arXiv subjects

Jiaxin Han

Publications and source records attributed to Jiaxin Han.

At least 91 records · Page 5Linked to original sources

arXiv2017

What to expect from dynamical modelling of galactic haloes

Many dynamical models of the Milky Way halo require assumptions that the distribution function of a tracer population should be independent of time (i.e., a steady state distribution function) and that the underlying potential is spherical. We study the limitations of such modelling by applying a general dynamical model with minimal assumptions to a large sample of galactic haloes from cosmological $N$-body and hydrodynamical simulations. Using dark matter particles as dynamical tracers, we find that the systematic uncertainties in the measured mass and concentration parameters typically have an amplitude of 25% to 40%. When stars are used as tracers, however, the systematic uncertainties can be as large as a factor of $2-3$. The systematic uncertainties are not reduced by increasing the tracer sample size and vary stochastically from halo to halo. These systematic uncertainties are mostly driven by underestimated statistical noise caused by correlated phase-space structures that violate the steady state assumption. The number of independent phase-space structures inferred from the uncertainty level sets a limiting sample size beyond which a further increase no longer significantly improves the accuracy of dynamical inferences. The systematic uncertainty level is determined by the halo merger history, the shape and environment of the halo. Our conclusions apply generally to any spherical steady-state model.

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arXiv2017

Environmental screening of dark matter haloes in $f(R)$ gravity

In certain theories of modified gravity, solar system constraints on deviations from general relativity (GR) are satisfied by virtue of a so-called screening mechanism, which enables the theory to revert to GR in regions where the matter density is high or the gravitational potential is deep. In the case of chameleon theories, the screening has two contributions -- self-screening, which is due to the mass of an object itself, and environmental screening, which is caused by the surrounding matter -- which are often entangled, with the second contribution being more crucial for less massive objects. A quantitative understanding of the effect of the environment on the screening can prove critical in observational tests of such theories using systems such as the Local Group and dwarf galaxies, for which the environment may be inferred in various ways. We use the high-resolution {\sc liminality} simulation of Shi et al. (2015) to test the fidelity of different definitions of environment. We find that, although the different ways to define environment in practice do not agree with one another perfectly, they can provide useful guidance, and cross checks about how well a dark matter halo is screened. In addition, the screening of subhaloes in dark matter haloes is primarily determined by the environment, with the subhalo mass playing a minor role, which means that lower-resolution simulations where subhaloes are not well resolved can still be useful for understanding the modification of gravity inside subhaloes.

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arXiv2016

Sussing Merger Trees: Stability and Convergence

Merger trees are routinely used to follow the growth and merging history of dark matter haloes and subhaloes in simulations of cosmic structure formation. Srisawat et al. (2013) compared a wide range of merger-tree-building codes. Here we test the influence of output strategies and mass resolution on tree-building. We find that, somewhat surprisingly, building the tree from more snapshots does not generally produce more complete trees; instead, it tends to short- en them. Significant improvements are seen for patching schemes which attempt to bridge over occasional dropouts in the underlying halo catalogues or schemes which combine the halo-finding and tree-building steps seamlessly. The adopted output strategy does not affec- t the average number of branches (bushiness) of the resultant merger trees. However, mass resolution has an influence on both main branch length and the bushiness. As the resolution increases, a halo with the same mass can be traced back further in time and will encounter more small progenitors during its evolutionary history. Given these results, we recommend that, for simulations intended as precursors for galaxy formation models where of order 100 or more snapshots are analysed, the tree-building routine should be integrated with the halo finder, or at the very least be able to patch over multiple adjacent snapshots.

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arXiv2015

A unified model for the spatial and mass distribution of subhaloes

N-body simulations suggest that the substructures that survive inside dark matter haloes follow universal distributions in mass and radial number density. We demonstrate that a simple analytical model can explain these subhalo distributions as resulting from tidal stripping which increasingly reduces the mass of subhaloes with decreasing halo-centric distance. As a starting point, the spatial distribution of subhaloes of any given infall mass is shown to be largely indistinguishable from the overall mass distribution of the host halo. Using a physically motivated statistical description of the amount of mass stripped from individual subhaloes, the model fully describes the joint distribution of subhaloes in final mass, infall mass and radius. As a result, it can be used to predict several derived distributions involving combinations of these quantities including, but not limited to, the universal subhalo mass function, the subhalo spatial distribution, the gravitational lensing profile, the dark matter annihilation radiation profile and boost factor. This model clarifies a common confusion when comparing the spatial distributions of galaxies and subhaloes, the so called "anti-bias", as a simple selection effect. We provide a Python code SubGen for populating haloes with subhaloes at http://icc.dur.ac.uk/data/

