arXiv subjects

Liang Gao

Publications and source records attributed to Liang Gao.

At least 145 records · Page 8Linked to original sources

arXiv2016

Alignments between galaxies, satellite systems and haloes

The spatial distribution of the satellite populations of the Milky Way and Andromeda are puzzling in that they are nearly perpendicular to the disks of their central galaxies. To understand the origin of such configurations we study the alignment of the central galaxy, satellite system and dark matter halo in the largest of the "Evolution and Assembly of GaLaxies and their Environments" (EAGLE) simulation. We find that centrals and their satellite systems tend to be well aligned with their haloes, with a median misalignment angle of $33^{\circ}$ in both cases. While the centrals are better aligned with the inner $10$ kpc halo, the satellite systems are better aligned with the entire halo indicating that satellites preferentially trace the outer halo. The central - satellite alignment is weak (median misalignment angle of $52^{\circ}$) and we find that around $20\%$ of systems have a misalignment angle larger than $78^{\circ}$, which is the value for the Milky Way. The central - satellite alignment is a consequence of the tendency of both components to align with the dark matter halo. As a consequence, when the central is parallel to the satellite system, it also tends to be parallel to the halo. In contrast, if the central is perpendicular to the satellite system, as in the case of the Milky Way and Andromeda, then the central - halo alignment is much weaker. Dispersion-dominated (spheroidal) centrals have a stronger alignment with both their halo and their satellites than rotation-dominated (disk) centrals. We also found that the halo, the central galaxy and the satellite system tend to be aligned with the surrounding large-scale distribution of matter, with the halo being the better aligned of the three.

astro-ph.GA↗

arXiv2016

Constraints on the identity of the dark matter from strong gravitational lenses

The cold dark matter (CDM) cosmological model unambigously predicts that a large number of haloes should survive as subhaloes when they are accreted into a larger halo. The CDM model would be ruled out if such substructures were shown not to exist. By contrast, if the dark matter consists of warm particles (WDM), then below a threshold mass that depends on the particle mass far fewer substructures would be present. Finding subhaloes below a certain mass would then rule out warm particle masses below some value. Strong gravitational lensing provides a clean method to measure the subhalo mass function through distortions in the structure of Einstein rings and giant arcs.Using mock lensing observations constructed from high-resolution N-body simulations, we show that measurements of approximately 100 strong lens systems with a detection limit of $10^7 h^{-1} M_{\odot}$ would clearly distinguish CDM from WDM in the case where this consists of 7 keV sterile neutrinos such as those that might be responsible for the 3.5 keV X-ray emission line recently detected in galaxies and clusters.

astro-ph.CO↗

arXiv2015

Distinguishing general relativity and $f(R)$ gravity with the gravitational lensing Minkowski functionals

We explore the Minkowski functionals of weak lensing convergence map to distinguish between $f(R)$ gravity and the general relativity (GR). The mock weak lensing convergence maps are constructed with a set of high-resolution simulations assuming different gravity models. It is shown that the lensing MFs of $f(R)$ gravity can be considerably different from that of GR because of the environmentally dependent enhancement of structure formation. We also investigate the effect of lensing noise on our results, and find that it is likely to distinguish F5, F6 and GR gravity models with a galaxy survey of $\sim3000$ degree$^2$ and with a background source number density of $n_g=30~{\rm arcmin}^{-2}$, comparable to an upcoming survey dark energy survey (DES). We also find that the $f(R)$ signal can be partially degenerate with the effect of changing cosmology, but combined use of other observations, such as the cosmic microwave background (CMB) data, can help break this degeneracy.

astro-ph.CO↗

arXiv2015

Assembly history of subhalo populations in galactic and cluster sized dark haloes

