Thermal regime of the baryons behind shock waves arising in the process of virialization of dark matter halos is governed at cetrain conditions by radiation of HD lines. A small fraction of the shocked gas can cool down to the temperature of the cosmic microwave background (CMB). We estimate an upper limit for this fraction: at $z=10$ it increases sharply from about $q_{_T}\sim 10^{-3}$ for dark halos of $M=5\times 10^7\msun$ to $\sim 0.1$ for halos with $M=10^8\msun$. Further increase of the halo mass does not lead however to a significant growth of $q_T$ -- the asymptotic value for $M\gg 10^8\msun$ is of 0.3. We estimate star formation rate associated with such shock waves, and show that they can provide a small but not negligible fraction of the star formation. We argue that extremely metal-poor low-mass stars in the Milky Way may have been formed from primordial gas behind such shocks.
According to the hierarchical scenario, galaxies form via merging and accretion of small objects. Using N-body simulations, we study the frequency of merging events in the history of the halos. We find that at z<~2 the merging rate of the overall halo population can be described by a simple power law (1+z)^3. The main emphasis of the paper is on the effects of environment of halos at the present epoch (z=0). We find that the halos located inside clusters have formed earlier (dz \approx 1) than isolated halos of the same mass. At low redshifts (z<1), the merger rate of cluster halos is 3 times …
We discuss the impact of the cosmological environment on the evolution of dark matter halos using a high-resolution simulation within a spatially flat LCDM cosmology.
We have constructed the merging history of dark matter halos in a SCDM cosmology, by means of a the {``Merging Cell Model''} (Rodrigues & Thomas 1996). It is based on the linear theory of growth of density fluctuations, and the Top-Hat model, and it takes into account mergers between halos through simplified criteria. The halo mass function, the progenitors and children mass distribution, the distribution of formation times, and the halo autocorrelation function (in the Lagrangian space of initial conditions) have been computed. To test the reliability of the model, results have been compared …
The merging history of dark matter halos is computed with the Merging Cell Model proposed by Rodrigues & Thomas (1996). While originally discussed in the case of scale-free power spectra, it is developed and tested here in the framework of the cold dark matter cosmology. The halo mass function, the mass distribution of progenitors and child halos, as well as the probability distribution of formation times, have been computed and compared to the available analytic predictions. The halo auto-correlation function has also been obtained (a first for a semi-analytic merging tree), and tested agains…
HD molecules can be an important cooling agent of the primordial gas behind the shock waves originated through mergings of the dark matter haloes at epochs when first luminous objects were to form. We study the necessary conditions for the HD cooling to switch on in the low temperature range $T<200$ K. We show that these conditions are fulfiled in merging haloes with the total (dark matter and baryon) mass in excess of $M_{\rm cr}\sim 10^7[(1+z)/20]^{-2}\msun$. Haloes with masses $M>M_{\rm cr}$ may be the sites of low-mass star formation.
