ترغب بنشر مسار تعليمي؟ اضغط هنا

Numerical convergence of simulations of galaxy formation: the abundance and internal structure of cold dark matter haloes

277   0   0.0 ( 0 )
 نشر من قبل Aaron Ludlow Ph.D.
 تاريخ النشر 2018
  مجال البحث فيزياء
والبحث باللغة English
 تأليف Aaron D. Ludlow




اسأل ChatGPT حول البحث

We study the impact of numerical parameters on the properties of cold dark matter haloes formed in collisionless cosmological simulations. We quantify convergence in the median spherically-averaged circular velocity profiles for haloes of widely varying particle number, as well as in the statistics of their structural scaling relations and mass functions. In agreement with prior work focused on single haloes, our results suggest that cosmological simulations yield robust halo properties for a wide range of gravitational softening parameters, $epsilon$, provided: 1) $epsilon$ is not larger than a convergence radius, $r_{rm conv}$, which is dictated by 2-body relaxation and determined by particle number, and 2) a sufficient number of timesteps are taken to accurately resolve particle orbits with short dynamical times. Provided these conditions are met, median circular velocity profiles converge to within $approx 10$ per cent for radii beyond which the local 2-body relaxation timescale exceeds the Hubble time by a factor $kappaequiv t_{rm relax}/t_{rm H}gt 0.177$, with better convergence attained for higher $kappa$. We provide analytic estimates of $r_{rm conv}$ that build on previous attempts in two ways: first, by highlighting its explicit (but weak) softening-dependence and, second, by providing a simpler criterion in which $r_{rm conv}$ is determined entirely by the mean inter-particle spacing, $l$; for example, better than $10$ per cent convergence in circular velocity for $rgt 0.05,l$. We show how these analytic criteria can be used to assess convergence in structural scaling relations for dark matter haloes as a function of their mass or maximum circular speed.



