On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10(-21). It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203,000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410(-180)(+160)  Mpc corresponding to a redshift z=0.09(-0.04)(+0.03). In the source frame, the initial black hole masses are 36(-4)(+5)M⊙ and 29(-4)(+4)M⊙, and the final black hole mass is 62(-4)(+4)M⊙, with 3.0(-0.5)(+0.5)M⊙c(2) radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

/pdf/the-observation-of-gravitational-waves-from-a-binary-black-58srpupgg9.pdf

The Observation of Gravitational Waves from a Binary Black Hole Merger

Cosmology at low frequencies: The 21 cm transition and the high-redshift Universe

Physical Processes in the Interstellar Medium

To constrain the nature of the very first stars, we investigate the collapse and fragmentation of primordial, metal-free gas clouds. We explore the physics of primordial star formation by means of three-dimensional simulations of the dark matter and gas components, using smoothed particle hydrodynamics, under a wide range of initial conditions, including the initial spin, the total mass of the halo, the redshift of virialization, the power spectrum of the DM fluctuations, the presence of HD cooling, and the number of particles employed in the simulation. We find characteristic values for the temperature, T ~ a few 100 K, and the density, n ~ 103-104 cm-3, characterizing the gas at the end of the initial free-fall phase. These values are rather insensitive to the initial conditions. The corresponding Jeans mass is MJ ~ 103 M?. The existence of these characteristic values has a robust explanation in the microphysics of H2 cooling, connected to the minimum temperature that can be reached with the H2 coolant, and to the critical density at which the transition takes place between levels being populated according to non-LTE (NLTE), and according to LTE. In all cases, the gas dissipatively settles into an irregular, central configuration that has a filamentary and knotty appearance. The fluid regions with the highest densities are the first to undergo runaway collapse due to gravitational instability, and to form clumps with initial masses ~103 M?, close to the characteristic Jeans scale. These results suggest that the first stars might have been quite massive, possibly even very massive with M* 100 M?. After a gas element has undergone runaway collapse, and has reached densities in excess of 108 cm-3, a sink particle is created. This procedure allows us to follow the evolution of the overall system beyond the point where the first nonlinear region would otherwise force the calculation to a halt. These later evolutionary stages, during which the clumps grow in mass due to accretion and merging with other clumps, are quite sensitive to the initial conditions. The key process in building up very massive clumps, with masses up to a few times 104 M?, is merging between clumps. Since the merging rate sensitively depends on the density of the gas, halos with the highest degree of central concentration are able to assemble the most massive clumps. Among these are halos with a low spin (? 0.01), and with DM fluctuations imprinted according to a white-noise spectrum.

https://iopscience.iop.org/article/10.1086/323947/pdf

The Formation of the First Stars. I. The Primordial Star-forming Cloud

Modeling galaxy formation in a cosmological context presents one of the greatest challenges in astrophysics today, due to the vast range of scales and numerous physical processes involved. Here we review the current status of models that employ two leading techniques to simulate the physics of galaxy formation: semi-analytic models and numerical hydrodynamic simulations. We focus on a set of observational targets that describe the evolution of the global and structural properties of galaxies from roughly Cosmic High Noon ($z\sim 2-3$) to the present. Although minor discrepancies remain, overall, models show remarkable convergence between different methods and make predictions that are in qualitative agreement with observations. Modelers seem to have converged on a core set of physical processes that are critical for shaping galaxy properties. This core set includes cosmological accretion, strong stellar-driven winds that are more efficient at low masses, black hole feedback that preferentially suppresses star formation at high masses, and structural and morphological evolution through merging and environmental processes. However, all cosmological models currently adopt phenomenological implementations of many of these core processes, which must be tuned to observations. Many details of how these diverse processes interact within a hierarchical structure formation setting remain poorly understood. Emerging multi-scale simulations are helping to bridge the gap between stellar and cosmological scales, placing models on a firmer, more physically grounded footing. Concurrently, upcoming telescope facilities will provide new challenges and constraints for models, particularly by directly constraining inflows and outflows through observations of gas in and around galaxies.