N-body simulations that follow only a collisionless dark matter component have failed to produce galaxy halos or substructure within dense environments. We investigate the `over-merging' problem analytically and with numerical simulations, by calculating dissolution timescales of halos due to physical and artificial dynamical effects. The numerical resolution that has recently been attained is such that mass-loss from two-body relaxation is negligible. We demonstrate that substructure is destroyed in present simulations as a result of large force softening combined with the heating sources of …
The excursion set theory of halo formation is modified by adopting the fractional Brownian motion, to account for possible correlation between merging steps. We worked out analytically the conditional mass function, halo merging rate and formation time distribution in the spherical collapse model. We also developed an approximation for the ellipsoidal collapse model and applied it to the calculation of the conditional mass function and the halo formation time distribution. For models in which the steps are positively correlated, the halo merger rate is enhanced when the accreted mass is less t…
The effects of merging histories of proto-objects on the angular momentum distributions of the present-time dark matter haloes are analysed. An analytical approach to the analysis of the angular momentum distributions assumes that the haloes are initially homogeneous ellipsoids and that the growth of the angular momentum of the haloes halts at their maximum expansion time. However, the maximum expansion time cannot be determined uniquely, because in the hierarchical clustering scenario each progenitor, or subunit, of the halo has its own maximum expansion time. Therefore the merging history of…
We discuss the merging rates in compact groups of 5 identical elliptical galaxies. All groups have the same mass and binding energy. We consider both cases with individual halos and cases where the halo is common to all galaxies and enveloping the whole group. In the latter situation the merging rate is slower if the halo is more massive. The mass of individual halos has little influence on the merging rates, due to the fact that all galaxies in our simulations have the same mass, and so the more extended ones have a smaller velocity dispersion. Groups with individual halos merge faster than g…
We investigate thermal regime of the baryons behind shock waves arising in the process of virialization of dark matter halos. We find a fraction of the shocked gas cooled by radiation of HD molecules down to the temperature of the cosmic microwave background (CMB): this fraction increases sharply from about $f_{\rm c}\sim 10^{-3}$ for dark halos of $M=5\times 10^7\msun$ to $\sim 0.1$ for halos with $M=10^8\msun$ at $z=10$. We show, however, that further increase of the mass does not lead to a significant growth of $f_{\rm c}$ -- the assymptotic value for $M\gg 10^8\msun$ is of 0.2. We estimate…
In the LCDM cosmology dark matter halos grow primarily through the accretion of smaller halos. Much of the mass in a halo of 10^14Msun comes in through accretion of ~10^13Msun halos. If each such halo hosted one luminous red galaxy (LRG) then the accretion of so many halos is at odds with the observed number of LRGs in clusters unless these accreted LRGs merge or disrupt on relatively short timescales (~2Gyr). These timescales are consistent with classical dynamical friction arguments, and imply that 2-3 LRGs have merged or disrupted within each halo more massive than 10^14Msun by z=0. The tot…
To account for the observed differential metallicity distribution (DMD) of the Milky Way halo, a semi-analytical model is presented in the framework of the hierarchical merging paradigm for structure formation. It is assumed that the Milky Way halo is composed of a number of sub-haloes with properties either as observed in the dwarf satellite galaxies of the Local group (shape of metallicity distribution, effective yield) or derived from calculations of structure formation (sub-halo distribution function). With reasonable assumptions for the parameters involved, we find that the overall shape …
A method of deriving and using merging history trees of dark matter galaxy haloes directly from pure gravity N-body simulations is presented. This combines the full non-linearity of N-body simulations with the flexibility of the semi-analytical approach. Merging history trees derived from power-law initial perturbation spectrum simulations (for indices $n=-2$ and $n=0$) by Warren et al. (1992) are shown. As an example of a galaxy formation model, these are combined with evolutionary stellar population synthesis, via simple scaling laws for star formation rates, showing that if most star format…
We present the preliminary results of 235 MHz, 327 MHz and 610 MHz observations of the galaxy cluster A3562 in the core of the Shapley Concentration. The purpose of these observations, carried out with the Giant Metrewave Radio Telescope (GMRT, Pune, India) was to study the radio halo located at the centre of A3562 and determine the shape of its radio spectrum at low frequencies, in order to understand the origin of this source. In the framework of the re--acceleration model, the preliminary analysis of the halo spectrum suggests that we are observing a young source (few $10^8$ yrs) at the beg…
