Primordial Gas Clouds Research Articles

Collapse of low-mass clouds in the presence of a UV radiation field

The collapse of marginally Jeans-unstable primordial gas clouds in the presence of a UV radiation field is discussed. Assuming that the dynamical collapse proceeds approximately in an isothermal self-similar fashion, we investigate the thermal evolution of the collapsing central core until H2 cooling dominates photoheating and the temperature drops to below 104 K. Consequently, the mass of the cooled core is evaluated as Mcool=3.6×106 M⊙ (I21/1)−0.32. This scale depends only on the incident UV intensity, and provides a lower limit to the mass of collapsed objects in the UV radiation field.

Does Deuterium Enable the Formation of Primordial Brown Dwarfs?

We investigate thermal and dynamical evolution of a primordial gas cloud with an updated deuterium chemistry. We consider a fragment of a postshock-cooled sheet that is expected to form by collapse of a massive cloud ( greater, similar108 M middle dot in circle) and by blast waves due to supernova explosions. At first we investigate molecule formation in a primordial shock. We show that almost all deuterium can be converted to HD within the age of the universe at the collapsed redshift in the case of a cloud that has a virial temperature of approximately 106 K and collapses at z>1. When the postshock sheet fragments owing to gravitational instability, the fractional H2 and HD abundances become approximately 10-2 and approximately 10-5, respectively, which are 103-104 times higher than the result of molecule formation in the expanding universe after recombination. To study the subsequent evolution of a fragment, we performed one-dimensional simulations of a spherical/cylindrical cloud, of which initial conditions (e.g., fractional abundances of chemical composition, temperature) are derived from the result of the shock. It is found that, in case of a cylindrical collapse, the cooling by HD molecules keeps the temperature of the cloud less than 100 K and the cloud evolves almost isothermally. When the cloud becomes optically thick to the HD line emission ( approximately 1010 cm-3) and the gravitational fragmentation of the cylindrical cloud becomes effective, the Jeans mass becomes comparable to 0.1 M middle dot in circle. This series of processes enables the formation of primordial low-mass stars, and possibly brown dwarfs, in primordial gas clouds.

The Space Density of Primordial Gas Clouds near Galaxies and Groups and their Relation to Galactic High-Velocity Clouds

The Arecibo H i Strip Survey probed the halos of approximately 300 cataloged galaxies and the environments of approximately 14 groups with sensitivity to neutral hydrogen masses >/=107 M middle dot in circle. The survey detected no objects with properties resembling the high-velocity clouds (HVCs) associated with the Milky Way or Local Group. If the HVCs were typically MHi=107.5 M middle dot in circle objects distributed throughout groups and galaxy halos at distances of approximately 1 Mpc, the survey should have made approximately 70 HVC detections in groups and approximately 250 detections around galaxies. The null detection implies that HVCs are deployed at typical distances of </=200 kpc from the galaxies or group barycenters. If the clouds are in virial equilibrium, their average dark matter fraction must be 98% or higher.

Photodissociative Regulation of Star Formation in Metal‐free Pregalactic Clouds

We study the H2 photodissociation regions around OB stars in primordial gas clouds whose virial temperatures are between a few hundred and a few thousand Kelvin. In such small objects, a single O star can photodissociate a mass equal to that of the cloud itself. As a result, the clouds deplete their molecular coolant and cannot cool in a free-fall time, and subsequent star formation is totally quenched. This indicates that stars do not form efficiently in small objects and that these objects contribute little to the reionization of the universe.

On the Mass of Population III Stars

Performing one-dimensional hydrodynamical calculations coupled with nonequilibrium processes for hydrogen molecule formation, we pursue the thermal and dynamical evolution of filamentary primordial gas clouds and attempt to make an estimate on the mass of Population III stars. The cloud evolution is computed from the central proton density nc~102-104 cm-3 up to ~1013 cm-3. It is found that, almost independent of initial conditions, a filamentary cloud continues to collapse nearly isothermally owing to H2 cooling until the cloud becomes optically thick against the H2 lines (nc~1010-1011 cm-3). During the collapse the cloud structure separates into two parts, i.e., a denser spindle and a diffuse envelope. The spindle contracts quasi-statically, and thus the line mass of the spindle keeps a characteristic value determined solely by the temperature (~800 K), which is ~1×103 M☉ pc-1 during the contraction from nc~105 cm-3 to 1013 cm-3. Applying a linear theory, we find that the spindle is unstable against fragmentation during the collapse. The wavelength of the fastest growing perturbation (λm) lessens as the collapse proceeds. Consequently, successive fragmentation could occur. When the central density exceeds nc~1010-1011 cm-3, the successive fragmentation may cease, since the cloud becomes opaque against the H2 lines and the collapse decelerates appreciably. Resultantly, the minimum value of λm is estimated to be ~2×10−3 pc. The mass of the first star is then expected to be typically ~3 M☉, which may grow up to ~16 M☉ by accreting the diffuse envelope. Thus, the first-generation stars are anticipated to be massive but not supermassive.

