arXiv subjects

Liang Gao

Publications and source records attributed to Liang Gao.

177 records · Page 10Linked to original sources

arXiv2007

The Nature of CO Emission From z~6 Quasars

We investigate the nature of CO emission from z~6 quasars by combining non-LTE radiative transfer calculations with merger-driven models of z~6 quasar formation that arise naturally in LCDM cosmological simulations. We consider four model quasars formed in 10^12-10^13 M_sun halos from different merging histories. Our main results follow. Owing to massive starbursts and funneling of dense gas into the nuclear regions of merging galaxies, the CO is highly excited and the flux density peaks between J=5-8. The CO morphology of z~6 quasars often exhibits multiple emission peaks which arise from H2 concentrations which have not yet fully coalesced. Quasars at z~6 display a large range of sightline dependent line widths such that the lines are narrowest when the rotating H2 gas associated with the quasar is viewed face-on (when L_B is largest), and broadest when the gas is seen edge-on (when L_B is lowest). Thus for all models selection effects exist such that quasars selected for optical luminosity are preferentially face-on which may result in detected CO line widths narrower than the median. The sightline averaged line width is reflective of the circular velocity (V_c) of the host halo, and ranges from sigma~300-650 km/s. For optically selected QSOs, 10-25% (halo-mass dependant) of sightlines have narrow line widths compatible with the sole CO detection at z>6, J1148+5251. When accounting for both the temporal evolution of CO line widths, as well as the redshift evolution of halo V_c, these models self-consistently account for the CO line widths of both z~2 sub-mm galaxies and QSO's. Finally, the dynamical mass derived from the sightline averaged line widths provides a good estimate of the total mass, and allows for a stellar bulge and SMBH consistent with the local M_BH-M_bulge relation. [abridged]

astro-ph↗

arXiv2007

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 ~10^9 Msun when the Universe was less than one billion years old. This finding presents several challenges for theoretical models. Here, we present the first multi-scale simulations that, together with a self-regulated model for the SMBH growth, produce a luminous quasar at z ~ 6.5 in the LCDM paradigm. We follow the hierarchical assembly history of the most massive halo in a ~ 3 Gpc^3 volume, and find that this halo of ~ 8x 10^{12} Msun 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 significant star formation (up to 10^4 Msun/yr) preceding the major quasar phase such that the stellar mass of the quasar host reaches 10^{12} Msun at z ~ 6.5, consistent with observations of significant metal enrichment in SDSS J1148+5251. Our results provide a viable formation mechanism for z ~ 6 quasars in the standard LCDM cosmology, and demonstrate a common, merger-driven origin for the rarest quasars and the fundamental SMBH-host correlation in a hierarchical Universe.(Abridged)

astro-ph↗

arXiv2007

The statistics of LCDM Halo Concentrations

We use the Millennium Simulation (MS) to study the statistics of LCDM halo concentrations at z = 0. Our results confirm that the average halo concentration declines monotonically with mass; a power-law fits well the concentration-mass relation for over 3 decades in mass, up to the most massive objects to form in a LCDM universe (~ 10^15 h^-1 Msol). This is in clear disagreement with the predictions of the model proposed by Bullock et al. for these rare objects, and agrees better with the original predictions of Navarro, Frenk, & White. The large volume surveyed, together with the unprecedented numerical resolution of the MS, allow us to estimate with confidence the distribution of concentrations and, consequently, the abundance of systems with unusual properties. About one in a hundred cluster haloes (M200 >~ 3x10^14 h^-1 Msol) have concentrations exceeding c200 = 7.5, a result that may be used to interpret the likelihood of unusually strong massive gravitational lenses, such as Abell 1689, in the LCDM cosmogony. A similar fraction (1 in 100) of galaxy-sized haloes (M200 ~ 10^12 h^-1 Msol) have c200 < 4.5, an important constraint on models that attempt to reconcile the rotation curves of low surface-brightness galaxies by appealing to haloes of unexpectedly low concentration. We find that halo concentrations are independent of spin once haloes manifestly out of equilibrium are removed from the sample. Compared to their relaxed brethren, the concentrations of out-of-equilibrium haloes tend to be lower and to have more scatter, while their spins tend to be higher... (continue)