We study the properties of hot gaseous halos in 10 nearby ultraluminous IRAS galaxies observed with the ACIS instrument on board Chandra. For all sample galaxies, diffuse soft X-ray emissions are found within ~10 kpc of the central region; their spectra are well fitted by a MEKAL model plus emission lines from alpha-elements and other ions. The temperature of the hot gas is about 0.7 keV and metallicity is about 1 solar. Outside the central region, extended hot gaseous halos are found for nine out of the ten ULIRGs. Most spectra of these extended halos can be fitted with a MEKAL model with a t…
We show an interesting correlation between the surface brightness and temperature structure of the relaxed clusters RXJ1720.1+2638 and MS1455.0+2232, hosting a pair of cold fronts, and their central core--halo radio source. We discuss the possibility that the origin of this diffuse radio emission may be strictly connected with the gas sloshing mechanism suggested to explain the formation of cold fronts in non major merging clusters. We show that the radiative lifetime of the relativistic electrons is much shorter than the timescale on which they can be transported from the central galaxy up to…
We evaluate the accuracy of semi-analytic merger-trees by comparing them with the merging histories of dark-matter halos in N-body simulations, focusing on the joint distribution of the number of progenitors and their masses. We first confirm that the halo mass function as predicted directly by the Press-Schechter (PS) model deviates from the simulations by up to 50% depending on the mass scale and redshift, while the means of the projected distributions of progenitor number and mass for a halo of a given mass are more accurately predicted by the Extended PS model. We then use the full merger …
We present a new algorithm (PINOCCHIO, PINpointing Orbit-Crossing Collapsed HIerarchical objects) to predict accurately the formation and evolution of individual dark matter haloes in a given realization of an initial linear density field. Compared with the halo population formed in a large (360^3 particles) collisionless simulation of a CDM universe, our method is able to predict to better than 10 per cent statistical quantities such as the mass function, two-point correlation function and progenitor mass function of the haloes. Masses of individual haloes are estimated accurately as well, wi…
The Cold Dark Matter paradigm successfully explains many phenomena on scales larger than galaxies, but seems to predict galaxy halos which are more centrally concentrated and have a lumpier substructure than observed. Endowing cosmic dark matter with a small primordial velocity dispersion preserves the successful predictions of the Cold Dark Matter scenario on large scales and improves the agreement with halo structure. A ``phase density'' $Q$, proportional to the inverse entropy for nonrelativistic matter, is estimated for relativistically decoupled thermal or degenerate relic particles of ma…
The timescale for galaxies within merging dark matter halos to merge with each other is an important ingredient in galaxy formation models. Accurate estimates of merging timescales are required for predictions of astrophysical quantities such as black hole binary merger rates, the build-up of stellar mass in central galaxies, and the statistical properties of satellite galaxies within dark matter halos. In this paper, we study the merging timescales of extended dark matter halos using N-body simulations. We compare these results to standard estimates based on the Chandrasekhar theory of dynami…
Hubble Space telescope (HST) images of merging galaxy system VV29 reveal the 0.3 Mpc baryonic-dark-matter (BDM) halo composed of primordial protoglobularstarcluster (PGC) clumps of planets. Star-cluster-wakes trace the merger by formation of stars from the planets. Aligned young globular star clusters (YGCs), star-wakes and dust-trails show the frictional, spiral passage of galaxy fragments VV29cdef in a long tail-like galaxy (VV29b) as the fragments merge on the accretion disk plane of the central spiral galaxy VV29a. The observations confirm the hydro-gravitational-dynamics (HGD) prediction …
In order to specify cosmologically motivated initial conditions for major galaxy mergers (mass ratios $\leq$ 4:1) that are supposed to explain the formation of elliptical galaxies we study the orbital parameters of major mergers of cold dark matter halos using a high-resolution cosmological simulation. Almost half of all encounters are nearly parabolic with eccentricities $e \approx 1$ and no correlations between the halo spin planes or the orbital planes. The pericentric argument $ω$ shows no correlation with the other orbital parameters and is distributed randomly. In addition we find that 5…
