arXiv subjects

Xufen Wu

Publications and source records attributed to Xufen Wu.

32 records · Page 2Linked to original sources

arXiv2016

SDSS J163459.82+204936.0: A Ringed Infrared-Luminous Quasar with Outflows in both Absorption and Emission Lines

SDSS J1634+2049 is a local (z = 0.1293) infrared-luminous quasar with LIR= 10^11.91 Lsun. We present a detailed multiwavelength study of both the host galaxy and the nucleus. The host galaxy demonstrates violent, obscured star formation activities with SFR ~ 140 Msun yr^-1, estimated from either the PAH emission or IR luminosity. The optical to NIR spectra exhibit a blueshifted narrow cuspy component in Hb, HeI5876,10830 and other emission lines consistently with an offset velocity of ~900 km/s, as well as additional blueshifting phenomena in high-ionization lines , while there exist blueshifted broad absorption lines (BALs) in NaID and HeI*3889,10830, indicative of the AGN outflows producing BALs and emission lines. Constrained mutually by the several BALs with CLOUDY, the physical properties of the absorption-line outflow are derived as follows: 10^4 < n_H <= 10^5 cm^-3, 10^-1.3 <= U <= 10^-0.7 and 10^22.5<= N_H <= 10^22.9 cm^-2 , similar to those derived for the emission-line outflows. The similarity suggests a common origin. Taking advantages of both the absorption lines and outflowing emission lines, we find that the outflow gas is located at a distance of 48 - 65 pc from the nucleus, and that the kinetic luminosity of the outflow is 10^44-10^46 erg s^-1. J1634+2049 has a off-centered galactic ring on the scale of ~ 30 kpc that is proved to be formed by a recent head-on collision by a nearby galaxy. Thus this quasar is a valuable object in the transitional phase emerging out of dust enshrouding as depicted by the co-evolution scenario.

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arXiv2015

An Excessive Core Collapse in Nbody Cosmological simulations

The particle mass used in cosmology N-body simulations is close to $10^{8}M_{\odot}$, which is about $10^{65}$ times larger than the GeV scale expected in particle physics. However, self-gravity interacting particle systems made up of different particles mass have different statistical and dynamical properties. Here we demonstrate that, due to this particle mass difference, the nowaday cosmology N-body simulations can have introduced an excessive core collapse process, especially for the low mass halos at high redshift. Such dynamical effect introduces an excessive cuspy center for these small halos, and it implies a possible connection to the so called "small scale crisis" for CDM models. Our results show that there exist a physical limit in cosmological simulations, by using about $10^3$ particles to describe smallest halos, and we provide a simple suggestion based on it to relieve those effects from the bias.

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arXiv2014

Galactic rotation curves, the baryon-to-dark-halo-mass relation and space-time scale invariance

Low-acceleration space-time scale invariant dynamics (SID, Milgrom 2009a) predicts two fundamental correlations known from observational galactic dynamics: the baryonic Tully-Fisher relation (BTFR) and a correlation between the observed mass discrepancy and acceleration (MDA) in the low acceleration regime for disc galaxies. SID corresponds to the deep MOdified Newtonian Dynamics (MOND) limit. The MDA data emerging in cold/warm dark matter (C/WDM) cosmological simulations disagree significantly with the tight MDA correlation of the observed galaxies. Therefore, the most modern simulated disc galaxies, which are delicately selected to have a quiet merging history in a standard dark-matter-cosmological model, still do not represent the correct rotation curves. Also, the observed tight correlation contradicts the postulated stochastic formation of galaxies in low-mass DM halos. Moreover, we find that SID predicts a baryonic to apparent virial halo (dark matter) mass relation which agrees well with the correlation deduced observationally assuming Newtonian dynamics to be valid, while the baryonic to halo mass relation predicted from CDM models does not. The distribution of the observed ratios of dark-matter halo masses to baryonic masses may be empirical evidence for the external field effect, which is predicted in SID as a consequence of the forces acting between two galaxies depending on the position and mass of a third galaxy. Applying the external field effect, we predict the masses of galaxies in the proximity of the dwarf galaxies in the Miller et al. sample. Classical non-relativistic gravitational dynamics is thus best described as being Milgromian, rather than Newtonian.