We make use of two suits of ultra high resolution N-body simulations of individual dark matter haloes from the Phoenix and the Aquarius Projects to investigate systematics of assembly history of subhaloes in dark matter haloes differing by a factor of $1000$ in the halo mass. We have found that real progenitors which built up present day subhalo population are relatively more abundant for high mass haloes, in contrast to previous studies claiming a universal form independent of the host halo mass. That is mainly because of repeated counting of the 're-accreted' (progenitors passed through and were later re-accreted to the host more than once) and inclusion of the 'ejected' progenitor population(progenitors were accreted to the host in the past but no longer members at present day) in previous studies. The typical accretion time for all progenitors vary strongly with the host halo mass, which is typical about $z \sim 5$ for the galactic Aquarius and about $z \sim 3$ for the cluster sized Phoenix haloes. Once these progenitors start to orbit their parent haloes, they rapidly lose their original mass but not their identifiers, more than $55$ ($50$) percent of them survive to present day for the Phoenix(Aquarius) haloes. At given redshift, survival fraction of the accreted subhalo is independent of the parent halo mass, whilst the mass-loss of the subhalo is more efficient in high mass haloes. These systematics results in similarity and difference in the subhalo population in dark matter haloes of different masses at present day.

astro-ph.CO↗

arXiv2015

Cluster gas fraction as a test of gravity

We propose a new cosmological test of gravity, by using the observed mass fraction of X-ray emitting gas in massive galaxy clusters. The cluster gas fraction, believed to be a fair sample of the average baryon fraction in the Universe, is a well-understood observable, which has previously mainly been used to constrain background cosmology. In some modified gravity models, such as $f(R)$ gravity, gas temperature in a massive cluster is determined by the effective mass of that cluster, which can be larger than its true mass. On the other hand, X-ray luminosity is determined by the true gas density, which in both modified gravity and $Λ$CDM models depends mainly on $Ω_{\rm b}/Ω_{\rm m}$ and hence the true total cluster mass. As a result, the standard practice of combining gas temperatures and X-ray surface brightnesses of clusters to infer their gas fractions can, in modified gravity models, lead to a larger - in $f(R)$ gravity this can be $1/3$ larger - value of $Ω_{\rm b}/Ω_{\rm m}$ than that inferred from other observations such as the CMB. A quick calculation shows that the Hu-Sawicki $n=1$ $f(R)$ model with $|\bar{f}_{R0}|=3\sim5\times10^{-5}$ is in tension with the gas fraction data of the 42 clusters analysed by Allen et al. (2008). We also discuss the implications for other modified gravity models.

astro-ph.CO↗

arXiv2015

Exploring the liminality: properties of haloes and subhaloes in borderline $f(R)$ gravity

We investigate the properties of dark matter haloes and subhaloes in an $f(R)$ gravity model with $|f_{R0}|=10^{-6}$, using a very high-resolution N-body simulation. The model is a borderline between being cosmologically interesting and yet still consistent with current data. We find that the halo mass function in this model has a maximum 20% enhancement compared with the $Λ$CDM predictions between $z=1$ and $z=0$. Because of the chameleon mechanism which screens the deviation from standard gravity in dense environments, haloes more massive than $10^{13}h^{-1}M_\odot$ in this $f(R)$ model have very similar properties to haloes of similar mass in $Λ$CDM, while less massive haloes, such as that of the Milky Way, can have steeper inner density profiles and higher velocity dispersions due to their weaker screening. The halo concentration is remarkably enhanced for low-mass haloes in this model due to a deepening of the total gravitational potential. Contrary to the naive expectation, the halo formation time $z_f$ is later for low-mass haloes in this model, a consequence of these haloes growing faster than their counterparts in $Λ$CDM at late times and the definition of $z_f$. Subhaloes, especially those less massive than $10^{11}h^{-1}M_\odot$, are substantially more abundant in this $f(R)$ model for host haloes less massive than $10^{13}h^{-1}M_\odot$. We discuss the implications of these results for the Milky Way satellite abundance problem. Although the overall halo and subhalo properties in this borderline $f(R)$ model are close to their $Λ$CDM predictions, our results suggest that studies of the Local Group and astrophysical systems, aided by high-resolution simulations, can be valuable for further tests of it.