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arXiv2015

The orbital PDF: general inference of the gravitational potential from steady-state tracers

We develop two general methods to infer the gravitational potential of a system using steady-state tracers, i.e., tracers with a time-independent phase-space distribution. Combined with the phase-space continuity equation, the time independence implies a universal Orbital Probability Density Function (oPDF) $\mathrm{d} P(λ|{\rm orbit})\propto \mathrm{d} t$, where $λ$ is the coordinate of the particle along the orbit. The oPDF is equivalent to Jeans theorem, and is the key physical ingredient behind most dynamical modelling of steady-state tracers. In the case of a spherical potential, we develop a likelihood estimator that fits analytical potentials to the system, and a non-parametric method ("phase-mark") that reconstructs the potential profile, both assuming only the oPDF. The methods involve no extra assumptions about the tracer distribution function and can be applied to tracers with any arbitrary distribution of orbits, with possible extension to non-spherical potentials. The methods are tested on Monte Carlo samples of steady-state tracers in dark matter haloes to show that they are unbiased as well as efficient. A fully documented \textsc{C/Python} code implementing our method is freely available at a GitHub repository linked from \url{http://icc.dur.ac.uk/data/#oPDF}.

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arXiv2015

The orbital PDF: the dynamical state of Milky Way sized haloes and the intrinsic uncertainty in the determination of their masses

Using realistic cosmological simulations of Milky Way sized haloes, we study their dynamical state and the accuracy of inferring their mass profiles with steady-state models of dynamical tracers. We use a new method that describes the phase-space distribution of a steady-state tracer population in a spherical potential without any assumption regarding the distribution of their orbits. Applying the method to five haloes from the Aquarius $Λ$CDM N-body simulation, we find that dark matter particles are an accurate tracer that enables the halo mass and concentration parameters to be recovered with an accuracy of $5\%$. Assuming a potential profile of the NFW form does not significantly affect the fits in most cases, except for halo A whose density profile differs significantly from the NFW form, leading to a $30\%$ bias in the dynamically fitted parameters. The existence of substructures in the dark matter tracers only affects the fits by $\sim 1\%$. Applying the method to mock stellar haloes generated by a particle-tagging technique, we find the stars are farther from equilibrium than dark matter particles, yielding a systematic bias of $\sim 20\%$ in the inferred mass and concentration parameter. The level of systematic biases obtained from a conventional distribution function fit to stars is comparable to ours, while similar fits to DM tracers are significantly biased in contrast to our fits. In line with previous studies, the mass bias is much reduced near the tracer half-mass radius.

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arXiv2015

A weak gravitational lensing recalibration of the scaling relations linking the gas properties of dark halos to their mass

We use weak gravitational lensing to measure mean mass profiles around Locally Brightest Galaxies (LBGs). These are selected from the SDSS/DR7 spectroscopic and photometric catalogues to be brighter than any neighbour projected within 1.0 Mpc and differing in redshift by $<1000$ km/s. Most ($> 83\%$) are expected to be the central galaxies of their dark matter halos. Previous stacking analyses have used this LBG sample to measure mean Sunyaev-Zeldovich flux and mean X-ray luminosity as a function of LBG stellar mass. In both cases, a simulation of the formation of the galaxy population was used to estimate effective halo mass for LBGs of given stellar mass, allowing the derivation of scaling relations between the gas properties of halos and their mass. By comparing results from a variety of simulations to our lensing data, we show that this procedure has significant model dependence reflecting: (i) the failure of any given simulation to reproduce observed galaxy abundances exactly; (ii) a dependence on the cosmology underlying the simulation; and (iii) a dependence on the details of how galaxies populate halos. We use our lensing results to recalibrate the scaling relations, eliminating most of this model dependence and explicitly accounting both for residual modelling uncertainties and for observational uncertainties in the lensing results. The resulting scaling relations link the mean gas properties of dark halos to their mass over an unprecedentedly wide range, $10^{12.5}<M_{500}/ \mathrm{M_\odot}<10^{14.5}$, and should fairly and robustly represent the full halo population.