Fragmentation of the Primordial Gas Clouds

We discuss the fragmentation of primordial gas clouds in the universe after decoupling. Comparing the timescale of collapse (tdyn) with that of fragmentation (tfrag), we obtained the minimum mass of a fragment analytically as following way.

A Cluster of Low-Redshift [CLC]Ly[/CLC]α Clouds toward PKS 2155−304. I. Limits on Metals and D/H

We report observations from the Hubble Space Telescope (HST) and the VLA on the galactic environment, metallicity, and D/H in strong low-redshift Lyα absorption systems toward the bright BL Lac object PKS 2155-304. These studies are intended to clarify the origin and chemical evolution of gas at large distances from galaxies. With GHRS/G160M data at ~20 km s-1 resolution, we detect a total of 14 Lyα absorbers, six of them clustered between cz = 16,100 and 18,500 km s-1. Although ORFEUS studies claimed Lyman continuum (Lyc) absorption at z ≈ 0.056 with N(H I) = (2–5) × 1016 cm-2, the Lyα data suggest a range, N(H I) = (3–10) × 1014 cm-2. Even higher columns, needed for consistency with the ORFEUS Lyc results, are possible if the Lyα line core at 17,000 ± 50 km s-1 contains narrow H I components. We identify the Lyα cluster with a group of five H I galaxies offset by 400–800 h kpc from the sight line. The two strongest absorption features cover the same velocity range as the H I emission in the two galaxies closest to the line of sight. If the Lyα is associated with these galaxies, they must have huge halos (400–500 h kpc) of highly turbulent, mostly ionized gas. The Lyα absorption could also arise from an extended sheet of intragroup gas, or from smaller primordial clouds and halos of dwarf galaxies. We see no absorption from Si III λ1206, C IV λ1548, or deuterium Lyα at the expected positions of the strongest Lyα absorbers. Photoionization models yield (4 σ) limits of (Si/H) ≤ 0.003(Si/H)☉, (C/H) ≤ 0.005(C/H)☉, and (D/H) ≤ 2.8 × 10-4 if N(H I) has the ORFEUS value of 2 × 1016 cm-2. The limits increase to 0.023 solar metallicity and D/H ≤ 2.8 × 10-3 if N(H I) is only 2 × 1015 cm-2. These limits can be improved with further studies by HST/STIS and measurements of the Lyc and higher Lyman series absorption by FUSE. However, the current data suggest that the intergalactic gas in this group has not been enriched to the levels suggested by X-ray studies of intracluster gas. Because of their low metallicity and large distance from galaxies, these absorbers could be primordial gas clouds.

Thermal and Dynamical Evolution of Primordial Gas Clouds: On the Formation of First Luminous Objects

We investigate the thermal and dynamical evolution of primordial gas clouds in the universe after decoupling. Comparing the time scale of dynamical evolution with that of fragmentation, we can estimate the typical fragmentation scale. We propose the following scenario of the formation process of first luminous objects consisting of large number stars. First, by pancake collapse of the overdensity regions in the expanding universe or collision between clouds in potential wells, quasi-plane shocks form. If the shock-heated temperature is higher than about 104 K, the post-shock gas cools down to several hundred K by H2 line cooling, and the shock-compressed layer fragments into filamentary clouds. The filamentary cloud collapses dynamically once more and fragments into cloud cores. Finally, a primordial star forms in a cloud core. We show that the minimum mass of the first star is essentially determined by the Chandrasekhar mass. Also, we investigate the dynamical collapse of cloud cores by numerical simulation and show that the evolution paths of the central regions of the cores depend only very weakly on the total core mass. After mass accretion, a massive star may be formed in a core, since the estimated mass accretion rate is very large. In such a case, it may be possible for many massive stars form almost simultaneously. Then the clouds can be luminous objects. On the other hand, if the shock-heated temperature is lower, effective star formation is delayed significantly.