astro-ph↗

arXiv2007

Assembly bias in the clustering of dark matter haloes

We use a very large simulation of structure growth in a LCDM universe -- the Millennium Simulation -- to study assembly bias, the fact that the large-scale clustering of haloes of given mass varies significantly with their assembly history. We extend earlier work based on the same simulation by superposing results for redshifts from 0 to 3, by defining a less noisy estimator of clustering amplitude, and by considering halo concentration, substructure mass fraction and spin, as well as formation time, as additional parameters. These improvements lead to results with less noise than previous studies and covering a wider range of halo masses and structural properties. We find significant and significantly different assembly bias effects for all the halo properties we consider, although in all cases the dependences on halo mass and on redshift are adequately described as a dependence on equivalent peak height nu(M,z). The nu-dependences for different halo properties differ qualitatively and are not related as might naively be expected given the relations between formation time, concentration, substructure fraction and spin found for the halo population as a whole. These results suggest that it will be difficult to build models for the galaxy populations of dark haloes which can robustly relate the amplitude of large-scale galaxy clustering to that for mass clustering at better than the 10% level.

astro-ph↗

arXiv2006

Halo assembly bias and its effects on galaxy clustering

The clustering of dark halos depends not only on their mass but also on their assembly history, a dependence we term `assembly bias'. Using a galaxy formation model grafted onto the Millennium Simulation of the LCDM cosmogony, we study how assembly bias affects galaxy clustering. We compare the original simulation to `shuffled' versions where the galaxy populations are randomly swapped among halos of similar mass, thus isolating the effects of correlations between assembly history and environment at fixed mass. Such correlations are ignored in the halo occupation distribution models often used populate dark matter simulations with galaxies, but they are significant in our more realistic simulation. Assembly bias enhances 2-point correlations by 10% for galaxies with M_bJ-5logh brighter than -17, but suppresses them by a similar amount for galaxies brighter than -20. When such samples are split by colour, assembly bias is 5% stronger for red galaxies and 5% weaker for blue ones. Halo central galaxies are differently affected by assembly bias than are galaxies of all types. It almost doubles the correlation amplitude for faint red central galaxies. Shuffling galaxies among halos of fixed formation redshift or concentration in addition to fixed mass produces biases which are not much smaller than when mass alone is fixed. Assembly bias must reflect a correlation of environment with aspects of halo assembly which are not encoded in either of these parameters. It induces effects which could compromise precision measurements of cosmological parameters from large galaxy surveys.

astro-ph↗

arXiv2005

The First Generation of Star-Forming Haloes

We model gas cooling in high-resolution N-body simulations in order to investigate the formation of the first generation of stars. We follow a region of a LCDM universe especially selected to contain a rich cluster by the present day. The properties of the dark haloes that form in these sub-solar mass resolution simulations are presented in a companion paper by Gao et al. The first gas clouds able to cool by molecular hydrogen line emission collapse at extremely high redshift, z ~ 47, when the mass of the dark halo is 2.4 x 10^5 Msun/h. By z ~ 30, a substantial population of haloes are capable of undergoing molecular hydrogen cooling although their ability to form stars is dependent on the efficiency of feedback processes such as dissociating Lyman-Werner radiation. The mass of the main halo grows extremely rapidly and, by z ~ 36, its virial temperature has reached 10^4K, at which point gas cooling becomes dominated by more effective atomic line processes. By z ~ 30, a small ``group'' of such potential galaxies will have formed unless prevented from doing so by feedback processes. By this redshift, massive (>~ 100 Msun) population III stars are able to ionise gas well beyond their own host halo and neighbouring HII regions can percolate to form an ionized superbubble. Such patches would be too widely separated to contribute significantly to reionisation at this time. The large number density of early cooling haloes in the pre-reionised universe raises the exciting prospect that this ultra-early generation of stars may be observable as gamma-ray bursts or supernovae.

