Radiative cooling implementations in simulations of primordial star formation

We study the thermal evolution of primordial star-forming gas clouds using three-dimensional cosmological simulations. We critically examine how assumptions and approximations made in calculating radiative cooling rates affect the dynamics of the collapsing gas clouds. We consider two important molecular hydrogen cooling processes that operate in a dense primordial gas; H_2 line cooling and continuum cooling by H_2 collision-induced emission. To calculate the optically thick cooling rates, we follow the Sobolev method for the former, whereas we perform ray-tracing for the latter. We also run the same set of simulations using simplified fitting functions for the net cooling rates. We compare the simulation results in detail. We show that the time- and direction-dependence of hydrodynamic quantities such as gas temperature and local velocity gradients significantly affects the optically thick cooling rates. Gravitational collapse of the cloud core is accelerated when the cooling rates are calculated by using the fitting functions. The structure and evolution of the central pre-stellar disk are also affected. We conclude that physically motivated implementations of radiative transfer are necessary to follow accurately the thermal and chemical evolution of a primordial gas to high densities.Comment: 25 pages, 12 figures, To appear in Ap

Supersonic Gas Streams Enhance the Formation of Massive Black Holes in the Early Universe

The origin of super-massive black holes in the early universe remains poorly understood.Gravitational collapse of a massive primordial gas cloud is a promising initial process,but theoretical studies have difficulty growing the black hole fast enough.We report numerical simulations of early black hole formation starting from realistic cosmological conditions.Supersonic gas motions left over from the Big Bang prevent early gas cloud formation until rapid gas condensation is triggered in a proto-galactic halo. A protostar is formed in the dense, turbulent gas cloud, and it grows by sporadic mass accretion until it acquires 34,000 solar masses.The massive star ends its life with a catastrophic collapse to leave a black hole -- a promising seed for the formation of a monstrous black hole.Comment: Published in Science, combined with updated SOM, additional images and movies are available at http://www-utap.phys.s.u-tokyo.ac.jp/naoki.yoshida/Blackhole/0929e.htm

Early Structure Formation from Primordial Density Fluctuations with a Blue, Tilted Power Spectrum: High-Redshift Galaxies

Recent observations by the James Webb Space Telescope (JWST) discovered unexpectedly abundant luminous galaxies at high redshift, posing possibly a severe challenge to popular galaxy formation models. We study early structure formation in a cosmological model with a blue, tilted power spectrum (BTPS) given by $P(k) \propto k^{m_{\rm s}}$ with $m_{\rm s} > 1$ at small length scales. We run a set of cosmological $N$-body simulations and derive the abundance of dark matter halos and galaxies under simplified assumptions on star formation efficiency. The enhanced small-scale power allows rapid nonlinear structure formation at $z>7$, and galaxies with stellar mass exceeding $10^{10}\,M_\odot$ can be formed by $z=9$. Because of frequent mergers, the structure of galaxies and galaxy groups appears clumpy. The BTPS model reproduces the observed stellar mass density at $z=7-9$, and thus eases the claimed tension between galaxy formation theory and recent JWST observations. The large-scale structure of the present-day Universe is largely unaffected by the modification of the small-scale power spectrum. We conduct a systematic study by varying the slope of the small-scale power spectrum to derive constraints on the BTPS model from a set of observations of high-redshift galaxies.Comment: 10 pages, 7 figures, 1 table, accepted for publication in Ap

Evolution of Primordial Stars Powered by Dark Matter Annihilation up to the Main-Sequence Stage

Primordial stars formed in the early universe are thought to be hosted by compact dark matter (DM) halos. If DM consists of Weakly Interacting Massive Particles (WIMPs), such stars may be powered by DM annihilation during the early phases of their evolutions. We study the pre-main sequence evolutions of the primordial star using a detailed stellar evolution code under the assumption that the annihilation of adiabatically contracted WIMPs DM within the star provides a sufficient energy to sustain the stellar equilibrium. We follow the evolution of accreting stars using several gas mass accretion rates derived from cosmological simulations. We show that the stellar mass becomes very large, up to 900 - 1000 M_sun when the star reaches the main-sequence phase for a reasonable set of model parameters such as DM particle mass and the annihilation cross section. During the dark star phase, the star expands over a thousand solar-radii, while the surface temperature remains below 10^4 K. The energy generated by nuclear reactions is not dominant during this phase. We also study models with different gas mass accretion rates and the DM particle masses. All our models for different DM particle masses pass the dark star phase. The final mass of the dark stars is essentially unchanged for DM mass of m_DM <= 10 GeV. Gravitational collapse of the massive dark stars will leave massive black holes with mass as large as 1000 M_sun in the early universe.Comment: 21 pages, 14 figures, accepted to Ap