Yuexing Li

Formation of z~6 Quasars from Hierarchical Galaxy Mergers

The discovery of luminous quasars at redshift z ~ 6 indicates the presence of supermassive black holes (SMBHs) of mass ~109 M? when the universe was less than 1 billion years old. This finding presents several challenges for theoretical models because whether such massive objects can form so early in the ?CDM cosmology, the leading theory for cosmic structure formation, is an open question. Furthermore, whether the formation process requires exotic physics such as super-Eddington accretion remains undecided. Here we present the first multiscale simulations that, together with a self-regulated model for the SMBH growth, produce a luminous quasar at z ~ 6.5 in the ?CDM paradigm. We follow the hierarchical assembly history of the most massive halo in a ~3 Gpc3 volume and find that this halo of ~8 ? 1012 M? forming at z ~ 6.5 after several major mergers is able to reproduce a number of observed properties of SDSS J1148+5251, the most distant quasar detected at z = 6.42 (Fan et al. 2003). Moreover, the SMBHs grow through gas accretion below the Eddington limit in a self-regulated manner owing to feedback. We find that the progenitors experience vigorous star formation (up to 104 M? yr-1) preceding the major quasar phase such that the stellar mass of the quasar host reaches 1012 M? at z ~ 6.5, consistent with observations of significant metal enrichment in SDSS J1148+5251. The merger remnant thus obeys a similar MBH-Mbulge scaling relation observed locally as a consequence of coeval growth and evolution of the SMBH and its host galaxy. Our results provide a viable formation mechanism for z ~ 6 quasars in the standard ?CDM cosmology and demonstrate a common, merger-driven origin for the rarest quasars and the fundamental MBH-Mbulge correlation in a hierarchical universe.

The Formation of Stellar Clusters in Turbulent Molecular Clouds: Effects of the Equation of State

We study the effect of varying the equation of state on the formation of stellar clusters in turbulent molecular clouds using three-dimensional, smoothed particle hydrodynamics simulations. Our results show that the equation of state helps determine how strongly self-gravitating gas fragments. The degree of fragmentation decreases with increasing polytropic exponent γ in the range 0.2 1 probably results in the formation of isolated and massive stars. Fragmentation and collapse ceases entirely for γ > 1.4, as expected from analytic arguments. The mass spectrum of overdense gas clumps is roughly lognormal for non-self-gravitating turbulent gas, but changes to a power law under the action of gravity. The spectrum of collapsed cores, on the other hand, remains lognormal for γ ≤ 1 but flattens markedly for γ > 1. The density probability function approaches lognormal, with widths that decrease with increasing γ. Primordial gas may have effective γ > 1, in which case these results could help explain why models of the formation of the first stars tend to produce isolated, massive objects.We study the effect of varying the equation of state on the formation of stellar clusters in turbulent molecular clouds, using three-dimensional, smoothed particle hydrodynamics simulations. Our results show that the equation of state helps determine how strongly self-gravitating gas fragments. The degree of fragmentation decreases with increasing \new{polytropic exponent}

SEDS: The Spitzer Extended Deep Survey: survey design, photometry, and deep IRAC source counts

\gamma

STAR FORMATION IN ISOLATED DISK GALAXIES. I. MODELS AND CHARACTERISTICS OF NONLINEAR GRAVITATIONAL COLLAPSE

in the range

Star Formation in Isolated Disk Galaxies. II. Schmidt Laws and Efficiency of Gravitational Collapse

0.2 1

Control of star formation in galaxies by gravitational instability

probably results in the formation of isolated and massive stars. Fragmentation and collapse ceases entirely for

Modeling the Dust Properties of z ~ 6 Quasars with ART2—All-Wavelength Radiative Transfer with Adaptive Refinement Tree

\gamma > 1.4

The stellar mass spectrum from non-isothermal gravoturbulent fragmentation

as expected from analytic arguments. The mass spectrum of overdense gas clumps is roughly log-normal for {\em non}-self-gravitating turbulent gas, but changes to a power-law under the action of gravity. The spectrum of collapsed cores, on the other hand, remains log-normal for

Escape of Lyα and continuum photons from star-forming galaxies

\gamma\le 1

Formation of Globular Clusters in Galaxy Mergers

, but flattens markedly for