We examine the effect of primordial dark matter velocity dispersion and/or particle self-interactions on the structure and stability of galaxy halos, especially with respect to the formation of substructure and central density cusps. Primordial velocity dispersion is characterised by a ``phase density'' $Q\equiv ρ/<v^2>^{3/2}$, which for relativistically-decoupled relics is determined by particle mass and spin and is insensitive to cosmological parameters. Finite $Q$ leads to small-scale filtering of the primordial power spectrum, which reduces substructure, and limits the maximum central dens…
We present the first study on the mass functions of Jenkins et al (J01) and an estimate of their corresponding largest virialized dark halos in the Universe for a variety of dark-energy cosmological models with a running spectral index. Compared with the PL-CDM model, the RSI-CDM model can raise the mass abundance of dark halos for small mass halos at lower redshifts, but it is not apparent on scales of massive mass halos. Particularly, this discrepancy increases largely with the decrease of redshift, and the RSI-CDM model can suppress the mass abundance on any scale of halo masses at higher r…
In this second paper in our series, we continue to test primordial scenarios of globular cluster formation which predict that globular clusters formed in the early universe in the potential of dark matter minihalos. In this paper we use high-resolution N-body simulations to model tidal stripping experienced by primordial dark-matter dominated globular clusters in the static gravitational potential of the host dwarf galaxy. We test both cuspy Navarro-Frenk-White (NFW) and flat-core Burkert models of dark matter halos. Our primordial globular cluster with an NFW dark matter halo survives severe …
Primordial stars are likely to be very massive >30 Msun, form in isolation, and will likely leave black holes as remnants in the centers of their host dark matter halos. We expect primordial stars to form in halos in the mass range 10^6-10^10 Msun. Some of these early black holes, formed at redshifts z>10, could be the seed black hole for a significant fraction of the supermassive black holes found in galaxies in the local universe. If the black hole descendants of the primordial stars exist, their mergers with nearby supermassive black holes may be a prime candidate for long wavelength gravit…
The radio galaxy MRC 1138-262 at z=2.16 is most likely a brightest cluster galaxy in an early stage of evolution. Here we present observations of the luminous emission line halo and the stellar components of this radio galaxy. Optical narrow band imaging shows a very extended (about 160 kpc) and luminous Lyman alpha halo. Infrared narrow band imaging reveals a much smaller H alpha halo with a very different morphology. We advocate a model in which the inner part of the halo is photoionized by direct AGN illumination or by UV photons from young stars. Far from the nucleus (about 25 kpc), there …
we present new VLA observations at 1.4 GHz confirming the presence of a radio halo at the centre of the cluster A3562, in the core of the Shapley Concentration. We also report a detailed multifrequency radio study of the head tail galaxy J1333--3141, which is completely embedded in the halo emission. The radio halo has an irregular shape, and a largest linear size of $\sim$ 620 kpc, which is among the smallest found in the literature. The source has a steep spectrum, i.e. $α_{843 MHz}^{1.4 GHz} \sim 2$, and its total radio power, P$_{1.4 GHz} \sim 2 \times10^{23}$ W Hz$^{-1}$, is the lowest kn…
A large number of stellar evolution models with [Fe/H] = -2.3 and -3.3 have been calculated in order to determine the primordial $^7$Li abundance and to test current stellar evolution models by a comparison to the extensive database of Li abundances in extremely metal poor halo stars observed by Thorburn (1994). Standard models do a good job of fitting the observed Li abundances in stars hotter than 5600 K. They predict a primordial $^7$Li abundance of Log N(Li) = 2.24\pm 0.03$. Models which include microscopic diffusion predict a downward curvature in the $^7$Li destruction isochrones at hot …
We present an analytic expression for the axis ratio distribution of triaxial dark matter halos driven from physical principles. Adopting the picture of triaxial collapse based on the Zel'dovich approximation, we derive analytically both the minor-to-major and the conditional intermediate-to-major axis ratio distributions, and examine how they depend on the halo mass, redshift, and cosmology. Our analytic model is tested against the simulation data given by Jing & Suto in 2002, and found to reproduce the conditional intermediate-to-major axis ratio distribution successfully and the minor-to-ma…
We present numerical simulations of primordial supernovae in cosmological minihalos at $z \sim$ 20. We consider Type II supernovae, hypernovae, and pair instability supernovae (PISN) in halos from 6.9 $\times$ 10$^5$ - 1.2 $\times$ 10$^7$ $\Ms$, those in which Population III stars are expected to form via H$_2$ cooling. The supernovae evolve along two evolutionary paths according to whether they explode in \ion{H}{2} regions or neutral halos. Those in \ion{H}{2} regions first expand adiabatically and then radiate strongly upon collision with baryons ejected from the halo during its photoevapor…