The Thermal Evolution of the Postshock Layer in Pregalactic Clouds

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 ion­ ization 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. The thermal evolution of primordial gas clouds has been investigated by many authors. 15),24),3),17),9),20),29) Almost all of those studies have been concerned with the formation of various kinds of galaxies, or primordial stars. In those papers, galaxies are assumed to grow out of small density perturbations present in the early universe. Because of the coldness of the growing density perturbations, the formed clouds, which are the progenitors of galaxies, experience strong shock heating at the bouncing epoch. Shock heating is also expected in the hierarchical clustering scenario of structure formation. In this case, shocks are expected in the collision between two clouds which are trapped in the gravitational potential well associated with the larger structure. In any case, shock heating is expected in the era of galaxy formation. The spatial structure and the thermal evolution of the postshock layer in primordial gas clouds were investigated by many authors. 8), 26), 27),14),23),10),1),30) In those studies the common and most important point is the over production of hydrogen molecules. For example, in Shapiro and Kang 23) (hereafter SK), the thermal evolution of steady postshock flow is investigated. They found that the postshock flow cools down so fast that the recombination process cannot catch up with the cooling. As a result, the ionization degree remains high (Ye rv 10- 3 ), even when the temperature has dropped below 10 4 K. Feeded these relic electrons, hydrogen molecules form through the processes

Destruction of Molecular Hydrogen during Cosmological Reionization

We investigate the ability of primordial gas clouds to retain molecular hydrogen (H_2) during the initial phase of the reionization epoch. We find that before the Stromgren spheres of the individual ionizing sources overlap, the UV background below the ionization threshold is able to penetrate large clouds and suppress their H_2 abundance. The consequent lack of H_2 cooling could prevent the collapse and fragmentation of clouds with virial temperatures T_vir < 10^4 K (or masses 10^8 Msun [(1+z_vir)/10]^{-3/2}). This negative feedback on structure-formation arises from the very first ionizing sources, and precedes the feedback due to the photoionization heating.

Fragmentation of the Primordial Gas Clouds and the Lower Limit on the Mass of the First Stars

We discuss the fragmentation of primordial gas clouds in the universe after decoupling. Comparing the time scale of collapse with that of fragmentation, we obtain the typical mass of a fragment both numerically and analytically. It is shown that the estimated mass gives the minimum mass of a fragment which is formed from the primordial gas cloud and is essentially determined by the Chandrasekhar mass.

The Fragment Mass Scale of the Primordial Gas Clouds. I: Non-Spherical Pressure-Free Collapse

The Fragment Mass Scale of the Primordial Gas Clouds. I: Non-Spherical Pressure-Free Collapse

Collapse of primordial gas clouds and the formation of quasar black holes

The formation of quasar black holes during the hydrodynamic collapse of protogalactic gas clouds is discussed. The dissipational collapse and long-term dynamical evolution of these systems is analyzed using three-dimensional numerical simulations. The calculations focus on the final collapse stages of the inner baryonic component and therefore ignore the presence of dark matter. Two types of initial conditions are considered: uniformly rotating spherical clouds, and iirotational ellipsoidal clouds. In both cases the clouds are initially cold, homogeneous, and not far from rotational support (T/(absolute value of W) approximately equals 0.1). Although the details of the dynamical evolution depend sensitively on the initial conditions, the qualitative features of the final configurations do not. Most of the gas is found to fragment into small dense clumps, that eventually make up a spheroidal component resembling a galactic bulge. About 5% of the initial mass remains in the form of a smooth disk of gas supported by rotation in the gravitational potential potential well of the outer spheroid. If a central seed black hole of mass approximately greater than 10(exp 6) solar mass forms, it can grow by steady accretion from the disk and reach a typical quasar black hole mass approximately 10(exp 8) solar mass in less than 5 x 10(exp 8) yr. In the absence of a sufficiently massive seed, dynamical instabilities in a strongly self-gravitating inner region of the disk will inhibit steady accretion of gas and may prevent the immediate formation of quasar.

On the Cosmological Origin of Population III Objects

AbstractIt is the aim of our letter to embedd the presently well understood scale of stellar mass into a simple cosmological scenario for the origin of Population III objects. The result of our consideration will be that if the first bound objects with typical stellar mass of the order of 30 M⊙ could evolve as ordinary stars ending up exploding (Type II supernovae) the problem of the pregalactic enrichment of a primordial gas cloud be solved.

Thermal-chemical instability in a pre-galactic gas cloud

The growth of thermal-chemical instability in a pre-galactic medium is followed by numerical simulation of gas dynamics. Results show that a primordial gas cloud breaks into self-gravitating subcondensations with the mass of normal stars.