astro-ph↗

arXiv2005

Early structure in LCDM

We use a novel technique to simulate the growth of the most massive progenitor of a protocluster region from redshift z~80, when its mass was about 10 Msun until the present day. Our nested sequence of N-body resimulations allows us to study in detail the structure both of the dark matter object itself and of its environment. Our effective resolution is optimal at redshifts of 49, 29, 12, 5 and 0 when the dominant object has mass 1.2e5, 5e7, 2e10, 3e12 and 8e14 Msun/hrespectively, and contains ~1e6 simulation particles within its virial radius. Extended Press-Schechter theory correctly predicts both this rapid growth and the substantial overabundance of massive haloes we find at early times in regions surrounding the dominant object. Although the large-scale structure in these regions differs dramatically from a scaled version of its present-day counterpart, the internal structure of the dominant object is remarkably similar. Molecular hydrogen cooling could start as early as z~49 in this object, while cooling by atomic hydrogen becomes effective at z~39. If the first stars formed in haloes with virial temperature ~2000, their comoving abundance by z=49 would be similar to that of dwarf galaxies today, while their comoving correlation length would be ~2.5Mpc/h.

astro-ph↗

arXiv2005

The age dependence of halo clustering

We use a very large simulation of the concordance LCDM cosmogony to study the clustering of dark matter haloes. For haloes less massive than about 1e13Msun/h, the amplitude of the two-point correlation function on large scales depends strongly on halo formation time. Haloes that assembled at high redshift are substantially more clustered than those that assembled more recently. The effect is a smooth function of halo formation time and its amplitude increases with decreasing halo mass. At 1e11 Msun/h the ``oldest'' 10% of haloes are more than 5 times more strongly correlated than the ``youngest'' 10%. This unexpected result is incompatible with the standard excursion set theory for structure growth, and it contradicts a fundamental assumption of the halo occupation distribution models often used to study galaxy clustering, namely that the galaxy content of a halo of given mass is statistically independent of its larger scale environment.

astro-ph↗

arXiv2005

Simulating the joint evolution of quasars, galaxies and their large-scale distribution

The cold dark matter model has become the leading theoretical paradigm for the formation of structure in the Universe. Together with the theory of cosmic inflation, this model makes a clear prediction for the initial conditions for structure formation and predicts that structures grow hierarchically through gravitational instability. Testing this model requires that the precise measurements delivered by galaxy surveys can be compared to robust and equally precise theoretical calculations. Here we present a novel framework for the quantitative physical interpretation of such surveys. This combines the largest simulation of the growth of dark matter structure ever carried out with new techniques for following the formation and evolution of the visible components. We show that baryon-induced features in the initial conditions of the Universe are reflected in distorted form in the low-redshift galaxy distribution, an effect that can be used to constrain the nature of dark energy with next generation surveys.

astro-ph↗

arXiv2005

Voids in a $Λ$CDM Universe

We study the formation and evolution of voids in the dark matter distribution using various simulations of the popular $Λ$ Cold Dark Matter cosmogony. We identify voids by requiring them to be regions of space with a mean overdensity of -0.8 or less. Each of the simulations contains thousands of voids. The distribution of void sizes in the different simulations shows good agreement. Voids very clearly correspond to minima in the smoothed initial density field. We find a universal void mass profile of the form $ρ(<r)/ρ(r_{\rm eff}) \propto \exp[(r/r_{\rm eff})^α]$ where $r_{\rm eff}$ is the effective radius of a void and $α\sim 2$. The mass function of haloes in voids is steeper than that of haloes that populate denser regions. The abundances of void haloes seem to evolve somewhat more strongly between redshifts $\sim 1$ and 0 than the global abundances of haloes. (abridged)

astro-ph↗

arXiv2004

Weighted networks of scientific communication: the measurement and geometrical role of weight

In order to take the weight of connection into consideration and to find a natural measurement of weight, we have collected papers in Econophysics and constructed a network of scientific communication to integrate idea transportation among econophysicists by collaboration, citation and personal discussion. Some basic statistics such as weight per degree are discussed in \cite{fan}. In this paper, by including the papers published recently, further statistical results for the network are reported. Clustering coefficient of weighted network is introduced and empirically studied in this network. We also compare the typical statistics on this network under different weight measurements, including random and inverse weight. The conclusion from weight-randomized network is helpful to the investigation of the geometrical role of weight.