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arXiv2015

Estimating the dark matter halo mass of our Milky Way using dynamical tracers

The mass of the dark matter halo of the Milky Way can be estimated by fitting analytical models to the phase-space distribution of dynamical tracers. We test this approach using realistic mock stellar halos constructed from the Aquarius N-body simulations of dark matter halos in the $Λ$CDM cosmology. We extend the standard treatment to include a Navarro-Frenk-White (NFW) potential and use a maximum likelihood method to recover the parameters describing the simulated halos from the positions and velocities of their mock halo stars. We find that the estimate of halo mass is highly correlated with the estimate of halo concentration. The best-fit halo masses within the virial radius, $R_{200}$, are biased, ranging from a 40\% underestimate to a 5\% overestimate in the best case (when the tangential velocities of the tracers are included). There are several sources of bias. Deviations from dynamical equilibrium can potentially cause significant bias; deviations from spherical symmetry are relatively less important. Fits to stars at different galactocentric radii can give different mass estimates. By contrast, the model gives good constraints on the mass within the half-mass radius of tracers even when restricted to tracers within 60kpc. The recovered velocity anisotropies of tracers, $β$, are biased systematically, but this does not affect other parameters if tangential velocity data are used as constraints.

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arXiv2015

Planes of satellite galaxies: when exceptions are the rule

The detection of planar structures within the satellite systems of both the Milky Way (MW) and Andromeda (M31) has been reported as being in stark contradiction to the predictions of the standard cosmological model ($Λ$CDM). Given the ambiguity in defining a planar configuration, it is unclear how to interpret the low incidence of the MW and M31 planes in $Λ$CDM. We investigate the prevalence of satellite planes around galactic mass haloes identified in high resolution cosmological simulations. We find that planar structures are very common, and that ~10% of $Λ$CDM haloes have even more prominent planes than those present in the Local Group. While ubiquitous, the planes of satellite galaxies show a large diversity in their properties. This precludes using one or two systems as small scale probes of cosmology, since a large sample of satellite systems is needed to obtain a good measure of the object-to-object variation. This very diversity has been misinterpreted as a discrepancy between the satellite planes observed in the Local Group and $Λ$CDM predictions. In fact, ~10% of $Λ$CDM galactic haloes have planes of satellites that are as infrequent as the MW and M31 planes. The look-elsewhere effect plays an important role in assessing the detection significance of satellite planes and accounting for it leads to overestimating the significance level by a factor of 30 and 100 for the MW and M31 systems, respectively.

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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.

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arXiv2015

Major Mergers Going Notts: Challenges for Modern Halo Finders

Merging haloes with similar masses (i.e., major mergers) pose significant challenges for halo finders. We compare five halo finding algorithms' (AHF, HBT, Rockstar, SubFind, and VELOCIraptor) recovery of halo properties for both isolated and cosmological major mergers. We find that halo positions and velocities are often robust, but mass biases exist for every technique. The algorithms also show strong disagreement in the prevalence and duration of major mergers, especially at high redshifts (z>1). This raises significant uncertainties for theoretical models that require major mergers for, e.g., galaxy morphology changes, size changes, or black hole growth, as well as for finding Bullet Cluster analogues. All finders not using temporal information also show host halo and subhalo relationship swaps over successive timesteps, requiring careful merger tree construction to avoid problematic mass accretion histories. We suggest that future algorithms should combine phase-space and temporal information to avoid the issues presented.

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arXiv2014

Galaxy and Mass Assembly (GAMA): The halo mass of galaxy groups from maximum-likelihood weak lensing