Molecules in the early universe

view Abstract Citations (236) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Molecules in the early universe Lepp, S. ; Shull, J. M. Abstract Calculations are presented of the formation of astrophysically interesting molecules (H2, HD, LiH, and HeH+) by gas-phase reactions during the postrecombination epoch (redshifts z = 300-30). In standard Friedmann cosmological models, H2/H is approximately equal to 10 to the -6th, HD/H2 is approximately equal to 10 to the -4.5th, and LiH/H2 is approximately equal to 10 to the -6.5th. These molecules may dominate the cooling and trigger the collapse of primordial gas clouds. The dipole rotational transitions of HD and LiH are particularly important at high density and low temperature. Additional molecules form during spherical collapse of these clouds, and their rotational cooling keeps the gas temperature between 400 and 1500 K over 12 decades of density increase until the H2 lines becomes optically thick. The existence of molecular coolants at high redshift has significant implications for the first generation of stars and for thermal instabilities in intergalactic matter. Publication: The Astrophysical Journal Pub Date: May 1984 DOI: 10.1086/162013 Bibcode: 1984ApJ...280..465L Keywords: Cosmochemistry; Cosmology; Gravitational Collapse; Interstellar Gas; Molecular Clouds; Universe; Astronomical Models; Chemical Evolution; Hydrogen Recombinations; Red Shift; Stellar Evolution; Astrophysics full text sources ADS |

Thick discs and the Formation of Disc Galaxies

The formation of disc galaxies is generally believed to involve the dissipative collapse of primordial gas clouds, moving in the potentials of dark, self-gravitating massive halos. Until now, most theories have considered only spherically symmetric collapse, with the luminous disc being formed out of a plane of cold gas which is in centrifugal equilibrium. However, it is unlikely that the halo potential will in fact be spherical.

On the fragmentation of cosmic gas clouds. I - The formation of galaxies and the first generation of stars

view Abstract Citations (554) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the fragmentation of cosmic gas clouds. I. The formation of galaxies and the first generation of stars. Silk, J. Abstract The formation of galaxies is studied in the context of the fragmentation of massive diffuse ionized gas clouds. A semiquantitative discussion of the role of cooling indicates that there is a characteristic mass of a galaxy (of the order of 500 billion solar masses) which can fragment out of an ionized medium of density not exceeding 10 to the -25th power g/cu cm, whereas more massive galaxies attain a characteristic radius of the order of 60 kpc. The relationship between binding energy and mass is investigated for spheroidal galaxies. The role of H2 and Ly-alpha cooling in the fragmentation of primordial gas clouds is evaluated, and relatively qualitative arguments indicate that the first stars have masses of at least 0.3 solar mass with a characteristic mass of approximately 20 solar masses. When the average heavy-element abundance by mass exceeds about 0.00001, heavy-element cooling prevails over Ly-alpha cooling, and subsequent fragmentation forms second-generation stars of low characteristic mass. Publication: The Astrophysical Journal Pub Date: February 1977 DOI: 10.1086/154972 Bibcode: 1977ApJ...211..638S Keywords: Fragmentation; Galactic Evolution; Ionized Gases; Stellar Evolution; Cooling; Elliptical Galaxies; Galactic Structure; Heavy Elements; Hydrogen Clouds; Stellar Mass; Astrophysics full text sources ADS |

Formation of Gravitationally Bound Primordial Gas Clouds. II

Formation of Gravitationally Bound Primordial Gas Clouds. II

Formation of Gravitationally Bound Primordial Gas Clouds

The formation of gravitationally bound systems from primordial gas is studied by . means of Tolman's solution for dust-like matter. The critical values of density contrast and its growth rate at an initial epoch are derived, which are necessary for an inhomogeneity to condense into a bound system before the appearance of the oldest stars. The matter distributions in an . isolated inhomogeneity and an inhomogeneity included in a larger one (which will be a cluster of gas clouds) are followed with time by assuming simple models for inhomogeneities, and it is shown how the bound region spreads outwards. Moreover, the minimum mass of fragments into which the gas clouds may break up is examined. The formation of galaxies which are dispersed everywhere at present in the universe has been studied by many authors. Some of them have assumed that small density fluctuations in an early stage of cosmic evolution grow owing to gravitational instability, until they condense into protogalaxies. However, on Lifshitz's assumption!) that the early stage was very quiet and the fluctua­ tions were statistical, gravitational instability was not effective. This is because the density contrasts of the statistical fluctuations corresponding to galaxies are too small (r-J 10- 34 ) to grow to gravitationally bound systems within the cosmic age. The linearized theories were sufficient for the description of such small fluctuations. After Lifshitz, the behavior of the fluctuations has been studied successively in the linear approximation,2) while Lifshitz's assumption was not always used. On the other hand, it has been assumed that an early stage of the UnIverse was very turbulent and density fluctuations were not small. S ) At the stage when radiation density was larger than matter density and matter was wholly ionized, matter was strongly kicked by photon particles and hindered from condensing. But, once matter was neutrarized, density fluctuations condensed promptly into bound systems. However, we do not know at present how quiet or turbulent the early stages were. Accordingly, still we could assume the existence of fluctuations with vari­ ous amplitudes. In the process of their condensation, gravitation would have played the main role, and, as the density contrasts increased to the order o£