Recent observational studies have revealed star-to-star abundance inhomogeneity among light elements (e.g., C, N, O, Na, and Al) of stars on the main sequence in the Galactic globular clusters (GCs). One of promising interpretations for this result is that the observed abundance inhomogeneity is due to the second generation of stars formed from ejecta of the first generation of evolved stars (e.g., AGB stars) within GCs. However it remains unclear whether and how this primordial pollution can occur within GCs. We here propose a new scenario in which primordial pollution of GCs is highly likely…
Future observations of CMB anisotropies will be able to probe high multipole regions of the angular power spectrum, corresponding to a resolution of a few arcminutes. Dust emission from merging haloes is one of the foregrounds that will affect such very small scales. We estimate the contribution to CMB angular fluctuations from objects that are bright in the sub-millimeter band due to intense star formation bursts following merging episodes. We base our approach on the Lacey-Cole merger model and on the Kennicutt relation which connects the star formation rate in galaxies with their infrared l…
The role of HD cooling in the formation of primordial objects is examined by means of a great number of 1-D models of the collapse of halos, exploring a wide range of masses and virialization redshifts. We find that HD has very little effect upon the critical mass separating the objects which are likely to form stars from those which are not. We also find that, once the proto-stellar collapse has started, HD effects are quite negligible. Instead, HD effects can be important during the intermediate stage of gas fragmentation: objects below a certain mass scale (~3x10^5 Msun at z_vir=20 in our `…
We report the discovery of spatially-extended, non-thermal-like emission components in Chandra X-ray spectra for five of a sample of seven massive, merging galaxy clusters with powerful radio halos. The emission components can be fitted by power-law models with mean photon indices in the range 1.5 < Gamma < 2.0. A control sample of regular, dynamically relaxed clusters, without radio halos but with comparable mean thermal temperatures and luminosities, shows no compelling evidence for similar components. Detailed X-ray spectral mapping reveals the complex thermodynamic states of the radio halo…
We reanalyze the galaxy-mass correlation function measured by the Sloan Digital Sky Survey to obtain host dark matter halo masses at galaxy and galaxy group scales. We extend the data to galaxy clusters in the 2MASS catalog and study the relation between central galaxy luminosity and halo mass. While the central galaxy luminosity scales as ~M^{0.7-0.8} at low masses, the relation flattens to ~M^{<0.3} above ~4x10^{13} M_sun. The total luminosity of galaxies in the halo, however, continues to grow as a power-law ~M^{0.8-0.9}. Starting from the hypothesis that the central galaxies grow by mergin…
CONTEXT.The mass assembly is believed to be the dominant process of early galaxy formation. This mechanism of galaxy building can proceed either by repeated major mergers with other systems, or by means of accretion of matter from the surrounding regions. AIMS.In this paper we compare the properties of local disk galaxies that appear isolated, i.e. not tidally affected by other galaxies during the last few Gyr within the volume given by cz<= 5000 km/s, with those galaxies at z values from 0.25 to 5. METHODS.Effective radii for 203 isolated galaxies and 1645 galaxies from the RC3 have been coll…
Chandra observations of the merging galaxy cluster A520 reveal a prominent bow shock with M=2.1+0.4-0.3. This is only the second clear example of a substantially supersonic merger shock front in clusters. Comparison of the X-ray image with that of the previously known radio halo reveals a coincidence of the leading edge of the halo with the bow shock, offering an interesting experimental setup for determining the role of shocks in the radio halo generation. The halo in A520 apparently consists of two spatially distinct parts, the main turbulence-driven component and a cap-like forward structur…
We determine the expected surface brightness and photometric signature of a white dwarf remnant population, issued from primordial low-mass stars formed at high redshifts, in today galactic halos. We examine the radial dependence of such a contribution as well as its redshift dependence. Such a halo diffuse radiation is below the detection limit of present large field ground-based surveys, but should be observable with the HST and with the future JWST project. Since the surface brightness does not depend on the distance, the integration of several galactic dark halos along the line of sight wi…
We study the population of satellite galaxies formed in a suite of N-body/gasdynamical simulations of galaxy formation in a LCDM universe. We find little spatial or kinematic bias between the dark matter and the satellite population. The velocity dispersion of the satellites is a good indicator of the virial velocity of the halo: σ_{sat}/V_{vir}=0.9 +/- 0.2. Applied to the Milky Way and M31 this gives V_{vir}^{MW}=109 +/- 22$ km/s and V_{vir}^{M31} = 138 +/- 35 km/s, respectively, substantially lower than the rotation speed of their disk components. The detailed kinematics of simulated satelli…