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arXiv2013

The mass and angular momentum distribution of simulated massive early-type galaxies to large radii

We study the dark and luminous mass distributions, circular velocity curves (CVC), line-of-sight kinematics, and angular momenta for a sample of 42 cosmological zoom simulations of massive galaxies. Using a temporal smoothing technique, we are able to reach large radii. We find that: (i)The dark matter halo density profiles outside a few kpc follow simple power-law models, with flat dark matter CVCs for lower-mass systems, and rising CVCs for high-mass haloes. The projected stellar density distributions at large radii can be fitted by Sersic functions with n>10, larger than for typical ETGs. (ii)The massive systems have nearly flat total CVCs at large radii, while the less massive systems have mildly decreasing CVCs. The slope of the CVC at large radii correlates with v_circ itself. (iii)The dark matter fractions within Re are in the range 15-30% and increase to 40-65% at 5Re. Larger and more massive galaxies have higher dark matter fractions. (iv)The short axes of simulated galaxies and their host dark matter haloes are well aligned and their short-to-long axis ratios are correlated. (v)The stellar vrms(R) profiles are slowly declining, in agreement with planetary nebulae observations in the outer haloes of most ETGs. (vi)The line-of-sight velocity fields v show that rotation properties at small and large radii are correlated. Most radial profiles for the cumulative specific angular momentum parameter lambda(R) are nearly flat or slightly rising, with values in [0.06,0.75] from 2Re to 5Re. (vii)Stellar mass, ellipticity at 5Re, and lambda(5Re) are correlated: the more massive systems have less angular momentum and are rounder, as for observed ETGs. (viii)More massive galaxies with a large fraction of accreted stars have radially anisotropic velocity distributions outside Re. Tangential anisotropy is seen only for galaxies with high fraction of in-situ stars. (Full abstract in PDF)

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arXiv2013

The specific frequency and the globular cluster formation efficiency in Milgromian dynamics

Previous studies of globular cluster (GC) systems show that there appears to be a universal specific GC formation efficiency $η$ which relates the total mass of GCs to the virial mass of host dark matter halos, $M_{vir}$ (Georgiev et al 2010, Spitler & Forbes2009). In this paper, the specific frequency, $S_N$, and specific GC formation efficiency, $η$, are derived as functions of $M_{vir}$ in Milgromian dynamics, i.e., in modified Newtonian dynamics (MOND). In Milgromian dynamics, for the galaxies with GCs, the mass of the GC system, $M_{GC}$, is a two-component function of $M_{vir}$ instead of a simple linear relation. An observer in a Milgromian universe, who interprets this universe as being Newtonian/Einsteinian, will incorrectly infer a universal constant fraction between the mass of the GC system and a (false) dark matter halo of the baryonic galaxy. In contrast to a universal constant of $η$, in a Milgromian universe, for galaxies with $M_{vir} <= 10^{12}\msun$, $η$ decreases with the increase of $M_{vir}$, while for massive galaxies with $M_{vir}>10^{12}\msun$, $η$ increases with the increase of $M_{vir}$.

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arXiv2013

The dynamical phase transitions of stellar systems and the corresponding kinetimacs

External fields in Migromian dynamics (MD or MOND, Milgrom 1983) break the Strong Equivalence Principle (SEP) and change the dynamics of self-bound stellar systems moving in space-varying background gravitational fields. We study two kinds of re-virialisation of the stellar systems: the violent phase transition and the adiabatic phase transition for systems moving on radial orbits, where the external field evolves from strong to weak and whose corresponding dynamics changes from Newtonian to Milgromian. We find that the time scale for the phase tranformation from Newtonian to Milgromian gravity lies only within one to a few crossing times for low density globular clusters with masses ranging from $10^4\msun$ to $10^6\msun$. Thus a globular cluster can appear frozen in the Newtonian regime despite being in the Milgromian regime for not longer than a few crossing times. We also study the kinematics and anisotropy profiles of the systems. The velocity dispersions of the systems are larger after the phase transitions, especially for the outer regions of the stellar systems. Moreover, the isotropic systems become radially anisotropic, especially for the outer parts, after the process caused by the dynamical phase transition. Deeper Milgromian systems have more radially anisotropic velocity dispersion functions. We also find that the final profiles of density, velocity dispersion and anisotropy do not depend on the details of the phase transition. I.e., the mass distribution and kinematics of the end-states of the globular clusters do not depend on the rapidity of the transition from Newtonian to Milgromian gravity. Thus, the transition from the Newtonian to the Milgromian regime naturally induces a significant radially anisotropic velocity distribution in a globular cluster.