We present a maximum-likelihood weak lensing analysis of the mass distribution in optically selected spectroscopic Galaxy Groups (G$^3$Cv5) in the Galaxy And Mass Assembly (GAMA) survey, using background Sloan Digital Sky Survey (SDSS) photometric galaxies. The scaling of halo mass, $M_h$, with various group observables is investigated. Our main results are: 1) the measured relations of halo mass with group luminosity, virial volume and central galaxy stellar mass, $M_\star$, agree very well with predictions from mock group catalogues constructed from a GALFORM semi-analytical galaxy formation model implemented in the Millennium $Λ$CDM N-body simulation; 2) the measured relations of halo mass with velocity dispersion and projected half-abundance radius show weak tension with mock predictions, hinting at problems in the mock galaxy dynamics and their small scale distribution; 3) the median $M_h|M_\star$ measured from weak lensing depends more sensitively on the lognormal dispersion in $M_\star$ at fixed $M_h$ than it does on the median $M_\star|M_h$. Our measurements suggest an intrinsic dispersion of $σ_{\log(M_\star)}\sim 0.15$; 4) Comparing our mass estimates with those in the catalogue, we find that the G$^3$Cv5 mass can give biased results when used to select subsets of the group sample. Of the various new halo mass estimators that we calibrate using our weak lensing measurements, group luminosity is the best single-proxy estimator of group mass.

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arXiv2014

Sussing Merger Trees : The Impact of Halo Merger Trees on Galaxy Properties in a Semi-Analytic Model

A halo merger tree forms the essential backbone of a semi-analytic model for galaxy formation and evolution. Recent studies have pointed out that extracting merger trees from numerical simulations of structure formation is non-trivial; different tree building algorithms can give differing merger histories. These differences should be carefully understood before merger trees are used as input for models of galaxy formation. We investigate the impact of different halo merger trees on a semi-analytic model. We find that the z=0 galaxy properties in our model show differences between trees when using a common parameter set. The star formation history of the Universe and the properties of satellite galaxies can show marked differences between trees with different construction methods. Independently calibrating the semi-analytic model for each tree can reduce the discrepancies between the z=0 global galaxy properties, at the cost of increasing the differences in the evolutionary histories of galaxies. Furthermore, the underlying physics implied can vary, resulting in key quantities such as the supernova feedback efficiency differing by factors of 2. Such a change alters the regimes where star formation is primarily suppressed by supernovae. Therefore, halo merger trees extracted from a common halo catalogue using different, but reliable, algorithms can result in a difference in the semi-analytic model. Given the uncertainties in galaxy formation physics, however, these differences may not necessarily be viewed as significant.

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arXiv2014

Subhaloes gone Notts: Subhaloes as tracers of the dark matter halo shape

We study the shapes of subhalo distributions from four dark-matter-only simulations of Milky Way type haloes. Comparing the shapes derived from the subhalo distributions at high resolution to those of the underlying dark matter fields we find the former to be more triaxial if theanalysis is restricted to massive subhaloes. For three of the four analysed haloes the increased triaxiality of the distributions of massive subhaloes can be explained by a systematic effect caused by the low number of objects. Subhaloes of the fourth halo show indications for anisotropic accretion via their strong triaxial distribution and orbit alignment with respect to the dark matter field. These results are independent of the employed subhalo finder. Comparing the shape of the observed Milky Way satellite distribution to those of high-resolution subhalo samples from simulations, we find an agreement for samples of bright satellites, but significant deviations if faint satellites are included in the analysis. These deviations might result from observational incompleteness.

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arXiv2014

The co-evolution of black hole growth and star formation from a cross-correlation analysis between quasars and the cosmic infrared background

We present the first cross-correlation measurement between Sloan Digital Sky Survey (SDSS) Type 1 quasars and the cosmic infrared background (CIB) measured by Herschel. The distribution of the quasars at 0.15 =1.4) is $11.1^{+1.6}_{-1.4}$, $7.1^{+1.6}_{-1.3}$ and $3.6^{+1.4}_{-1.0}$ mJy at 250, 350 and 500 microns, respectively, while the mean sub-mm flux densities of the DR9 quasars ( =2.5) is $5.7^{+0.7}_{-0.6}$, $5.0^{+0.8}_{-0.7}$ and $1.8^{+0.5}_{-0.4}$ mJy. We find that the correlated sub-mm emission includes both the emission from satellite DSFGs in the same halo as the central quasar and the emission from DSFGs in separate halos (correlated with the quasar-hosting halo). The amplitude of the one-halo term is ~10 times smaller than the sub-mm emission of the quasars, implying the the satellites have a lower star-formation rate than the quasars. The satellite fraction for the DR7 quasars is $0.008^{+0.008}_{-0.005}$ and the host halo mass scale for the central and satellite quasars is $10^{12.36\pm0.87}$ M$_{\odot}$ and $10^{13.60\pm0.38}$ M$_{\odot}$, respectively. The satellite fraction of the DR9 quasars is $0.065^{+0.021}_{-0.031}$ and the host halo mass scale for the central and satellite quasars is $10^{12.29\pm0.62}$ M$_{\odot}$ and $10^{12.82\pm0.39}$ M$_{\odot}$, respectively. Thus, the typical halo environment of the SDSS Type 1 quasars is found to be similar to that of DSFGs, which supports the generally accepted view that dusty starburst and quasar activity are evolutionarily linked.