astro-ph.CO↗

arXiv2015

Surface photometry of BCGs and intracluster stars in Lambda-CDM

We simulate the phase-space distribution of stellar mass in 9 massive Lambda-CDM galaxy clusters by applying the semi-analytic particle tagging method of Cooper et al. to the Phoenix suite of high-resolution N-body simulations (M200 = 7.5 to 33 x 10^14 Msol). The resulting surface brightness (SB) profiles of brightest cluster galaxies (BCGs) match well to observations. On average, stars formed in galaxies accreted by the BCG account for ~90 per cent of its total mass (the remainder is formed in situ). In circular BCG-centred apertures, the superposition of multiple debris clouds (each ~10 per cent of the total BCG mass) from different progenitors can result in an extensive outer diffuse component, qualitatively similar to a 'cD envelope'. These clouds typically originate from tidal stripping at z < 1 and comprise both streams and the extended envelopes of other massive galaxies in the cluster. Stars at very low SB contribute a significant fraction of the total cluster stellar mass budget: in the central 1 Mpc^2 of a z ~ 0.15 cluster imaged at SDSS-like resolution, our fiducial model predicts 80-95 per cent of stellar mass below a SB of mu_V = 26.5 mag arcsec^2 is associated with accreted stars in the envelope of the BCG. The ratio of BCG stellar mass (including this diffuse component) to total cluster stellar mass is ~30 per cent.

astro-ph.GA↗

arXiv2015

Traffic incident analysis on urban arterials using ESE: A method for moderate length of time window

Moderate length of time window can get the best accurate result in detecting the key incident time using extended spectral envelope. This paper presents a method to calculate the moderate length of time window. Two factors are mainly considered: (1) The significant vertical lines consist of negative elements of eigenvectors; (2) the least amount of interruption. The elements of eigenvectors are transformed into binary variable to eliminate the interruption of positive elements. Sine transform is introduced to highlight the significant vertical lines of negative elements. A novel Quality Index (QI) is proposed to measure the effect of different lengths of time window. Empirical studies on four real traffic incidents in Beijing verify the validity of this method.

physics.soc-ph↗

arXiv2015

Traffic Incident Analysis on Urban Arterials Using Extended Spectral Envelope Method

A traffic incident analysis method based on extended spectral envelope (ESE) method is presented to detect the key incident time. Sensitivity analysis of parameters (the length of time window, the length of sliding window and the study period) are discussed on four real traffic incidents in Beijing. The results show that: (1) Moderate length of time window got the best accurate in detection. (2) The shorter the sliding window is, the more accurate the key incident time are detected. (3) If the study period is too short, the end time of an incident cannot be detected. Empirical studies show that the proposed method can effectively discover the key incident time, which can provide a theoretic basis for traffic incident management.

physics.soc-ph↗

arXiv2015

Star forming filaments in warm dark matter models

We performed a hydrodynamical cosmological simulation of the formation of a Milky Way-like galaxy in a warm dark matter (WDM) cosmology. Smooth and dense filaments, several co-moving mega parsec long, form generically above z 2 in this model. Atomic line cooling allows gas in the centres of these filaments to cool to the base of the cooling function, resulting in a very striking pattern of extended Lyman-limit systems (LLSs). Observations of the correlation function of LLSs might hence provide useful limits on the nature of the dark matter. We argue that the self-shielding of filaments may lead to a thermal instability resulting in star formation. We implement a sub-grid model for this, and find that filaments rather than haloes dominate star formation until z 6. Reionisation decreases the gas density in filaments, and the more usual star formation in haloes dominates below z 6, although star formation in filaments continues until z=2. Fifteen per cent of the stars of the z=0 galaxy formed in filaments. At higher redshift, these stars give galaxies a stringy appearance, which, if observed, might be a strong indication that the dark matter is warm.

astro-ph.CO↗

arXiv2015

How well can cold-dark-matter substructures account for the observed radio flux-ratio anomalies?