قيم البحث

اقرأ أيضاً

113 - Aaron D. Ludlow 2019
We address the issue of numerical convergence in cosmological smoothed particle hydrodynamics simulations using a suite of runs drawn from the EAGLE project. Our simulations adopt subgrid models that produce realistic galaxy populations at a fiducial mass and force resolution, but systematically vary the latter in order to study their impact on galaxy properties. We provide several analytic criteria that help guide the selection of gravitational softening for hydrodynamical simulations, and present results from runs that both adhere to and deviate from them. Unlike dark matter-only simulations, hydrodynamical simulations exhibit a strong sensitivity to gravitational softening, and care must be taken when selecting numerical parameters. Our results--which focus mainly on star formation histories, galaxy stellar mass functions and sizes--illuminate three main considerations. First, softening imposes a minimum resolved escape speed, $v_epsilon$, due to the binding energy between gas particles. Runs that adopt such small softening lengths that $v_epsilon gt 10,{rm km s^{-1}}$ (the sound speed in ionised $sim 10^4,{rm K}$ gas) suffer from reduced effects of photo-heating. Second, feedback from stars or active galactic nuclei may suffer from numerical over-cooling if the gravitational softening length is chosen below a critical value, $epsilon_{rm eFB}$. Third, we note that small softening lengths exacerbate the segregation of stars and dark matter particles in halo centres, often leading to the counter-intuitive result that galaxy sizes {em increase} as softening is reduced. The structure of dark matter haloes in hydrodynamical runs respond to softening in a way that reflects the sensitivity of their galaxy populations to numerical parameters.
We study the effect of baryons on the abundance of structures and substructures in a Lambda-CDM cosmology, using a pair of high resolution cosmological simulations from the GIMIC project. Both simulations use identical initial conditions, but while o ne contains only dark matter, the other also includes baryons. We find that gas pressure, reionisation, supernova feedback, stripping, and truncated accretion systematically reduce the total mass and the abundance of structures below ~10^12 solar masses compared to the pure dark matter simulation. Taking this into account and adopting an appropriate detection threshold lowers the abundance of observed galaxies with maximum circular velocities below 100 km/s, significantly reducing the reported discrepancy between Lambda-CDM and the measured HI velocity function of the ALFALFA survey. We also show that the stellar-to-total mass ratios of galaxies with stellar masses of ~10^5 - 10^7 solar masses inferred from abundance matching of the (sub)halo mass function to the observed galaxy mass function increase by a factor of ~2. In addition, we find that an important fraction of low-mass subhaloes are completely devoid of stars. Accounting for the presence of dark subhaloes below 10^10 solar masses further reduces the abundance of observable objects, and leads to an additional increase in the inferred stellar-to-total mass ratio by factors of 2 - 10 for galaxies in haloes of 10^9 - 10^10 solar masses. This largely reconciles the abundance matching results with the kinematics of individual dwarf galaxies in Lambda-CDM. We propose approximate corrections to the masses of objects derived from pure dark matter calculations to account for baryonic effects.
174 - Mark Vogelsberger 2009
We simulate the growth of isolated dark matter haloes from self-similar and spherically symmetric initial conditions. Our N-body code integrates the geodesic deviation equation in order to track the streams and caustics associated with individual sim ulation particles. The radial orbit instability causes our haloes to develop major-to-minor axis ratios approaching 10 to 1 in their inner regions. They grow similarly in time and have similar density profiles to the spherical similarity solution, but their detailed structure is very different. The higher dimensionality of the orbits causes their stream and caustic densities to drop much more rapidly than in the similarity solution. This results in a corresponding increase in the number of streams at each point. At 1% of the turnaround radius (corresponding roughly to the Suns position in the Milky Way) we find of order 10^6 streams in our simulations, as compared to 10^2 in the similarity solution. The number of caustics in the inner halo increases by a factor of several, because a typical orbit has six turning points rather than one, but caustic densities drop by a much larger factor. This reduces the caustic contribution to the annihilation radiation. For the region between 1% and 50% of the turnaround radius, this is 4% of the total in our simulated haloes, as compared to 6.5% in the similarity solution. Caustics contribute much less at smaller radii. These numbers assume a 100 GeV c^-2 neutralino with present-day velocity dispersion 0.03 cm s^-1, but reducing the dispersion by ten orders of magnitude only doubles the caustic luminosity. We conclude that caustics will be unobservable in the inner parts of haloes. Only the outermost caustic might potentially be detectable.
We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investigate the response of cold dark matter (CDM) haloes to baryonic processes. Previous work has shown that the halo response is primarily a function of the r atio between galaxy stellar mass and total virial mass, and the density threshold above which gas is eligible to form stars, $n [{rm cm}^{-3}]$. At low $n$ all simulations in the literature agree that dwarf galaxy haloes are cuspy, but at high $nge 100$ there is no consensus. We trace halo contraction in dwarf galaxies with $nge 100$ reported in some previous simulations to insufficient spatial resolution. Provided the adopted star formation threshold is appropriate for the resolution of the simulation, we show that the halo response is remarkably stable for $nge 5$, up to the highest star formation threshold that we test, $n=500$. This free parameter can be calibrated using the observed clustering of young stars. Simulations with low thresholds $nle 1$ predict clustering that is too weak, while simulations with high star formation thresholds $nge 5$, are consistent with the observed clustering. Finally, we test the CDM predictions against the circular velocities of nearby dwarf galaxies. Low thresholds predict velocities that are too high, while simulations with $nsim 10$ provide a good match to the observations. We thus conclude that the CDM model provides a good description of the structure of galaxies on kpc scales provided the effects of baryons are properly captured.
We constrain cold dark energy of negligible sound speed using galaxy cluster abundance observations. In contrast to standard quasi-homogeneous dark energy, negligible sound speed implies clustering of the dark energy fluid at all scales, allowing us to measure the effects of dark energy perturbations at cluster scales. We compare those models and set the stage for using non-linear information from semi-analytical modelling in cluster growth data analyses. For this, we recalibrate the halo mass function with non-linear characteristic quantities, the spherical collapse threshold and virial overdensity, that account for model and redshift dependent behaviours, as well as an additional mass contribution for cold dark energy. We present the first constraints from this cold dark matter plus cold dark energy mass function using our cluster abundance likelihood, which self-consistently accounts for selection effects, covariances and systematic uncertainties. We combine cluster growth data with CMB, SNe Ia and BAO data, and find a shift between cold versus quasi-homogeneous dark energy of up to $1sigma$. We make a Fisher matrix forecast of constraints attainable with cluster growth data from the on-going Dark Energy Survey (DES). For DES, we predict $sim$50$%$ tighter constraints on $left(Omega_mathrm{m},w right)$ for cold dark energy versus $w$CDM models, with the same free parameters. Overall, we show that cluster abundance analyses are sensitive to cold dark energy, an alternative, viable model that should be routinely investigated alongside the standard dark energy scenario.
التعليقات
جاري جلب التعليقات جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
mircosoft-partner

هل ترغب بارسال اشعارات عن اخر التحديثات في شمرا-اكاديميا