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arXiv2014

A scaling relation between merger rate of galaxies and their close pair count

We study how to measure the galaxy merger rate from the observed close pair count. Using a high-resolution N-body/SPH cosmological simulation, we find an accurate scaling relation between galaxy pair counts and merger rates down to a stellar mass ratio of about 1:30. The relation explicitly accounts for the dependence on redshift (or time), on pair separation, and on mass of the two galaxies in a pair. With this relation, one can easily obtain the mean merger timescale for a close pair of galaxies. The use of virial masses, instead of stellar masses, is motivated by the fact that the dynamical friction time scale is mainly determined by the dark matter surrounding central and satellite galaxies. This fact can also minimize the error induced by uncertainties in modeling star formation in the simulation. Since the virial mass can be read from the well-established relation between the virial masses and the stellar masses in observation, our scaling relation can be easily applied to observations to obtain the merger rate and merger time scale. For major merger pairs (1:1-1:4) of galaxies above a stellar mass of 4*10^10 M_sun/h at z=0.1, it takes about 0.31 Gyr to merge for pairs within a projected distance of 20 kpc/h with stellar mass ratio of 1:1, while the time taken goes up to 1.6 Gyr for mergers with stellar mass ratio of 1:4. Our results indicate that a single timescale usually used in literature is not accurate to describe mergers with the stellar mass ratio spanning even a narrow range from 1:1 to 1:4.

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arXiv2014

Sussing Merger Trees: the influence of the halo finder

Merger tree codes are routinely used to follow the growth and merger of dark matter haloes in simulations of cosmic structure formation. Whereas in Srisawat et. al. we compared the trees built using a wide variety of such codes here we study the influence of the underlying halo catalogue upon the resulting trees. We observe that the specifics of halo finding itself greatly influences the constructed merger trees. We find that the choices made to define the halo mass are of prime importance. For instance, amongst many potential options different finders select self-bound objects or spherical regions of defined overdensity, decide whether or not to include substructures within the mass returned and vary in their initial particle selection. The impact of these decisions is seen in tree length (the period of time a particularly halo can be traced back through the simulation), branching ratio (essentially the merger rate of \subhalos) and mass evolution. We therefore conclude that the choice of the underlying halo finder is more relevant to the process of building merger trees than the tree builder itself. We also report on some built-in features of specific merger tree codes that (sometimes) help to improve the quality of the merger trees produced.

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arXiv2014

The Revised IRAS-FSC Redshift Catalogue (RIFSCz)

We present a Revised IRAS-FSC Redshift Catalogue (RIFSCz) of 60,303 galaxies selected at 60 microns from the IRAS Faint Source Catalogue (FSC). This revision merges in data from the WISE All-Sky Data Release, the Tenth SDSS Data Release (DR10), the GALEX All-Sky Survey Source Catalog (GASC), the 2MASS Redshift Survey (2MRS) and the Planck Catalogue of Compact Sources (PCCS). The RIFSCz consists of accurate position, ultra-violet (UV), optical, near-, mid- and far-infrared, sub-millimetre (sub-mm) and/or radio identifications, spectroscopic redshift (if available) or photometric redshift (if possible), predicted far-infrared and sub-mm fluxes ranging from 12 to 1380 microns based upon the best-fit infrared template. We also provide stellar masses, star-formation rates and dust masses derived from the optical and infrared template fits, where possible. 56 of the galaxies in the RIFSCz have spectroscopic redshifts and a further 26 have photometric redshifts obtained through the template-fitting method. At S60 > 0.36 Jy, the 90% completeness limit of the FSC, 93 of the sources in the RIFSCz have either spectroscopic or photometric redshifts. An interesting subset of the catalogue is the sources detected by Planck at sub-mm wavelengths. 1200 sources have a detection at better than 5 sigma in at least one Planck band and a further 1186 sources have detections at 3-5 sigma in at least one Planck band.

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