Discrepancies between the observed and model-predicted radio flux ratios are seen in a number of quadruply-lensed quasars. The most favored interpretation of these anomalies is that CDM substructures present in lensing galaxies perturb the lens potentials and alter image magnifications and thus flux ratios. So far no consensus has emerged regarding whether or not the predicted CDM substructure abundance fully accounts for the lensing flux anomaly observations. Accurate modeling relies on a realistic lens sample in terms of both the lens environment and internal structures and substructures. In this paper we construct samples of generalised and specific lens potentials, to which we add (rescaled) subhalo populations from the galaxy-scale Aquarius and the cluster-scale Phoenix simulation suites. We further investigate the lensing effects from subhalos of masses several orders of magnitude below the simulation resolution limit. The resulting flux ratio distributions are compared to the currently best available sample of radio lenses. The observed anomalies in B0128+437, B0712+472 and B1555+375 are more likely to be caused by propagation effects or oversimplified lens modeling, signs of which are already seen in the data. Among the quadruple systems that have closely located image triplets/pairs, the anomalous flux ratios of MG0414+0534 can be reproduced by adding CDM subhalos to its macroscopic lens potential, with a probability of 5%-20%; for B0712+472, B1422+231, B1555+375 and B2045+265, these probabilities are only of a few percent. We hence find that CDM substructures are unlikely to be the whole reason for radio flux anomalies. We discuss other possible effects that might also be at work.

astro-ph.CO↗

arXiv2014

An exponent tunable network model for reproducing density driven superlinear relation

Previous works have shown the universality of allometric scalings under density and total value at city level, but our understanding about the size effects of regions on them is still poor. Here, we revisit the scaling relations between gross domestic production (GDP) and population (POP) under total and density value. We first reveal that the superlinear scaling is a general feature under density value crossing different regions. The scaling exponent $β$ under density value falls into the range $(1.0, 2.0]$, which unexpectedly goes beyond the range observed by Pan et al. (Nat. Commun. vol. 4, p. 1961 (2013)). To deal with the wider range, we propose a network model based on 2D lattice space with the spatial correlation factor $α$ as parameter. Numerical experiments prove that the generated scaling exponent $β$ in our model is fully tunable by the spatial correlation factor $α$. We conjecture that our model provides a general platform for extensive urban and regional studies.

physics.soc-ph↗

arXiv2014

How well can cold-dark-matter substructures account for the observed lensing flux-ratio anomalies?

Lensing flux-ratio anomalies are most likely caused by gravitational lensing by small-scale dark matter structures. These anomalies offer the prospect of testing a fundamental prediction of the cold dark matter (CDM) cosmological model: the existence of numerous substructures that are too small to host visible galaxies. In two previous studies we found that the number of subhalos in the six high-resolution simulations of CDM galactic halos of the Aquarius project is not sufficient to account for the observed frequency of flux ratio anomalies seen in selected quasars from the CLASS survey. These studies were limited by the small number of halos used, their narrow range of masses (1-2E12 solar masses) and the small range of lens ellipticities considered. We address these shortcomings by investigating the lensing properties of a large sample of halos with a wide range of masses in two sets of high resolution simulations of cosmological volumes and comparing them to a currently best available sample of radio quasars. We find that, as expected, substructures do not change the flux-ratio probability distribution of image pairs and triples with large separations, but they have a significant effect on the distribution at small separations. For such systems, CDM substructures can account for a substantial fraction of the observed flux-ratio anomalies. For large close-pair separation systems, the discrepancies existing between the observed flux ratios and predictions from smooth halo models are attributed to simplifications inherent in these models which do not take account of fine details in the lens mass distributions.

astro-ph.CO↗

arXiv2014

The Size Evolution of Elliptical Galaxies

Recent work has suggested that the amplitude of the size mass relation of massive early type galaxies evolves with redshift. Here we use a semi-analytical galaxy formation model to study the size evolution of massive early type galaxies. We find this model is able to reproduce the amplitude of present day amplitude and slope of the relation between size and stellar mass for these galaxies, as well as its evolution. The amplitude of this relation reflects the typical compactness of dark halos at the time when most of the stars are formed. This link between size and star formation epoch is propagated in galaxy mergers. Mergers of high or moderate mass ratio (less than 1:3) become increasingly important with increasing present day stellar mass for galaxies more massive than $10^{11.4}M_{\odot}$. At lower masses, low mass ratio mergers play a more important role. In situ star formation contribute more to the size growth than it does to stellar mass growth. We also find that, for ETGs identified at $z=2$, minor mergers dominate subsequent growth both for stellar mass and in size, consistent with earlier theoretical results.