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arXiv2012

Stirring N-body systems: Universality of end states

We study the evolution of the phase-space of collisionless N-body systems under repeated stirrings or perturbations. We find convergence towards a limited solution group, in accordance with Hansen 2010, that is independent of the initial system and environmental conditions, paying particular attention to the assumed gravitational paradigm (Newtonian and MOND). We examine the effects of changes to the perturbation scheme and in doing so identify a large group of perturbations featuring radial orbit instability (ROI) which always lead to convergence. The attractor is thus found to be a robust and reproducible effect under a variety of circumstances.

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arXiv2010

Lopsidedness of cluster galaxies in modified gravity

We point out an interesting theoretical prediction for elliptical galaxies residing inside galaxy clusters in the framework of modified Newtonian dynamics (MOND), that could be used to test this paradigm. Apart from the central brightest cluster galaxy, other galaxies close enough to the centre experience a strong gravitational influence from the other galaxies of the cluster. This influence manifests itself only as tides in standard Newtonian gravity, meaning that the systematic acceleration of the centre of mass of the galaxy has no consequence. However, in the context of MOND, a consequence of the breaking of the strong equivalence principle is that the systematic acceleration changes the own self-gravity of the galaxy. We show here that, in this framework, initially axisymmetric elliptical galaxies become lopsided along the external field's direction, and that the centroid of the galaxy, defined by the outer density contours, is shifted by a few hundreds parsecs with respect to the densest point.

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arXiv2009

On the separation between baryonic and dark matter: evidence for phantom dark matter?

The recent years have seen combined measurements of X-ray and (weak) lensing contours for colliding galaxy clusters such as, for instance, the famous "Bullet" cluster. These observations have revealed offsets in the peaks of the baryonic and (dominant) gravitational matter component of order ~(100-200) kpc. Such discrepancies are difficult to explain using modified theories for gravity other than dark matter. Or are they not? Here we use the concept of "phantom dark matter" that is based upon a Newtonian interpretation of the MONDian gravitational potential. We show that this idea is in fact capable of producing substantial offsets in idealistic density configurations, involving a uniform external field. However, when analysed in a MONDian cosmological framework we deduce that the size (and probablity) of the effect is too small to explain the observed offsets found in the most recent observations, at least in the simplest incarnation of phantom dark matter as discussed here. The lensing centers in merging galaxy clusters are likely very close to the centers of true mass even in a MONDian cosmology. This gives the support to the idea that neutrino-like non-collisional matter might be responsible for the observed offsets of lensing and X-ray peaks.

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arXiv2009

N-body simulations for testing the stability of triaxial galaxies in MOND

We perform a stability test of triaxial models in MOdified Newtonian Dynamics (MOND) using N-body simulations. The triaxial models considered here have densities that vary with $r^{-1}$ in the center and $r^{-4}$ at large radii. The total mass of the model varies from $10^8\Msun$ to $10^{10}\Msun$, representing the mass scale of dwarfs to medium-mass elliptical galaxies, respectively, from deep MOND to quasi-Newtonian gravity. We build triaxial galaxy models using the Schwarzschild technique, and evolve the systems for 200 Keplerian dynamical times (at the typical length scale of 1.0 kpc). We find that the systems are virial overheating, and in quasi-equilibrium with the relaxation taking approximately 5 Keplerian dynamical times (1.0 kpc). For all systems, the change of the inertial (kinetic) energy is less than 10% (20%) after relaxation. However, the central profile of the model is flattened during the relaxation and the (overall) axis ratios change by roughly 10% within 200 Keplerian dynamical times (at 1.0kpc) in our simulations. We further find that the systems are stable once they reach the equilibrium state.