Physical Models of Galaxy Formation in a Cosmological Framework

We present the merger rate density of Population (Pop.) III binary black holes (BHs) by means of a widely-used binary population synthesis code BSE with extensions to very massive and extreme metal-poor stars. We consider not only low-mass BHs (lBHs: $5-50 M_\odot$) but also high-mass BHs (hBHs: $130-200 M_\odot$), where lBHs and hBHs are below and above the pair-instability mass gap ($50-130 M_\odot$), respectively. Pop. III BH-BHs can be categorized into three subpopulations: BH-BHs without hBHs (hBH0s: $m_{\rm tot} \lesssim 100 M_\odot$), with one hBH (hBH1s: $m_{\rm tot} \sim 130-260 M_\odot$), and with two hBHs (hBH2s: $m_{\rm tot} \sim 270-400 M_\odot$), where $m_{\rm tot}$ is the total mass of a BH-BH. Their merger rate densities at the current universe are $\sim 0.1$ yr$^{-1}$ Gpc$^{-3}$ for hBH0s, and $\sim 0.01$ yr$^{-1}$ Gpc$^{-3}$ for the sum of hBH1s and hBH2s, provided that the mass density of Pop. III stars is $\sim 10^{13} M_\odot$ Gpc$^{-3}$. These rates are modestly insensitive to initial conditions and single star models. The hBH1 and hBH2 mergers can dominate BH-BHs with hBHs discovered in near future. They have low effective spins $\lesssim 0.2$ in the current universe. The number ratio of the hBH2s to the hBH1s is high, $\gtrsim 0.1$. We also find BHs in the mass gap (up to $\sim 85 M_\odot$) merge. These merger rates can be reduced to nearly zero if Pop. III binaries are always wide ($\gtrsim 100 R_\odot$), and if Pop. III stars always enter into chemically homogeneous evolution. The presence of close Pop. III binaries ($\sim 10 R_\odot$) are crucial for avoiding the worst scenario.

Merger rate density of Population III binary black holes below, above, and in the pair-instability mass gap

Recent observations on the cosmic microwave background by Wilkinson Microwave Anisotropy Probe(WMAP) strongly suggest that the reionization of the universe took place quite early (z \sim 17). On the other hand, it has been pointed out that the cold dark matter cosmology suffers from the substructure problem that subgalactic halos are overproduced than the observed dwarf galaxies in the Local Group. In this paper, as a potential mechanism to solve this problem, we consider the feedback effects of the early reionization on the formation of small-scale structures. For this purpose, we perform 3D radiation hydrodynamic simulations with incorporating the radiative transfer for ionizing photons. As a result, it is found that the early reionization is so devastating for low-mass systems with M_{vir}\la 10^8 M_\odot or v_{circ}\la 20km/s, and almost all gas is photo-evaporated in more than 95% of low-mass systems. Such a strong negative feedback on the formation of low-mass galaxies may solve the substructure problem and support the picture that Local Group dwarf galaxies are descendants of the more massive halos that experienced and survived tidal stripping.

https://arxiv.org/pdf/astro-ph/0406305.pdf

Effects of Early Cosmic Reionization on the Substructure Problem in Galactic Halo

We investigate the evolution of cosmological low-mass (low virial temperature) objects and the formation of the first luminous objects. First, the "cooling diagram" for low-mass objects is shown. We assess the cooling rate taking into account the contribution of H2, which is not in chemical equilibrium generally, with a simple argument of timescales. The reaction rates and the cooling rate of H2 are taken from the recent results by Galli & Palla. Using this cooling diagram, we also estimate the formation condition of luminous objects taking into account the supernova (SN) disruption of virialized clouds. We find that the mass of the first luminous object is several times 107 M☉ because smaller objects may be disrupted by the SNe before they become luminous. Metal pollution of low-mass (Lyα) clouds is also discussed. The resultant metallicity of the clouds is Z/Z☉ ~ 10-3.

http://iopscience.iop.org/article/10.1086/312277/pdf

Formation and Disruption of Cosmological Low-Mass Objects

We re-examine the thermal evolution of the postshock layer in primordial gas clouds. Comparing the time scales, we find that the evolutionary paths of postshock regions in primordial gas clouds can be basically understood in terms of the diagram drawn in the ionization degree vs temperature plane. The results obtained from the diagram are independent of the density in the case that we do not include photodissociation and photoionization. We also argue that the diagram is not only relevant to the case of the steady postshock flow, but also to the isochorically cooling gas.

/pdf/the-thermal-evolution-of-the-postshock-layer-in-pregalactic-d2mcot97gm.pdf

The Thermal Evolution of the Postshock Layer in Pregalactic Clouds

We investigate the star formation process in primordial environment in the presence of radiative feedback by other population III stars formed earlier. In this paper, we focus our attention on the effects by photodissociative radiation toward the full understanding of the radiative feedback effects. We perform three dimensional radiation hydrodynamics simulations on this issue as well as analytic estimates, paying special attention on the self-shielding effect and dynamics of the star-forming cloud. As a result, we find that the ignition timing of the source star is crucial. If the ignition is later than the epoch when the central density of the collapsing cloud exceeds $\sim 10^3-10^4{\rm cm^{-3}}$, the collapse cannot be reverted, even if the source star is located at $\la$ 100pc. The uncertainty of the critical density comes from the variety of initial conditions of the collapsing cloud. We also find the analytic criteria for a cloud to collapse with given central density, temperature and the Lyman-Werner(LW) band flux which irradiates the cloud. 
Although we focus on the radiation from neighbor stars, this result can also be applied to the effects of diffuse LW radiation field, that is expected to be built up prior to the reionization of the universe. We find that self-gravitating clouds can easily self-shield from diffuse LW radiation and continue their collapse for densities larger than $\sim 10^3 {\rm cm^{-3}}$.

Photodissociation feedback of Population III stars \\on their neighbor prestellar cores

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