cond-mat.other↗

arXiv2004

The subhalo populations of LCDM dark haloes

We use a large cosmological simulation and a variety of high resolution resimulations of individual cluster and galaxy haloes to study the systematics of subhalo populations over ranges of 1000 in halo mass and 1000 in the ratio of subhalo to parent halo mass. The subhalo populations of different haloes are not scaled copies of each other, but vary systematically with halo properties. On average, the amount of substructure increases with halo mass. At fixed mass, it decreases with halo concentration and with halo formation redshift. These trends are comparable in size to the scatter in subhalo abundance between similar haloes. Averaged over all haloes of given mass, the abundance of low mass subhaloes per unit parent halo mass is independendent of parent mass.It is very similar to the abundance per unit mass of low mass haloes in the universe as a whole, once differing boundary definitions for subhaloes and haloes are accounted for. The radial distribution of subhaloes within their parent haloes is substantially less centrally concentrated than that of the dark matter. It varies at most weakly with the mass(or concentration) of the parent halo and not at all with subhalo mass. It does depend on the criteria used to define the subhalo population considered. About 90% of present-day subhaloes were accreted after z=1 and about 70% after z=0.5. Only about 8% the total mass of all haloes accreted at z=1 survives as bound subhaloes at z=0. For haloes accreted at z=2, the survival mass fraction is just 2%. Subhaloes seen near the centre of their parent typically were accreted earlier and retain less of their original mass than those seen near the edge. These strong systematics mean that comparison with galaxies in real clusters is only possible if the formation of the luminous component is modelled appropriately.

astro-ph↗

arXiv2004

Early Formation and Late Merging of the Giant Galaxies

The most luminous galaxies in the present Universe are found at the centers of the most massive dark matter haloes, rich galaxy clusters. In the LCDM cosmology, such massive halo cores are present at redshift z=6 with a comoving number density (as a function of mass interior to ~10 kpc) that is comparable to today's value. The identity of the matter in these central regions is, however, predicted to change as major mergers bring together stars and dark matter from initially well separated sub-units. We use N-body simulations to investigate how these mergers push pre-existing matter outwards in the dominant galaxy while preserving the inner density profile of collisionless matter. It appears that the central regions of large galaxies end up dominated by stars formed in a number of dense cores, well before the last major mergers. The density profile of collisionless matter (stars and dark matter combined) in these central regions appears to be stable and to have attractor-like behavior under merging. This suggests that the baryon loading associated with dissipative contraction and star formation may be erased as subsequent mergers drive the mass distribution back to a universal profile. Such suppression of the effects of baryon loading, along with the early assembly of mass concentrations, may help resolve some apparent challenges to the CDM model for structure formation.

astro-ph↗

arXiv2004

Galaxies and subhaloes in LCDM galaxy clusters

We combine 10 high resolution resimulations of cluster--sized dark haloes with semi--analytic galaxy formation modelling in order to compare the number density and velocity dispersion profiles of cluster galaxies with those of dark matter substructures (subhaloes). While the radial distribution of galaxies follows closely that of the dark matter, the distribution of dark matter subhaloes is much less centrally concentrated. The velocity dispersion profiles of galaxies are also very similar to those of the dark matter, while those for subhaloes are biased high, particularly in the inner regions of clusters. We explain how these differences, already clearly visible in earlier work, are a consequence of the formation of galaxies at the centres of dark matter haloes. Galaxies and subhaloes represent different populations and are not directly comparable. Evolution produces a complex and strongly position--dependent relation between galaxies and the subhaloes in which they reside. This relation can be properly modelled only by appropriate physical representation of the galaxy formation process.

astro-ph↗

arXiv2004

Network of Econophysicists: a weighted network to investigate the development of Econophysics

The development of Econophysics is studied from the perspective of scientific communication networks. Papers in Econophysics published from 1992 to 2003 are collected. Then a weighted and directed network of scientific communication, including collaboration, citation and personal discussion, is constructed. Its static geometrical properties, including degree distribution, weight distribution, weight per degree, and betweenness centrality, give a nice overall description of the research works. The way we introduced here to measure the weight of connections can be used as a general one to construct weighted network.

cond-mat.soft↗