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arXiv2009

The Galactic potential and the asymmetric distribution of hypervelocity stars

In recent years several hypervelocity stars (HVSs) have been observed in the halo of our Galaxy. Such HVSs have possibly been ejected from the Galactic center and then propagated in the Galactic potential up to their current position. The recent survey for candidate HVSs show an asymmetry in the kinematics of candidate HVSs (position and velocity vectors), where more outgoing stars than ingoing stars (i.e. positive Galactocentric velocities vs. negative ones) are observed. We show that such kinematic asymmetry, which is likely due to the finite lifetime of the stars and Galactic potential structure, could be used in a novel method to probe and constrain the Galactic potential, identify the stellar type of the stars in the survey and estimate the number of HVSs. Kinematics-independent identification of the stellar types of the stars in such surveys (e.g. spectroscopic identification) could further improve these results. We find that the observed asymmetry between ingoing and outgoing stars favors specific Galactic potential models. It also implies a lower limit of ~54+-8 main sequence HVSs in the survey sample (>=648+-96 in the Galaxy), assuming that all of the main sequence stars in the survey originate from the Galactic center. The other stars in the survey are likely to be hot blue horizontal branch stars born in the halo rather than stars ejected from the Galactic center.

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arXiv2008

Milky Way potentials in CDM and MOND. Is the Large Magellanic Cloud on a bound orbit?

We compute the Milky Way potential in different cold dark matter (CDM) based models, and compare these with the modified Newtonian dynamics (MOND) framework. We calculate the axis ratio of the potential in various models, and find that isopotentials are less spherical in MOND than in CDM potentials. As an application of these models, we predict the escape velocity as a function of the position in the Galaxy. This could be useful in comparing with future data from planned or already-underway kinematic surveys (RAVE, SDSS, SEGUE, SIM, GAIA or the hypervelocity stars survey). In addition, the predicted escape velocity is compared with the recently measured high proper motion velocity of the Large Magellanic Cloud (LMC). To bind the LMC to the Galaxy in a MOND model, while still being compatible with the RAVE-measured local escape speed at the Sun's position, we show that an external field modulus of less than $0.03 a_0$ is needed.

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arXiv2008

Self-consistent models of triaxial galaxies in MOND gravity

The Bekenstein-Milgrom gravity theory with a modified Poisson equation is tested here for the existence of triaxial equilibrium solutions. Using the non-negative least square method, we show that self-consistent triaxial galaxies exist for baryonic models with a mild density cusp $ρ\sim {Σ\over r}$. Self-consistency is achieved for a wide range of central concentrations, $Σ\sim 10-1000\mathrm{M_{\odot}pc^{-2}}$, representing low-to-high surface brightness galaxies. Our results demonstrate for the first time that the orbit superposition technique is fruitful for constructing galaxy models beyond Newtonian gravity, and triaxial cuspy galaxies might exist without the help of Cold dark Matter.

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arXiv2007

Loss of mass and stability of galaxies in MOND

The self-binding energy and stability of a galaxy in MOND-based gravity are curiously decreasing functions of its center of mass acceleration towards neighbouring mass concentrations. A tentative indication of this breaking of the Strong Equivalence Principle in field galaxies is the RAVE-observed escape speed in the Milky Way. Another consequence is that satellites of field galaxies will move on nearly Keplerian orbits at large radii (100 - 500 kpc), with a declining speed below the asymptotically constant naive MOND prediction. But consequences of an environment-sensitive gravity are even more severe in clusters, where member galaxies accelerate fast: no more Dark-Halo-like potential is present to support galaxies, meaning that extended axisymmetric disks of gas and stars are likely unstable. These predicted reappearance of asymptotic Keplerian velocity curves and disappearance of "stereotypic galaxies" in clusters are falsifiable with targeted surveys.

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