M31 rotation curve, inner velocity dispersion profile, and surface brightness profile. Models are obtained for a large range of bulge and disk masses; we find that the disk mass must be ≲8 × 1010 M☉ and that the preferred value for the bulge mass is 2.5 × 1010 M☉. N-body simulations are carried out to test the stability of our models against the formation of a bar within the disk. We also calculate the baryon fraction and halo concentration parameter for a subset of our models and show that the results are consistent with the predictions from cosmological theories of structure formation. In addition, we describe how gravitational microlensing surveys and dynamical studies of globular clusters and satellites can further constrain the models.

Wilkinson Microwave Anisotropy Probe data and the curvature of space

Inter alia, the high-precision Wilkinson Microwave Anisotropy Probe (WMAP) data on cosmic background radiation marginally indicate that the Universe has positively curved (and hence spherical) spatial sections. In this Letter, we take this data seriously and consider some of the consequences for the background dynamics. In particular, we show that this implies a limit to the number of e-foldings that could have taken place in the inflationary epoch; however, this limit is consistent with some inflationary models that solve all the usual cosmological problems and that are consistent with standard structure formation theory.

DENSITY CONTRAST-PECULIAR VELOCITY RELATION IN THE NEWTONIAN GAUGE

We investigate relation between density contrast and peculiar velocity in the Newtonian gauge, in general relativistic framework of theory of large scale structure formation in the universe. According to the gauge-invariant property of the energy-momentum tensor in this gauge, the velocity perturbation behaves like the Newtonian peculiar velocity. In this framework, the relation between peculiar velocity and density contrast with respect to the Newtonian–Peebles formula has an extra correction term which is ignorable for the small scale structures. The relativistic correction of peculiar velocity for structures of the extent of hundred mega parsec is about few percents which is smaller than the accuracy of the recent peculiar velocity measurements. We also study CMB anisotropies due to the Doppler effect in the Newtonian gauge compared with those obtained using Newtonian gravity.

Cosmological dark matter annihilations intoγrays: A closer look

We investigate the prospects of detecting weakly interacting massive particle (WIMP) dark matter by measuring the contribution to the extragalactic gamma-ray radiation induced, in any dark matter halo and at all redshifts, by WIMP pair annihilations into high-energy photons. We perform a detailed analysis of the very distinctive spectral features of this signal, recently proposed in a short letter by three of the authors: The gamma-ray flux which arises from the decay of ${\ensuremath{\pi}}^{0}$ mesons produced in the fragmentation of annihilation final states shows a severe cutoff close to the value of the WIMP mass. An even more spectacular signature appears for the monochromatic gamma-ray components, generated by WIMP annihilations into two-body final states containing a photon: the combined effect of cosmological redshift and absorption along the line of sight produces sharp bumps, peaked at the rest frame energy of the lines and asymmetrically smeared to lower energies. The level of the flux depends both on the particle physics scenario for WIMP dark matter (we consider, as our template case, the lightest supersymmetric particle in a few supersymmetry breaking schemes), and on the question of how dark matter clusters. Uncertainties introduced by the latter are thoroughly discussed implementing a realistic model inspired by results of the state-of-the-art N-body simulations and semianalytic modeling in the cold dark matter structure formation theory. We also address the question of the potential gamma-ray background originating from active galaxies, presenting a novel calculation and critically discussing the assumptions involved and the induced uncertainties. Furthermore, we apply a realistic model for the absorption of gamma-rays on the optical and near-IR intergalactic radiation field to derive predictions for both the signal and background. Comparing the two, we find that there are viable configurations, in the combined parameter space defined by the particle physics setup and the structure formation scenario, for which the WIMP induced extragalactic gamma-ray signal will be detectable in the new generation of gamma-ray telescopes such as GLAST.

Neutrino masses in astroparticle physics

The case for small neutrino mass differences from atmospheric and solar neutrino oscillation experiments has become compelling, but leaves the overall neutrino mass scale m ν undetermined. The most restrictive limit of m ν<0.8 eV arises from the 2dF galaxy redshift survey in conjunction with the standard theory of cosmological structure formation. A relation between the hot dark matter fraction and m ν depends on the cosmic number density n ν of neutrinos. If solar neutrino oscillations indeed correspond to the favored large mixing angle MSW solution, then big-bang nucleosynthesis gives us a restrictive limit on all neutrino chemical potentials, removing the previous uncertainty of n ν . Therefore, a possible future measurement of m ν will directly establish the cosmic neutrino mass fraction Ω ν . Cosmological neutrinos with sub-eV masses can play an interesting role for producing the highest-energy cosmic rays ( Z-burst scenario). Sub-eV masses also relate naturally to leptogenesis scenarios of the cosmic baryon asymmetry. Unfortunately, the time-of-flight dispersion of a galactic or local-group supernova neutrino burst is not sensitive in the sub-eV range.