We forecast constraints on primordial non-Gaussianity achievable from forthcoming surveys by exploiting the scale-dependent halo bias introduced on large scales by non-Gaussian initial conditions. We explore the performance of exploiting both the shape of the galaxy power-spectrum on large scales and the cross-correlation of galaxies with Cosmic Microwave Background maps through the Integrated Sachs Wolfe effect. We find that future surveys can detect primordial non-Gaussianity of the local form with a non-Gaussianity parameter $|f_{\rm NL}|$ of order unity. This is particularly exciting becau…
Dark matter (DM) decays and annihilations might heat and partially reionize the Universe at high redshift. Although this effect is not important for the cosmic reionization, the gas heating due to DM particles might affect the structure formation. In particular, the critical halo mass for collapse is increased up to a factor of ~2. Also the fraction of gas which collapses inside the smallest halos is substantially reduced with respect to the cosmological value. These effects imply that DM decays and annihilations might delay the formation of the first structures and reduce the total star mass …
It was suggested by several authors that hypothetical primordial black holes (PBHs) may contribute to the dark matter in our Galaxy. There are strong constraints based on the Hawking evaporation that practically exclude PBHs with masses m~1e15-1e16g and smaller as significant contributors to the Galactic dark matter. Similarly, PBHs with masses greater than about 1e26g are practically excluded by the gravitational lensing observation. The mass range between 10e16g<m<10e26g is unconstrained. In this paper, we examine possible observational signatures in the unexplored mass range, investigating …
The lithium plateau observed in halo stars has long appeared as a paradox in the general context of the lithium abundance behavior in stellar outer layers. First, the plateau is flat, second, the lithium abundance dispersion is extremely small. This seems in contradiction with the large lithium variations observed in younger stars. It is also difficult to understand theoretically as lithium nuclei are easily destroyed by nuclear reactions, and subject to microscopic diffusion which, in the case of halo stars, should also lead to depletion. Several ideas have been proposed to account for the li…
Dark-matter halos are the scaffolding around which galaxies and clusters are built. They form when the gravitational instability of primordial density fluctuations causes regions which are denser than average to slow their cosmic expansion, recollapse, and virialize. Objects as different in size and mass as dwarf spheroidal galaxies and galaxy clusters are predicted by the CDM model to have halos with a universal, self-similar equilibrium structure whose parameters are determined by the halo's total mass and collapse redshift. These latter two are statistically correlated, however, since halos…
Recent spectroscopic observations of planetary nebulae (PNe) in several elliptical galaxies have revealed structural and kinematical properties of the outer stellar halo regions. In order to elucidate the origin of the properties of these planetary nebula systems (PNSs), we consider the merger scenario in which an elliptical galaxy is formed by merging of spiral galaxies. Using numerical simulations, we particularly investigate radial profiles of projected PNe number densities, rotational velocities, and velocity dispersions of PNSs extending to the outer halo regions of elliptical galaxies fo…
We use simulations of merging galaxies to explore the sensitivity of the morphology of tidal tails to variations of the halo mass distributions in the parent galaxies. Our goal is to constrain the mass of dark halos in well-known merging pairs. We concentrate on prograde encounters between equal mass galaxies which represent the best cases for creating tidal tails, but also look at systems with different relative orientations, orbital energies and mass ratios. As the mass and extent of the dark halo increase in the model galaxies, the resulting tidal tails become shorter and less massive, even…
We perform numerical simulations for the formation of the Galactic stellar halo, based on the currently favored cold dark matter (CDM) theory of galaxy formation. Our numerical models, taking into account both dynamical and chemical evolution processes in a consistent manner, are aimed at explaining observed structure and kinematics of the stellar halo in the context of hierarchical galaxy formation. The main results of the present simulations are summarized as follows. (1) Basic physical processes involved in the formation of the stellar halo, composed of metal-deficient stars with [Fe/H] $\l…
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 po…