astro-ph.GA↗

arXiv2014

The properties of warm dark matter haloes

Well-motivated elementary particle candidates for the dark matter, such as the sterile neutrino, behave as warm dark matter (WDM).For particle masses of order a keV, free streaming produces a cutoff in the linear fluctuation power spectrum at a scale corresponding to dwarf galaxies. We investigate the abundance and structure of WDM haloes and subhaloes on these scales using high resolution cosmological N-body simulations of galactic haloes of mass similar to the Milky Way's. On scales larger than the free-streaming cutoff, the initial conditions have the same power spectrum and phases as one of the cold dark matter (CDM) haloes previously simulated by Springel et al as part of the Virgo consortium Aquarius project. We have simulated four haloes with WDM particle masses in the range 1.4-2.3keV and, for one case, we have carried out further simulations at varying resolution. N-body simulations in which the power spectrum cutoff is resolved are known to undergo artificial fragmentation in filaments producing spurious clumps which, for small masses (<10^7Msun in our case) outnumber genuine haloes. We have developed a robust algorithm to identify these spurious objects and remove them from our halo catalogues. We find that the WDM subhalo mass function is suppressed by well over an order magnitude relative to the CDM case for masses <10^9Msun. Requiring that there should be at least as many subhaloes as there are observed satellites in the Milky Way leads to a conservative lower limit to the (thermal equivalent) WDM particle mass of ~1.5\rmn{keV}. WDM haloes and subhaloes have cuspy density distributions that are well described by NFW or Einasto profiles. Their central densities are lower for lower WDM particle masses and none of the models we have considered suffer from the "too big to fail" problem recently highlighted by Boylan-Kolchin et al.

astro-ph.CO↗

arXiv2014

The Local Void: for or against LCDM?

The emptiness of the Local Void has been put forward as a serious challenge to the current standard paradigm of structure formation in LCDM. We use a high resolution cosmological N-body simulation, the Millennium-II run, combined with a sophisticated semi-analytic galaxy formation model, to explore statistically whether the local void is allowed within our current knowledge of galaxy formation in LCDM. We find that about 15 percent of the Local Group analogue systems (11 of 77) in our simulation are associated with nearby low density regions having size and 'emptiness' similar to those of the observed Local Void. This suggests that, rather than a crisis of the LCDM, the emptiness of the Local Void is indeed a success of the standard LCDM theory. The paucity of faint galaxies in such voids results from a combination of two factors: a lower amplitude of the halo mass function in the voids than in the field, and a lower galaxy formation efficiency in void haloes due to halo assembly bias effects. While the former is the dominated factor, the later also plays a sizable role. The halo assembly bias effect results in a stellar mass fraction 25 percent lower for void galaxies when compared to field galaxies with the same halo mass.

astro-ph.CO↗

arXiv2014

Characterizing and Modeling the Dynamics of Activity and Popularity

Social media, regarded as two-layer networks consisting of users and items, turn out to be the most important channels for access to massive information in the era of Web 2.0. The dynamics of human activity and item popularity is a crucial issue in social media networks. In this paper, by analyzing the growth of user activity and item popularity in four empirical social media networks, i.e., Amazon, Flickr, Delicious and Wikipedia, it is found that cross links between users and items are more likely to be created by active users and to be acquired by popular items, where user activity and item popularity are measured by the number of cross links associated with users and items. This indicates that users generally trace popular items, overall. However, it is found that the inactive users more severely trace popular items than the active users. Inspired by empirical analysis, we propose an evolving model for such networks, in which the evolution is driven only by two-step random walk. Numerical experiments verified that the model can qualitatively reproduce the distributions of user activity and item popularity observed in empirical networks. These results might shed light on the understandings of micro dynamics of activity and popularity in social media networks.

physics.soc-ph↗

arXiv2014

Quantifying Information Flow During Emergencies

Recent advances on human dynamics have focused on the normal patterns of human activities, with the quantitative understanding of human behavior under extreme events remaining a crucial missing chapter. This has a wide array of potential applications, ranging from emergency response and detection to traffic control and management. Previous studies have shown that human communications are both temporally and spatially localized following the onset of emergencies, indicating that social propagation is a primary means to propagate situational awareness. We study real anomalous events using country-wide mobile phone data, finding that information flow during emergencies is dominated by repeated communications. We further demonstrate that the observed communication patterns cannot be explained by inherent reciprocity in social networks, and are universal across different demographics.

physics.soc-ph↗