It has long been known how to analytically relate the clustering properties of the collapsed structures (halos) to those of the underlying dark matter distribution for Gaussian initial conditions. Here we apply the same approach to physically motivated non-Gaussian models. The techniques we use were developed in the 1980s to deal with the clustering of peaks of non-Gaussian density fields. The description of the clustering of halos for non-Gaussian initial conditions has recently received renewed interest, motivated by the forthcoming large galaxy and cluster surveys. For inflationary-motivate…
The apparent absence of stars in the Milky Way halo with -5 ~< [Fe/H] ~< -4 suggests that the gas out of which the halo stars were born experienced a period of low or delayed star formation after the local universe was lit up by the first, metal-free generation of stars (Pop III). Negative feedback owed to the Pop III stars could initially have prevented the pre-Galactic halo from cooling, which thereby delayed the collapse and inhibited further star formation. During this period, however, the nucleosynthesis products of the first supernovae (SNe) had time to mix with the halo gas. As a result…
I present a scenario for the production of low mass, degenerate dwarfs of mass $>0.1 M_{\odot}$ via the mechanism of Lenzuni, Chernoff & Salpeter (1992). Such objects meet the mass limit requirements for halo dark matter from microlensing surveys while circumventing the chemical evolution constraints on normal white dwarf stars. I describe methods to observationally constrain this scenario and suggest that such objects may originate in small clusters formed from the thermal instability of shocked, heated gas in dark matter haloes, such as suggested by Fall & Rees (1985) for globular clusters.
The simplest interpretation of the microlensing events observed towards the Large Magellanic Clouds is that approximately half of the mass of the Milky Way halo is in the form of MAssive Compact Halo Objects with $M \sim 0.5 M_{\odot}$. It is not possible, due to limits from star counts and chemical abundance arguments, for faint stars or white dwarves to comprise such a large fraction of the halo mass. This leads to the consideration of more exotic lens candidates, such as primordial black holes, or alternative lens locations. If the lenses are located in the halo of the Milky Way, then const…
Using a hydrodynamics plus N-body simulation of galaxy cluster formation within a large volume and mock Chandra X-ray observations, we study the form and evolution of the intrinsic scatter about the best-fit X-ray temperature-mass relation for clusters. We investigate the physical origin of the scatter by correlating it with quantities that are closely related to clusters' formation and merging histories. We also examine the distribution of the scatter for merging and nonmerging populations, identified using halo merger trees derived from the simulation as well as X-ray substructure measures. …
We use the algorithm of Cole et al. (2000) to generate merger trees for the first star clusters in a Lambda CDM cosmology under an isotropic UV background radiation field, parametrized by J_21. We have investigated the problem in two ways: a global radiation background and local radiative feedback surrounding the first star clusters. Cooling in the first halos at high redshift is dominated by molecular hydrogen, H_2 - we call these Generation 1 objects. At lower redshift and higher virial temperature, T_vir > 10^4K, electron cooling dominates - we call these generation 2. Radiation fields act …
The excursion set model provides a convenient theoretical framework to derive dark matter halo abundances. This paper generalizes the model by introducing a more realistic merging and collapse process. A new parameter regulates the influence of the environment and thus the coherence (non-Markovianity) of the merging and the collapse of individual mass shells. The model mass function also includes the effects of an ellipsoidal collapse. Analytic approximations of the halo mass function are derived for scale-invariant power spectra with the slopes $n=0,-1,-2$. The $n=-2$ mass function can be com…
We simulate the ionization environment of z ~ 20 luminous objects formed within the framework of the current CDM 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 hydrodynamical calculations of the evolution of these first HII regions in the universe. The initially D--type ionization front evolves to become R--type within $\lesssim 10^5$ yrs at a distance $\sim1$ pc. This ionization front then completely overruns the halo, accelerating an expandin…
We consider the formation of horizon-size primordial black holes (PBH's) from pre-existing density fluctuations during cosmic phase transitions. It is pointed out that the formation of PBH's should be particularly efficient during the QCD epoch due to a substantial reduction of pressure forces during adiabatic collapse, or equivalently, a significant decrease in the effective speed of sound during the color-confinement transition. Our considerations imply that for generic initial density perturbation spectra PBH mass functions are expected to exhibit a pronounced peak on the QCD-horizon mass s…
